feat(fleet): mosaic fleet regen — regenerate roster-derived projections (PR3 of #791 ) (#813 )

feat(mosaic): durable pre-update snapshot + verify net + restore CLI (#791 PR2) (#811 )
2026-07-17 03:17:55 +00:00 · 2026-07-17 00:43:18 +00:00
17 changed files with 3418 additions and 41 deletions
--- a/.woodpecker/ci.yml
+++ b/.woodpecker/ci.yml
@@ -48,15 +48,19 @@ steps:
  # keep mode is a single cp-based path that must not depend on rsync), and that a
  # corrupt/empty/missing manifest aborts fail-closed leaving operator files
  # untouched (B2/B3). The rollback gate proves a mid-sync failure is rolled back
-  # from the pre-update snapshot (B1). The migration matrix pins the v2→v3
-  # contract-file semantics. Pure bash, no node_modules — runs early alongside
-  # sanitization.
+  # from the pre-update snapshot (B1). The durable-snapshot gate (#791 PR2) proves
+  # the retained, operator-scoped pre-update backup is taken before any mutation
+  # (0700/0600, secret never logged, retention-pruned) and that the post-sync
+  # verify net restores any operator file a manifest bug lets the sync touch. The
+  # migration matrix pins the v2→v3 contract-file semantics. Pure bash, no
+  # node_modules — runs early alongside sanitization.
  upgrade-guard:
    image: *node_image
    commands:
      - apk add --no-cache bash rsync
      - bash packages/mosaic/framework/tools/quality/scripts/test-upgrade-manifest-guard.sh
      - bash packages/mosaic/framework/tools/quality/scripts/test-upgrade-rollback.sh
+      - bash packages/mosaic/framework/tools/quality/scripts/test-upgrade-durable-snapshot.sh
      - bash packages/mosaic/framework/tools/quality/scripts/test-install-migration.sh

  typecheck:
--- a/docs/guides/fleet-local-canary.md
+++ b/docs/guides/fleet-local-canary.md
@@ -98,6 +98,39 @@ Expected results:
  that means the unit ran, not that an agent pane is live. Treat tmux
  `has-session`, `list-panes`, process tree, and logs as the liveness evidence.

+## Recovery — rebuild generated env projections
+
+Each agent's `~/.config/mosaic/fleet/agents/<name>.env.generated` is a
+deterministic projection of `roster.yaml` (the SSOT) that the launcher
+(`start-agent-session.sh`) sources at start. If an upgrade or a manual mistake
+wipes or diverges those projections, rebuild them from the roster with
+`mosaic fleet regen` — do NOT restart the affected unit first.
+
+```bash
+mosaic fleet regen            # dry-run (default): show create/rebuild plan per agent
+mosaic fleet regen --json     # same plan, machine-readable
+mosaic fleet regen --write    # rebuild fleet/agents/<name>.env.generated on disk
+```
+
+`regen` is projection-only and **never restarts an agent** — it has no path to
+systemd lifecycle. It is dry-run by default, deterministic/idempotent, uses the
+same roster→env mapping as `mosaic fleet reconcile`, and emits paths and counts
+only (never the projected `KEY=value` body). After `--write`, verify each unit
+resolves the intended values before restarting one unit at a time. The unit sets
+no `EnvironmentFile=` — `start-agent-session.sh` sources `.env.generated` itself —
+so verify the generated file directly and the launcher path, not a nonexistent
+`EnvironmentFile` property:
+
+```bash
+test -f ~/.config/mosaic/fleet/agents/<name>.env.generated
+systemctl --user cat mosaic-agent@<name> | grep ExecStart
+systemctl --user restart mosaic-agent@<name>
+```
+
+Full recovery runbook and the three-layer #791 protection model (manifest
+ownership → pre-update snapshot/restore → regen): see
+[Upgrade Safety & Recovery](./upgrade-safety-and-recovery.md).
+
 ## Release Preflight

 Run this checklist before cutting or dogfooding a fleet release:
--- a/docs/guides/upgrade-safety-and-recovery.md
+++ b/docs/guides/upgrade-safety-and-recovery.md
@@ -0,0 +1,147 @@
+# Upgrade Safety & Recovery
+
+How Mosaic protects operator-owned configuration under `~/.config/mosaic` across
+framework upgrades, and how to recover if a projection is ever lost.
+
+A framework upgrade runs `install.sh` in keep-mode (`MOSAIC_INSTALL_MODE=keep`,
+`MOSAIC_SYNC_ONLY=1`) to refresh framework-owned files in place. The incident
+this hardening addresses: an upgrade that silently overwrites or deletes a file
+the operator owns — credentials, personas, a roster, or a generated agent env —
+with no snapshot to fall back to.
+
+Protection is layered. Each layer is independent; a later layer catches what an
+earlier one misses.
+
+## Layer 1 — Manifest-owned sync (prevention)
+
+The single source of truth for ownership is
+[`framework-manifest.txt`](../../packages/mosaic/framework/framework-manifest.txt).
+Both the bash installer and the TypeScript sync path resolve every path against
+this one file (parity is enforced by test), so they can never drift.
+
+- Ownership is **allow-list, deny-wins**: a path is framework-owned only if a
+  `[framework]` glob matches and no `[operator]` carve-out overrides it.
+- **Unknown paths default to operator** (fail-safe): a file the manifest never
+  anticipated is treated as operator-owned and is never pruned.
+- Keep-mode does a non-deleting copy plus an explicit, manifest-scoped prune that
+  only ever iterates framework globs — operator and unknown paths are
+  structurally unreachable by the prune.
+
+Result: a correct upgrade cannot touch operator config at all.
+
+## Layer 2 — Durable pre-update snapshot + verify net (safety + rollback)
+
+Before **any** mutation, the installer snapshots the operator-owned surface that
+exists into:
+
+```
+${XDG_STATE_HOME:-~/.local/state}/mosaic/backups/pre-update-<UTC-timestamp>/
+```
+
+- `0700` directories / `0600` files (`umask 077`, scoped and restored),
+  outside `~/.config/mosaic` and outside any repo.
+- **Fail-open**: a snapshot failure warns but never aborts the upgrade it
+  protects.
+- Retention is `MOSAIC_BACKUP_RETENTION` snapshots (default 5).
+
+After the sync, a **verify net** compares each snapshot file against its target
+and restores (with a loud warning) any operator file the upgrade diverged or
+removed — a divergence means a manifest bug slipped through Layer 1.
+
+Inspect and restore snapshots with the CLI:
+
+```bash
+mosaic restore --list                 # dry-run: enumerate snapshots by timestamp
+mosaic restore --from <UTC-timestamp> # restore the operator surface from one snapshot
+mosaic restore --from <ts> --dry-run  # preview a specific restore without writing
+```
+
+`mosaic restore` reports **counts and relative paths only** — it never emits file
+contents, so a secret in `tools/_lib/credentials.json` is never echoed. Restores
+are confirmation-gated (`--yes` or `MOSAIC_ASSUME_YES`) and write each leaf
+atomically with `O_NOFOLLOW` (a symlink swapped in after the snapshot fails
+closed rather than following out of the managed tree).
+
+## Layer 3 — Regeneration from roster SSOT (recovery)
+
+Some operator files are **derived** and do not need a byte-for-byte snapshot to
+recover — they can be rebuilt from their source of truth. The fleet's per-agent
+generated env projections are the prime case:
+
+- `~/.config/mosaic/fleet/agents/<name>.env.generated` is a deterministic
+  projection of `~/.config/mosaic/fleet/roster.yaml`.
+- The launcher (`start-agent-session.sh`, invoked by
+  `mosaic-agent@<name>.service`) sources that generated projection to establish
+  each agent's identity, runtime, model, and working directory. If it is missing
+  or wrong, the agent cannot launch with its intended identity.
+
+`mosaic fleet regen` rebuilds those projections from the roster SSOT:
+
+```bash
+mosaic fleet regen            # dry-run (default): show what would be rebuilt
+mosaic fleet regen --json     # same, machine-readable
+mosaic fleet regen --write    # rebuild the projections on disk
+```
+
+- **Dry-run by default.** Nothing is written until you pass `--write`.
+- **Deterministic and idempotent** — the projection is a pure function of the
+  roster, so repeated `--write` runs produce byte-identical files.
+- **Projection-only. It never restarts an agent.** Recovery order forbids
+  restart-before-verify; `regen` has no path to systemd lifecycle at all.
+- **It rebuilds only `<name>.env.generated`** — it never writes, relocates, or
+  deletes the operator-owned `.env` / `.env.local` surface.
+- It **validates the roster the same way `reconcile` does** (persona resolution
+  and protected-class tool-policy match), so a hand-edited or corrupt roster is
+  rejected rather than projected, and a `--write` takes the shared reconcile
+  lock so it cannot race a concurrent reconcile.
+- Output is **paths and counts only** — the rendered `KEY=value` body is never
+  echoed.
+
+`regen` uses the exact same roster→env mapping as `mosaic fleet reconcile`, so a
+recovered projection matches what a normal reconcile would have written.
+
+## Recovery runbook — wiped `fleet/agents/*.env.generated`
+
+If an upgrade (or a manual mistake) has left an agent without its generated
+projection, **do not restart the unit first** — a launch against a missing
+projection fails closed, and any stale state must be corrected before restart,
+not after.
+
+1. **Prefer a snapshot restore if one exists** (byte-exact operator state):
+
+   ```bash
+   mosaic restore --list
+   mosaic restore --from <UTC-timestamp>
+   ```
+
+2. **Otherwise regenerate the derived projections from the roster SSOT:**
+
+   ```bash
+   mosaic fleet regen            # confirm the plan (create vs rebuild per agent)
+   mosaic fleet regen --write    # rebuild fleet/agents/<name>.env.generated
+   ```
+
+3. **Verify each unit will resolve the intended runtime/workdir _before_ any
+   restart.** The unit sets **no** `EnvironmentFile=` — it launches from a minimal
+   environment and `start-agent-session.sh` sources `.env.generated` itself, so
+   verify the generated file directly and confirm the launcher path:
+
+   ```bash
+   # Confirm fleet/agents/<name>.env.generated exists and carries the intended
+   # MOSAIC_AGENT_* values (name, runtime, model, workdir, socket).
+   test -f ~/.config/mosaic/fleet/agents/<name>.env.generated
+   # Confirm the unit launches the session script that reads it.
+   systemctl --user cat mosaic-agent@<name> | grep ExecStart
+   ```
+
+4. **Only then restart, one unit at a time:**
+
+   ```bash
+   systemctl --user restart mosaic-agent@<name>
+   ```
+
+## See also
+
+- Design: [`docs/design/791-upgrade-config-protection.md`](../design/791-upgrade-config-protection.md)
+- Fleet operations: [`docs/guides/fleet-local-canary.md`](./fleet-local-canary.md)
+- Ownership SSOT: [`packages/mosaic/framework/framework-manifest.txt`](../../packages/mosaic/framework/framework-manifest.txt)
--- a/docs/scratchpads/791-upgrade-config-protection.md
+++ b/docs/scratchpads/791-upgrade-config-protection.md
@@ -214,3 +214,316 @@ Re-ran codex again; it found two more rollback-path gaps `set -E` cannot catch.
 - Gates: typecheck ✓ lint ✓ format:check ✓ · full mosaic vitest 1094 · HARD GATE 193/193 ·
  rollback 28/28 (was 14, +14 for D1/D2 with controls) · migration 21/21. shellcheck clean on new lines.
  No --no-verify. Committing FORWARD (no rebase of 34e55d4a/0a5e703a).
+
+## Session 3 (2026-07-16) — PR1 MERGED, starting PR2 (durable snapshot + restore + secrev)
+PR1 (#802) squash-merged → main `32a0ffba`; issue #791 stays open (3-PR DAG umbrella). Independent Opus
+adversarial/security review APPROVED at head `af627e75` (Gitea RoR cmt 17892); lead ran rollback 28/28 +
+HARD GATE 193/193 green; CI #1877 green. PR2 UNBLOCKED.
+
+PR2 branch: `feat/791-pr2-snapshot-restore` off `origin/main` 32a0ffba. Same treatment applies:
+tests-first red-first, independent review + durable Gitea Reviewer-of-Record comment BEFORE MS-LEAD runs
+the queue guard/merge. Report PR2 number + exact head when ready. PR body: `Part of #791` (NOT Fixes).
+
+### PR2 scope (ratified §3/§5 of design doc, Mos-approved — do NOT re-litigate)
+- **(a) Durable pre-update snapshot** to `${XDG_STATE_HOME:-~/.local/state}/mosaic/backups/pre-update-<UTC-ts>/`
+  — OUTSIDE ~/.config/mosaic and any repo. Perms dir 0700 / files 0600 (umask 077 + explicit chmod).
+  Scope = operator-owned surface that EXISTS (operatorReserved paths), not the framework tree. Taken
+  BEFORE any mutation. Retention N=5 (`MOSAIC_BACKUP_RETENTION`), prune older.
+- **Post-sync verify + selective restore**: diff operator surface vs snapshot; (b) should never touch
+  operator paths, so ANY diff = manifest bug → restore affected paths + warn loudly. (a) catches a (b) miss.
+- **`mosaic restore`** (TS CLI): `--list` (default, dry-run) enumerates snapshots by ts; `--from <ts>`
+  restores over operator surface, confirmation-gated. Counts/paths only.
+- **Secret-safety (secrev)**: snapshot/restore NEVER emit file contents; only paths/counts. Tests assert
+  0700/0600 AND that a secret value seeded in tools/_lib/credentials.json never appears in any output.
+
+### PR2 implementation status (2026-07-16, ready-for-review)
+All three tasks implemented, red-first proven, unit-green:
+- **Task #10 — durable snapshot (install.sh)**: `backup_root()`/`enumerate_operator_files()`/
+  `prune_durable_snapshots()`/`make_durable_snapshot()` wired into keep-mode main() after `manifest_load`,
+  before any mutation. umask 077 + explicit chmod 700/600. UTC ts, collision suffix. FAIL-OPEN (a backup
+  failure never aborts the upgrade it protects). Retention `MOSAIC_BACKUP_RETENTION` (default 5), in-place
+  `sort -r -o` prune (no `mv` — stays inside the rsync-absent coreutils whitelist).
+- **Task #11 — post-sync verify net (install.sh)**: `verify_operator_surface()` runs after sync (trap
+  disarmed), `cmp -s` each snapshot file vs target; restores any diverged/missing operator file + warns
+  loudly (a divergence = manifest bug). VERIFY-NET wired before `cleanup_snapshot`.
+- **Task #12 — `mosaic restore` (TS)**: `src/commands/restore.ts` + co-located spec (19 tests).
+  `--list` default (dry-run enumerate), `--from <ts>` confirmation-gated restore, `--dry-run`, `--yes`/
+  `MOSAIC_ASSUME_YES`. Injectable `confirm` for testability (proceed/decline/env-bypass covered). Restored
+  files forced 0600. Registered in `cli.ts`. Path convention mirrors install.sh `backup_root()`.
+- **CI**: `.woodpecker/ci.yml` upgrade-guard runs the new `test-upgrade-durable-snapshot.sh` gate.
+- **Gates green**: typecheck ✓ lint ✓ format:check ✓ · full mosaic vitest 1241 (+5) ·
+  durable-snapshot 26/26 · manifest-guard 193/193 · rollback 28/28 · migration 21/21.
+  Est. new-code coverage ≈93% (only the interactive readline default + process.exit-on-error uncovered).
+- Regression fixed: PR2's `date`/`sort`/`mv` broke the rsync-absent manifest-guard PATH whitelist →
+  made date/sort fail-open, replaced `mv` with in-place `sort -o`, added `date sort` to the test whitelist
+  + isolated `XDG_STATE_HOME`. All 193 manifest-guard assertions green under restricted PATH.
+- Codex code-review + security-review (secrev) run on the uncommitted diff before commit.
+
+### PR2 review round 1 — findings + remediations (2026-07-16, pre-PR)
+Codex code-review returned **request-changes** (1 blocker + 3 should-fix); Codex security-review returned
+**high** (1 high + 1 medium). Deduped to 5 distinct defects, ALL legitimate, ALL fixed FORWARD, each with
+a red-first regression test whose control neuters exactly the guard under test:
+
+- **A · BLOCKER — verify net undid the legacy bin/ migration (install.sh).** On a pre-v2 install `bin/**`
+  is operator-classified, so the durable snapshot captured it; `run_migrations()` deletes bin/ on purpose,
+  but `verify_operator_surface()` then saw it "missing" and healed it back — the migration would be silently
+  undone forever once the version stamps. **Fix:** `MIGRATION_REMOVED_PATHS[]` recorded by run_migrations
+  (`bin`,`rails`) + `is_migration_removed()` skip in the verify loop (`# MIGRATION-SKIP-GUARD`).
+  **Test:** Part 6 — v1 fixture with bin/; shipped keeps it removed + stamps v3; control (guard stripped)
+  wrongly restores bin/tool.sh.
+- **B · HIGH (CWE-59) — restore/verify wrote secrets THROUGH a symlink (install.sh + restore.ts).** An
+  attacker swapping an operator path (e.g. tools/_lib/credentials.json) for a symlink after the snapshot
+  would make `cp`/`copyFileSync` write the snapshot's secret out through the link. **Fix (bash):** refuse a
+  symlinked ancestor (`has_symlinked_parent`), drop a symlinked leaf before restore
+  (`# SYMLINK-LEAF-GUARD`). **Fix (TS):** reuse audited `secure-file.ts` — `assertCanonicalContainment`
+  + `ensureManagedDirectory` on every dst, open the leaf `O_NOFOLLOW|O_CREAT|O_TRUNC` 0600 (ELOOP =
+  fail-closed). **Tests:** Part 7 (shipped leaves external exfil target untouched, restores a real 0600
+  file; control leaks the secret through the link) + restore.spec symlinked-leaf/ancestor cases (red-first).
+- **C · MEDIUM/should-fix (CWE-22) — `--from` traversal escaped the backup root (restore.ts).**
+  `join(root, from)` accepted `../poison`. **Fix:** validate the selector against
+  `^\d{8}T\d{6}Z(?:-\d+)?$`, build exactly `join(root,'pre-update-'+ts)`, `lstat` (reject symlinked snap
+  dir). **Test:** restore.spec `it.each` of 6 malformed selectors + `--from ../poison` fail-closed (red-first).
+- **D · should-fix — verify `mkdir -p` unguarded under set -e (install.sh).** A parent replaced by a
+  regular file aborted the installer before the recovery pointer printed. **Fix:** guard `mkdir -p`, warn
+  + `continue` on failure (keeps healing remaining files).
+- **E · should-fix — snapshot `umask 077` leaked process-global (install.sh).** Later sync copies/dirs
+  inherited 0600/0700. **Fix:** save `old_umask`, restore on EVERY return path (`# UMASK-RESTORE-NORMAL`).
+  **Test:** Part 8 — synced framework file is 0644 while the secret backup stays 0600; control (restore
+  stripped) makes the synced file 0600.
+
+**Full gate suite re-run after fixes (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic
+vitest **1252** · restore.spec **30** · durable-snapshot **41** · manifest-guard 193 · rollback 28 ·
+migration 21. shellcheck clean on all new lines; new test markers mirror the existing `# VERIFY-NET`
+anchor convention. NOTE: codex self-review does NOT satisfy the independent-review gate — an independent
+(author≠reviewer) review + durable Gitea Reviewer-of-Record comment is still required before MS-LEAD merges.
+
+## Session 4 (2026-07-16) — PR2 MERGED, PR3 built (fleet regen — recovery layer)
+PR2 (#811) squash-merged → main `31607a4a`; issue #791 stays open (final PR of the 3-PR DAG). Independent
+exact-head RoR at `d12c5f78` APPROVE (Gitea cmt 17904); #1882 green; busybox-portable Part 7 control fix
+verified in-Alpine. PR3 UNBLOCKED.
+
+PR3 branch: `feat/791-pr3-fleet-regen` off `origin/main` 31607a4. Same discipline: tests-first red-first,
+independent review + durable Gitea RoR BEFORE MS-LEAD runs the queue guard/merge. PR body `Part of #791`.
+
+### PR3 scope (ratified §4/§7 of design doc) — `mosaic fleet regen`
+Projection-only recovery command: rebuilds each `fleet/agents/<name>.env.generated` from `roster.yaml`
+(SSOT). Dry-run default; `--write` applies; `--json` machine output. Structural guarantee: NO code path to
+systemd lifecycle — **never restarts an agent**. Single-SSOT: reuses `projectRosterV2AgentGeneratedEnv`
+(extracted, shared with the reconciler apply path) so regen and reconcile cannot drift. Secrev: paths +
+counts only, never the rendered KEY=value body.
+
+New files: `commands/fleet-regen-command.ts` (+ `.spec.ts`), guide `docs/guides/upgrade-safety-and-recovery.md`
+(three-layer model: PR1 manifest ownership → PR2 snapshot/restore → PR3 regen; do-NOT-restart-before-verify
+runbook), regen reference added to `docs/guides/fleet-local-canary.md`. Wired in `commands/fleet.ts`.
+
+### Independent review (3 reviewers: subagent code-reviewer + codex code-review + codex security) → 4 fixes, red-first
+- **A · BLOCKER (codex) — regen mutated/deleted legacy operator env.** `applyPreparedAgentEnvironmentProjection`
+  also writes `.env.local`/`.env.quarantine` and unlinks legacy `.env`. Violated projection-only contract.
+  **Fix:** NEW generated-only boundary primitives `prepareGeneratedAgentEnvironmentProjection` +
+  `applyPreparedGeneratedAgentEnvironmentProjection` (write ONLY `<name>.env.generated`). regen now has no
+  code path that touches `.env`/`.env.local`/`.env.quarantine`. **Test:** projection-only leaves legacy `.env`
+  verbatim, no local/quarantine fabricated.
+- **B · should-fix (codex + subagent + security) — partial write on mid-loop failure.** Interleaved
+  prepare/apply left earlier agents written when a later agent failed prepare. **Fix:** PREPARE ALL agents
+  before writing ANY (mirrors reconciler `defaultPrepareProjections`). **Test:** 2nd agent's projection
+  pre-seeded 0644 → prepare rejects → coder0 NOT written, exit 1.
+- **C · subagent — semantic-validation bypass.** Default readRoster skipped `validateRosterV2Semantics`, so
+  a tampered protected-class `tool_policy` would be silently projected. **Fix:** default readRoster now runs
+  `validateRosterV2Semantics` (persona resolution + protected-class match), rolesDir/overrideDir defaults
+  mirroring the reconciler. **Test:** merge-gate agent w/ tool_policy=code → fails closed, no write.
+- **D · MEDIUM (codex security, CWE-362) — concurrent-reconcile race.** regen `--write` wrote without the
+  reconcile lock. **Fix:** `--write` acquires `acquirePrivateReconcileLock(mosaicHome)` for the whole
+  read-prepare-apply sequence, released in `finally`; dry-run stays lock-free. **Test:** pre-held lock →
+  regen fails closed, no write.
+
+**Gate suite after fixes (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest **1265**
+(regen spec 13, incl. 4 new red-first regressions). NOTE: codex self-review does NOT satisfy the
+independent-review gate — an independent (author≠reviewer) review + durable Gitea RoR is still required
+before MS-LEAD merges. STOP at PR-open for MS-LEAD's exact-head review; do NOT self-merge.
+
+## Session 5 — PR3 review round 2 (finding L + M1/M2/M3), red-first fixes
+
+Second review pass on the lock-cleanup plumbing surfaced one round-1 residual (L) and three round-2
+findings (M1 blocker, M2/M3 should-fix). All fixed red-first (RED proven per-finding, then GREEN).
+
+- **L · should-fix (codex r1) — mutation-lock release swallowed unlink failures.** regen's
+  `acquirePrivateRosterMutationLock` release copied CRUD's `unlink().catch(()=>{})`, hiding a stale
+  `roster.yaml.mutation.lock`. **Fix:** its release PROPAGATES the unlink fault (finding-J stale-lock
+  warning then fires for this lock too). **Test:** acquire real lock, `rm` it, assert `release()` rejects.
+- **M1 · BLOCKER (codex r2) — replacement-lock race.** The propagating release from L did an
+  UNCONDITIONAL `unlink(lockPath)` without proving ownership. If the lock is cleared + re-created by
+  another writer mid-op, regen deletes the STRANGER's live lock → a third writer enters → mutual
+  exclusion defeated. **Fix (reuse, not reimplement):** generalized the reconciler's ownership-proving
+  lock body into shared `acquirePrivateManagedRosterLock(mosaicHome, lockLeaf, busyMessage, openLock)`;
+  `acquirePrivateReconcileLock` delegates to it (behavior-identical: same leaf/codes/messages), and a NEW
+  hardened `acquirePrivateRosterMutationLock` (now in fleet-reconciler.ts, leaf `roster.yaml.mutation.lock`)
+  records dev/ino + ownership token and RE-PROVES ownership (`assertLockOwnership`) before unlinking —
+  fails closed as `lock-cleanup-failed` if replaced. Removed the crud-based export; reverted
+  `acquireMutationLock` (fleet-agent-crud.ts) to its original inline empty-file/swallowing-release form
+  (CRUD behavior intentionally unchanged). Compatibility: CRUD empty-file `wx` and regen tokened `wx`
+  contend on the same path but never co-own (wx winner owns; loser → concurrent-mutation), so the token
+  is only ever read back by the same regen invocation. **Test:** acquire, `rm`+recreate lock (new inode),
+  assert `release()` rejects AND the replacement survives (not unlinked).
+- **M2 · should-fix (codex r2) — acquire-unwind fault dropped.** The acquire-failure catch discarded
+  `releaseFleetLocks`' return (a possible fault on the already-held first lock). **Fix:** capture and
+  augment — `const releaseFault = await releaseFleetLocks(releases); throw augmentWithLockCleanupFault(error, releaseFault);`
+  (symmetric to finding J). **Test:** mutation lock acquires w/ faulting release + reconcile acquire
+  throws → thrown error mentions stale/lock, nothing written.
+- **M3 · should-fix (codex r2 + subagent REQUEST-CHANGES) — cleanup warning named only reconcile lock.**
+  Finding L made the mutation-lock release fault reachable, so the `cleanup` marker can originate from
+  EITHER lock. **Fix:** `formatFleetRegenReport`'s WARNING now names BOTH `roster.yaml.mutation.lock` and
+  `roster.yaml.reconcile.lock`, matching `augmentWithLockCleanupFault`. **Test:** fault the mutation-lock
+  release specifically → report names both lock files.
+
+**Refactor note (no cycle):** neither fleet-reconciler nor fleet-agent-crud imports the other; regen
+imports lock acquirers from fleet-reconciler and the projection mapping from fleet-reconciler. The two
+reconcile-lock reviewers reconciled: independent reviewer validated acquire-time empty-file compatibility
+(preserved), codex flagged RELEASE-time replacement race (closed by ownership proof) — non-contradictory.
+
+**Gate suite after fixes (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest **1275**
+(regen spec 23, incl. 7 red-first lock regressions E/F/G/K/L/M1/M2/M3). RED proven per-finding by
+temporary revert before re-applying each fix. Independent (author≠reviewer) review of M1/M2/M3 + codex
+code/security re-run in flight. STOP at PR-open for MS-LEAD's exact-head review + durable Gitea RoR; do
+NOT self-merge; #791 umbrella stays OPEN; PR body `Part of #791`.
+
+### Round 3 review (after M1/M2/M3) — independent review PASS + codex residual-TOCTOU disposition
+Three reviewers on the post-M1/M2/M3 head:
+- **Independent (subagent, author≠reviewer) — PASS.** Verified M1/M2/M3 all correctly fixed; "never
+  restarts" is STRUCTURAL (runner never referenced in executable code); no secrets; no deadlock (only
+  regen holds both locks); tests meaningful (assert inode preservation + exact lock-file names). Raised:
+  - **should-fix #1 (fixed, red-first):** generalizing the lock helper left `assertSafeLockLeafIfPresent`/
+    `assertLockOwnership` hardcoding "reconciliation lock" in thrown messages → a MUTATION-lock fault
+    misreported as the reconcile lock, undercutting M3's accurate-diagnosis goal. **Fix:** thread
+    `lockLabel = fleet/<leaf>` through both helpers + the generic lock-io messages, so every fault names
+    the actual lock file. Red-first: strengthened the M1 test to assert `/roster\.yaml\.mutation\.lock/`
+    (RED: got "reconciliation lock"; GREEN after). Also resolves nit #3 (generic-message drift).
+  - **nit #2 (fixed):** `FleetRegenResult.cleanup` JSDoc still said "the shared reconcile lock"; now names
+    both locks (regen holds both).
+  - **nit #4 (fixed):** removed the redundant duplicate `assertLockOwnership` call before unlink
+    (pre-existing in merged main; harmless but dead — dropped since the fn was already being touched).
+- **Codex security — clean (risk: none).** Validates roster semantics, constrains env values, no shell
+  eval, no secret output, generated-only writes, serialized against both locks.
+- **Codex code — request-changes, 1 "blocker": residual check-then-unlink TOCTOU.** Between the final
+  `assertLockOwnership` and the path-based `unlink`, an external actor could vacate our inode and a new
+  writer grab the path, so the unlink deletes the stranger's lock. **Disposition: documented known
+  limitation, NOT fixed in PR3.** Rationale: (1) byte-identical to the MERGED, shipped reconcile-lock
+  release on origin/main (fleet-reconciler.ts L654-659) — not introduced here; (2) UNREACHABLE within the
+  `wx` writer protocol — no Mosaic writer removes a lock it doesn't own (wx fails EEXIST while our inode
+  exists), so only external interference can vacate our inode in the sub-instruction window; (3) the
+  ownership guard DOES close the reachable case (stale-lock reaper/operator cleared our lock + another
+  writer took it BEFORE release began → fail closed, don't delete stranger's lock); (4) the true atomic
+  fix — fd-held advisory lock (flock/lockf) adopted by ALL fleet writers (CRUD + reconcile + regen) — is
+  a cross-cutting mechanism change touching merged CRUD + reconciler, out of scope for a projection-only
+  recovery PR. Documented honestly in the acquirer doc + M1 test comment. **The binding independent
+  review did NOT treat this as a blocker.** Recommendation to MS-LEAD: proceed to PR-open + spin a
+  SEPARATE follow-up issue for the fd-advisory-lock migration; MS-LEAD adjudicates scope at exact-head
+  review (merge authority).
+
+**Gates after round-3 fixes (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest
+**1275** (regen spec 23). Fresh codex code re-run in flight to confirm no NEW issues from the label fix.
+
+---
+
+## Session 6 — Round 4/5 convergence (stranded-lock robustness)
+
+**Two independent reviewers converged on the SAME should-fix** on the init-failure cleanup path,
+strengthening confidence it was real:
+
+- **Codex code-review-5 — 0 blockers, 1 should-fix.** "Stat failure after lock creation strands the new
+  lock." When `handle.stat()` ITSELF fails right after the `wx` create (transient EIO/EBADF), `created`
+  is `undefined`, so `removeOwnedLockLeafBestEffort` had `if (!created) return;` → no cleanup → the
+  just-created `roster.yaml.mutation.lock`/`reconcile.lock` is stranded, permanently blocking future
+  regen + CRUD. (Notably NO blocker, and the TOCTOU is no longer flagged in code-review as of r5.)
+- **Independent delta reviewer (author≠reviewer, pr-review-toolkit) — no blockers, same should-fix.**
+  Independently flagged the identical `!created` gap; validated FIX 1 (label threading — no call site
+  missed, codes unchanged, no test depended on old text) and FIX 2 (dev/ino-guarded cleanup, best-effort,
+  happy-path release reuses captured dev/ino) as correct. Suggested an unconditional best-effort unlink
+  in the `!created` branch; I took the **safer** variant below.
+- **Codex security-review-5 — 0 crit / 0 high / 1 medium.** The single medium is the SAME residual
+  check-then-unlink TOCTOU already dispositioned in round 3 (its own remediation = "migrate every writer
+  to an fd-held advisory lock" = the follow-up issue). No new security finding. No secrets.
+
+**Fix (red-first, safer than an unconditional unlink):** thread the persisted random `token` into
+`removeOwnedLockLeafBestEffort`. Two independent ownership proofs now: primary dev/ino (unchanged), and a
+**fallback** when the post-create stat failed — read the leaf and unlink ONLY if its content equals our
+`randomUUID()` token. Only OUR lock carries that token, so a CRUD (empty) or differently-tokened
+replacement is never deleted. `tokenPersisted` guards passing the token (only after `writeFile` lands).
+Doubly-degenerate case (stat fails AND token write never landed) leaves the lock in place rather than
+risk deleting a stranger's file — requires two independent fs faults on a just-created fd; documented.
+
+- **Red-first proof:** new test `does not strand the lock file when the post-create stat itself fails`
+  injects a real `wx` create + a Proxy handle whose `stat()` rejects (writeFile/close succeed), asserts
+  `exists(lockPath) === false`. RED before fix (`expected true to be false` — lock stranded); GREEN after.
+- **Also fixed (delta nit #3):** `fleet-regen-command.ts` `acquireRosterMutationLock` JSDoc said "CRUD's
+  private lock"; the default is the reconciler's hardened ownership-proving acquirer for the same
+  `fleet/roster.yaml.mutation.lock` path. Corrected.
+- **PR-description note (delta nit #2):** FIX 1 also collapsed a pre-existing duplicate back-to-back
+  `assertLockOwnership` call in the release closure (identical args, no intervening logic) into one — a
+  no-op simplification of merged code, not a behavior change. Called out so a future reader doesn't
+  wonder if the duplicate had a purpose.
+
+**Gates after round-4 fixes (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest
+**1277** (regen spec now 25: +1 stat-failure stranded-lock regression). Residual TOCTOU still deferred to
+the fd-advisory-lock follow-up issue; MS-LEAD adjudicates scope at exact-head review (merge authority).
+
+---
+
+## Session 6 — Round 6 (persona-root wiring)
+
+**Codex code-review-6 — 0 blockers, 1 should-fix (NEW, distinct from the lock work).** "Forward
+configured persona directories to regen." `registerFleetRegenCommand` was registered at
+`fleet.ts:2069` with only `{ runner, mosaicHome }`, discarding `deps.reconcileDeps.rolesDir` /
+`overrideDir`. The regen command ALREADY has those seams (validates roster semantics via
+`validateRosterV2Semantics({ rolesDir, overrideDir })`, defaulting to `<mosaicHome>/fleet/roles{,.local}`),
+but the top-level wiring never forwarded the configured roots. **Impact:** in a deployment with custom
+persona roots, `fleet reconcile` (which honors the overrides) would ACCEPT a roster while `fleet regen`
+REJECTS the same roster (persona resolution against the wrong default dir) — blocking the recovery
+command and violating the documented "resolves personas the SAME way reconcile does" contract.
+
+**Fix (red-first):** forward `rolesDir`/`overrideDir` from `deps.reconcileDeps` into
+`registerFleetRegenCommand` at `fleet.ts:2069`. Red-first test `forwards configured persona roots
+(rolesDir/overrideDir) from reconcileDeps into regen`: seeds personas ONLY under a custom root, leaves
+the default `<home>/fleet/roles` empty, registers with `reconcileDeps: { rolesDir, overrideDir }`, and
+requires `fleet regen` to SUCCEED. RED before fix (`expected 1 not to be 1` — regen validated against the
+empty default and exited 1); GREEN after.
+
+**Codex security-review-6 — 0 crit / 0 high / 1 medium.** Same residual check-then-unlink TOCTOU, now
+noted at BOTH the release closure and the init-cleanup path; remediation = fd-held advisory lock across
+all writers = the SAME deferred follow-up item. No new security finding, no secrets.
+
+**Independent confirmation review of the token-fallback fix (Session 6/round 4) — PASS, no findings.**
+All 7 verification points confirmed; reviewer mechanically reverted `removeOwnedLockLeafBestEffort` to
+the pre-fix `if (!created) return;` and re-ran the new test → RED (`expected true to be false`),
+confirming the test genuinely pins the fix; restored after. No lint/type issues; doc-comment accurate.
+
+**Gates after round-6 fix (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest
+**1278** (regen spec now 26: +1 persona-root wiring regression).
+
+---
+
+## Session 6 — Round 7 convergence (review CLOSED for PR-open)
+
+- **Codex code-review-7 — 0 blockers, 1 should-fix = the residual TOCTOU** (previously a "blocker" in r3,
+  dropped in r4/r5, now re-surfaced as a should-fix). **Codex security-review-7 — 0 crit / 0 high /
+  1 medium = the SAME residual TOCTOU.** Codex has CONVERGED: the only remaining finding across both
+  streams is that one race, whose own remediation is "fd-held advisory lock shared by all fleet writers"
+  = the deferred follow-up. No new distinct finding; the wiring fix introduced nothing.
+- **Independent confirmation review of the persona-root wiring fix — PASS, no findings.** Reviewer
+  mechanically reverted the two forwarded lines → RED (`Roster v2 agent "coder0" class "code" does not
+  resolve to a readable persona` → exit 1), restored → GREEN (26 regen + 204 fleet tests). Confirmed the
+  optional-chaining fallback preserves default-deployment behavior and no type/lint issue.
+
+**Review disposition for PR-open:** ALL actionable findings fixed red-first across rounds 3–6 (label
+threading, stranded-lock on init failure, stat-failure strand, persona-root wiring). The residual
+check-then-unlink TOCTOU is the ONLY open item and is DEFERRED to a follow-up issue (fd-advisory-lock
+migration across CRUD + reconcile + regen) — byte-identical to merged origin/main's reconcile-lock
+release, unreachable within the `wx` writer protocol (no Mosaic writer removes a lock it doesn't own;
+only external `rm`/a stale-lock reaper can vacate the inode mid-release), and its true fix is a
+cross-cutting mechanism change out of scope for a projection-only recovery PR. Two independent human-agent
+reviews (author≠reviewer) treated it as non-blocking. MS-LEAD adjudicates scope at exact-head review
+(merge authority); recommendation = proceed to PR-open + spin the follow-up issue.
+
+**Final gates (all green):** typecheck ✓ · lint ✓ · format:check ✓ · full mosaic vitest **1278**
+(regen spec 26). No secret values in any snapshot/projection/report output (counts + paths only). Regen
+NEVER issues a lifecycle/restart call (load-bearing recordingRunner gate). STOP at PR-open for MS-LEAD's
+exact-head review + durable Reviewer-of-Record before any merge; do NOT self-merge.
--- a/packages/mosaic/framework/install.sh
+++ b/packages/mosaic/framework/install.sh
@@ -109,6 +109,225 @@ restore_snapshot() {
 }
 cleanup_snapshot() { [[ -n "$SNAPSHOT_DIR" && -d "$SNAPSHOT_DIR" ]] && rm -rf "$SNAPSHOT_DIR"; SNAPSHOT_DIR=""; }

+# ─── durable operator-config snapshot (#791 PR2) ─────────────────────────────
+# A SECOND, independent safety layer, distinct from SNAPSHOT_DIR above:
+#   • SNAPSHOT_DIR is ephemeral (/tmp, deleted on success) and mirrors the WHOLE
+#     target for CRASH rollback if the sync aborts mid-write.
+#   • DURABLE_SNAPSHOT_DIR is RETAINED, holds only the operator-owned surface, and
+#     lives OUTSIDE the framework tree and any repo. It exists for the failure the
+#     crash-rollback cannot see: a sync that finishes "successfully" yet a
+#     manifest/logic bug let it modify an operator file. verify_operator_surface()
+#     (post-sync) heals from it; `mosaic restore` recovers from it days later.
+# Path convention is mirrored in packages/mosaic/src/commands/restore.ts — keep
+# the two in sync (there is no shared code across the bash/TS boundary).
+DURABLE_SNAPSHOT_DIR=""
+backup_root() { printf '%s/mosaic/backups' "${XDG_STATE_HOME:-$HOME/.local/state}"; }
+
+# Relative paths that a migration INTENTIONALLY removes from the target (e.g. the
+# legacy bin/ tree). Such a path is operator-classified by the manifest (unknown⇒
+# operator), so the durable snapshot captures it — but its post-migration absence
+# is correct, NOT a manifest bug. run_migrations() records each removal here so
+# verify_operator_surface() does not "heal" it back and silently undo the
+# migration (which would then be skipped forever once the version is stamped).
+MIGRATION_REMOVED_PATHS=()
+
+# True (0) if $1 (a path relative to TARGET_DIR) equals or lives under a path a
+# migration deliberately removed this run.
+is_migration_removed() {
+  local rel="$1" removed
+  for removed in ${MIGRATION_REMOVED_PATHS[@]+"${MIGRATION_REMOVED_PATHS[@]}"}; do
+    [[ -n "$removed" ]] || continue
+    [[ "$rel" == "$removed" || "$rel" == "$removed"/* ]] && return 0
+  done
+  return 1
+}
+
+# True (0) if any parent directory of $1 (relative to TARGET_DIR) is a symlink.
+# Restoring THROUGH a symlinked ancestor would let cp write snapshot contents —
+# possibly secrets — outside the target (CWE-59), so the verify net refuses it.
+has_symlinked_parent() {
+  local rel="$1" dir p seg
+  dir="$(dirname "$rel")"
+  [[ "$dir" == "." ]] && return 1
+  p="$TARGET_DIR"
+  local IFS='/'
+  for seg in $dir; do
+    [[ -n "$seg" ]] || continue
+    p="$p/$seg"
+    [[ -L "$p" ]] && return 0
+  done
+  return 1
+}
+
+# Emit (NUL-delimited, into file $1) the operator-owned relative paths that exist
+# under TARGET_DIR, classified via the shared manifest (deny-wins; unknown⇒
+# operator). Returns non-zero if the filesystem walk itself failed — we must
+# NEVER snapshot from a truncated scan (a `< <(find …)` process substitution
+# would hide that error; capture-then-check does not — cf. #791 blocker-D1).
+enumerate_operator_files() {
+  local out="$1" scan abs rel
+  scan="$(mktemp)"
+  if ! find "$TARGET_DIR" -type f -print0 > "$scan"; then
+    rm -f "$scan"
+    return 1  # OP-SCAN-GUARD
+  fi
+  : > "$out"
+  while IFS= read -r -d '' abs; do
+    rel="${abs#"$TARGET_DIR"/}"
+    # Not operator config: version marker and any VCS metadata.
+    case "$rel" in .framework-version|.git|.git/*) continue ;; esac
+    manifest_is_framework "$rel" || printf '%s\0' "$rel" >> "$out"
+  done < "$scan"
+  rm -f "$scan"
+}
+
+# Retain only the newest MOSAIC_BACKUP_RETENTION (default 5) snapshots. The
+# pre-update-<UTC-ts> names sort lexicographically = chronologically, so a
+# reverse sort is newest-first. Pruning failures are non-fatal (they only leave
+# extra old backups); the enclosing find's status is still honored, not swallowed.
+prune_durable_snapshots() {
+  local root keep list d i=0
+  root="$(backup_root)"
+  keep="${MOSAIC_BACKUP_RETENTION:-5}"
+  [[ "$keep" =~ ^[0-9]+$ ]] && (( keep >= 1 )) || keep=5
+  list="$(mktemp)"
+  if ! find "$root" -maxdepth 1 -type d -name 'pre-update-*' > "$list"; then
+    rm -f "$list"; return 0
+  fi
+  # Newest-first ordering needs `sort` (`-o` writes back in place — no `mv`
+  # dependency); if it is somehow unavailable, leave the backups untouched rather
+  # than risk pruning in an undefined order.
+  if ! LC_ALL=C sort -r -o "$list" "$list" 2>/dev/null; then
+    rm -f "$list"; return 0
+  fi
+  while IFS= read -r d; do
+    [[ -n "$d" ]] || continue
+    i=$((i + 1))
+    (( i > keep )) && rm -rf "$d"
+  done < "$list"
+  rm -f "$list"
+}
+
+# Take the durable pre-update snapshot BEFORE any mutation. Fail-OPEN: the durable
+# snapshot is a recovery bonus on top of the manifest (which already keeps the
+# sync out of operator paths) and the crash-rollback — so an un-writable backup
+# location warns and continues rather than blocking the upgrade. Everything it
+# creates is private (umask 077 + explicit 0700 dirs / 0600 files): the snapshot
+# mirrors operator config, which may hold secrets, and must never be world-readable.
+make_durable_snapshot() {
+  is_existing_install || return 0
+  local root ts dir list rel src dst count=0 old_umask
+  root="$(backup_root)"
+  # Fail-open if we cannot even stamp a timestamp: the durable snapshot is a
+  # recovery bonus and must never be the thing that aborts an upgrade.
+  ts="$(date -u +%Y%m%dT%H%M%SZ 2>/dev/null || true)"
+  if [[ -z "$ts" ]]; then
+    warn "Durable snapshot skipped: no UTC timestamp available (upgrade continues)."
+    return 0
+  fi
+  # umask 077 makes every dir/file the snapshot creates private from birth (it
+  # mirrors operator config, which may hold secrets). It is PROCESS-global, so we
+  # save and restore it around exactly this block — otherwise every later sync
+  # copy and new framework dir would inherit 0600/0700 instead of 0644/0755.
+  old_umask="$(umask)"
+  umask 077
+  if ! mkdir -p "$root"; then
+    umask "$old_umask"
+    warn "Durable snapshot skipped: cannot create backup dir $root (upgrade continues; operator files remain manifest-protected)."
+    return 0
+  fi
+  chmod 700 "$root" 2>/dev/null || true
+  dir="$root/pre-update-$ts"
+  if [[ -e "$dir" ]]; then           # same-second re-run: disambiguate
+    local n=1; while [[ -e "$dir-$n" ]]; do n=$((n + 1)); done; dir="$dir-$n"
+  fi
+  if ! mkdir -p "$dir"; then
+    umask "$old_umask"
+    warn "Durable snapshot skipped: cannot create $dir (upgrade continues)."
+    return 0
+  fi
+  chmod 700 "$dir"
+  list="$(mktemp)"
+  if ! enumerate_operator_files "$list"; then
+    umask "$old_umask"
+    warn "Durable snapshot skipped: could not enumerate operator files (upgrade continues)."
+    rm -f "$list"; rmdir "$dir" 2>/dev/null || true
+    return 0
+  fi
+  while IFS= read -r -d '' rel; do
+    src="$TARGET_DIR/$rel"; dst="$dir/$rel"
+    [[ -f "$src" ]] || continue
+    mkdir -p "$(dirname "$dst")"
+    if ! cp "$src" "$dst"; then
+      warn "Durable snapshot: could not copy operator file '$rel' (skipped)."
+      continue
+    fi
+    chmod 600 "$dst" 2>/dev/null || true
+    count=$((count + 1))
+  done < "$list"
+  rm -f "$list"
+  # Tighten every dir the copy created (mkdir -p honors umask, but be explicit).
+  find "$dir" -type d -exec chmod 700 {} + 2>/dev/null || true
+  umask "$old_umask"   # UMASK-RESTORE-NORMAL — restore before the upgrade proper resumes (see above)
+  DURABLE_SNAPSHOT_DIR="$dir"
+  ok "Durable pre-update snapshot: $count operator file(s) saved to $dir (recover with: mosaic restore --list)"
+  prune_durable_snapshots
+}
+
+# Post-sync safety net: a keep-mode upgrade must NEVER modify an operator file.
+# Compare every file in the durable snapshot to its current target counterpart;
+# any that changed (or vanished) was touched by a framework bug — restore it from
+# the snapshot and warn loudly. This does NOT abort: the framework itself synced
+# correctly; we only heal the operator collateral. Runs after the restore trap is
+# disarmed so its corrective copies can't spuriously trip a full rollback, and
+# every step is guarded so `set -e` cannot exit silently mid-heal (cf. blocker-D2).
+verify_operator_surface() {
+  [[ -n "$DURABLE_SNAPSHOT_DIR" && -d "$DURABLE_SNAPSHOT_DIR" ]] || return 0
+  local scan snap rel cur healed=0
+  scan="$(mktemp)"
+  if ! find "$DURABLE_SNAPSHOT_DIR" -type f -print0 > "$scan"; then
+    rm -f "$scan"
+    warn "Post-upgrade verify skipped: could not enumerate the pre-update snapshot at $DURABLE_SNAPSHOT_DIR."
+    return 0
+  fi
+  while IFS= read -r -d '' snap; do
+    rel="${snap#"$DURABLE_SNAPSHOT_DIR"/}"
+    cur="$TARGET_DIR/$rel"
+    # A migration may legitimately delete an operator-classified path (e.g. legacy
+    # bin/). Its absence is intended — do not heal it back, or the migration is
+    # silently undone and never re-runs once the version is stamped (#791 PR2).
+    is_migration_removed "$rel" && continue   # MIGRATION-SKIP-GUARD
+    if [[ ! -e "$cur" ]] || ! cmp -s "$snap" "$cur"; then
+      # Never restore THROUGH a symlink: an operator path swapped for a link would
+      # otherwise let cp write snapshot contents (possibly secrets) outside the
+      # target (CWE-59). Refuse a symlinked parent; drop a symlinked leaf and write
+      # a real file in its place.
+      if has_symlinked_parent "$rel"; then
+        warn "Operator path '$rel' has a symlinked parent under $TARGET_DIR; refusing to restore through it (possible tampering) — recover it manually from $DURABLE_SNAPSHOT_DIR."
+        continue
+      fi
+      [[ -L "$cur" ]] && rm -f "$cur"   # SYMLINK-LEAF-GUARD
+      # Guard mkdir too: under set -e (trap already disarmed) a bare failure would
+      # exit the whole installer before the recovery pointer below is emitted.
+      if ! mkdir -p "$(dirname "$cur")"; then
+        warn "Operator file '$rel' was modified by the upgrade but could NOT be auto-restored (parent dir unavailable) — recover it manually from $DURABLE_SNAPSHOT_DIR."
+        continue
+      fi
+      if cp "$snap" "$cur"; then
+        chmod 600 "$cur" 2>/dev/null || true
+        warn "Operator file was modified by the upgrade and has been restored from the pre-update snapshot: $rel"
+        healed=$((healed + 1))
+      else
+        warn "Operator file '$rel' was modified by the upgrade but could NOT be auto-restored — recover it manually from $DURABLE_SNAPSHOT_DIR."
+      fi
+    fi
+  done < "$scan"
+  rm -f "$scan"
+  if (( healed > 0 )); then
+    warn "$healed operator file(s) were unexpectedly changed by this upgrade and were restored from the pre-update snapshot. A keep-mode upgrade must never modify operator files — this indicates a framework manifest bug; please report it (#791)."
+  fi
+}
+
 # Reconcile contract files after sync: framework-owned overwrite (backup-once),
 # user-seeded seed-if-absent.
 reconcile_framework_files() {
@@ -326,6 +545,10 @@ run_migrations() {
  # Remove bin/ directory — all executables now live in the npm CLI.
  # Scripts that were in bin/ are now in tools/_scripts/.
  if [[ "$from_version" -lt 2 ]]; then
+    # bin/ and the rails symlink are operator-classified by the manifest (unknown⇒
+    # operator) and thus captured in the durable snapshot; record them as
+    # intentional removals so the post-sync verify net does not restore them.
+    MIGRATION_REMOVED_PATHS+=("bin" "rails")
    if [[ -d "$TARGET_DIR/bin" ]]; then
      ok "Removing legacy bin/ directory (executables now in npm CLI)"
      rm -rf "$TARGET_DIR/bin"
@@ -383,6 +606,9 @@ fi
 # not for a validation failure that has touched nothing yet (#791 blocker-1).
 if [[ "$INSTALL_MODE" == "keep" ]]; then
  manifest_load
+  # Durable, operator-scoped backup taken BEFORE any mutation (#791 PR2). Kept
+  # outside the framework tree; recovered later via `mosaic restore`. Fail-open.
+  make_durable_snapshot
 fi

 # Snapshot before any destructive file operation; restore on interrupt/failure.
@@ -421,6 +647,10 @@ run_migrations

 # File-system phase complete and consistent — clear the restore trap.
 trap - ERR INT TERM
+# Post-sync safety net: heal any operator file a manifest bug let the sync touch,
+# using the durable pre-update snapshot (#791 PR2). Runs with the trap disarmed so
+# a corrective copy can't spuriously trigger a full rollback.
+verify_operator_surface   # VERIFY-NET (#791 PR2)
 cleanup_snapshot

 # Testability / minimal-install hook: stop after the file-system phase, before any
--- a/packages/mosaic/framework/tools/quality/scripts/test-upgrade-durable-snapshot.sh
+++ b/packages/mosaic/framework/tools/quality/scripts/test-upgrade-durable-snapshot.sh
@@ -0,0 +1,313 @@
+#!/usr/bin/env bash
+# test-upgrade-durable-snapshot.sh — the #791 PR2 regression gate.
+#
+# PR1 gave keep-mode upgrades two protections: the manifest (a keep-sync only
+# ever writes framework-owned paths — operator config is structurally untouched)
+# and an EPHEMERAL /tmp snapshot that rolls the whole target back if the sync
+# CRASHES mid-write. PR2 adds a third, independent layer for the case neither
+# covers: a "successful" upgrade that a manifest/logic bug silently let touch an
+# operator file. That layer is a DURABLE, operator-scoped pre-update snapshot:
+#
+#   Part 1 (scope):      before any mutation, the installer copies exactly the
+#                        operator-owned files that exist into a retained backup
+#                        under $XDG_STATE_HOME/mosaic/backups/pre-update-<ts>/ —
+#                        framework files are NOT captured.
+#   Part 2 (perms):      the backup root, snapshot dir and every nested dir are
+#                        0700; every backed-up file is 0600 (never world-readable,
+#                        even though operator config may hold secrets).
+#   Part 3 (no leak):    a secret seeded into credentials.json is copied into the
+#                        snapshot (proving coverage) but its value never appears
+#                        on stdout/stderr — the snapshot reports counts/paths only.
+#   Part 4 (retention):  only the newest MOSAIC_BACKUP_RETENTION snapshots survive;
+#                        older ones are pruned.
+#   Part 5 (verify net): if the upgrade DID modify an operator file (injected here
+#                        with a cp shim that scribbles on SOUL.md while a framework
+#                        file is copied), the post-sync verify restores that file
+#                        from the durable snapshot and warns loudly. The control —
+#                        the same installer with the verify call stripped — leaves
+#                        the corruption in place, proving the net is load-bearing.
+#
+# Usage:  bash test-upgrade-durable-snapshot.sh
+set -uo pipefail
+
+FW="$(cd "$(dirname "${BASH_SOURCE[0]}")/../../.." && pwd)"   # packages/mosaic/framework
+INSTALL="$FW/install.sh"
+ORIG_PATH="$PATH"
+FRAMEWORK_VERSION="$(grep -m1 '^FRAMEWORK_VERSION=' "$INSTALL" | cut -d= -f2)"
+
+# Control installers must live INSIDE $FW: install.sh derives SOURCE_DIR from its
+# own path and sources tools/_lib/manifest.sh relative to it, so a copy anywhere
+# else aborts before the sync. Each control is a shipped installer with one guard
+# line stripped (keyed off a `# <MARKER>` anchor), proving that guard load-bearing.
+# All controls share the .install-*.tmp.sh glob so one trap sweeps them on exit.
+VERIFYCTRL="$FW/.install-verifynet-control.tmp.sh"
+rm -f "$FW"/.install-*.tmp.sh
+trap 'rm -f "$FW"/.install-*.tmp.sh' EXIT
+
+# mk_control <marker-regex> <name> — echo a control installer path ($FW-local) that
+# is $INSTALL with every line matching /<marker-regex>/ deleted.
+mk_control() {
+  local path="$FW/.install-$2.tmp.sh"
+  sed "/$1/d" "$INSTALL" > "$path"
+  printf '%s' "$path"
+}
+
+pass=0; fail=0
+chk() { if eval "$2"; then echo "  ✓ $1"; pass=$((pass + 1)); else echo "  ✗ $1"; fail=$((fail + 1)); fi; }
+
+SECRET='SUPER-SECRET-TOKEN-do-not-log-pr2'
+SOUL_ORIG='# persona'
+# A framework file the sync copies (source ships it; the seeded target omits it,
+# so the bytes differ and cp is attempted). The Part-5 shim keys off this path.
+POISON_REL='guides/E2E-DELIVERY.md'
+
+# Seed a recognized keep-mode install holding four operator-owned files across
+# the identity file, an operator subtree, memory, and the credentials carve-out.
+seed_home() {
+  local H="$1"
+  mkdir -p "$H/agents" "$H/tools/_lib" "$H/memory"
+  printf '%s\n' "$SOUL_ORIG" > "$H/SOUL.md"    # recognized install → keep mode
+  printf 'MODEL=opus\n'      > "$H/agents/coder0.conf"
+  printf '# operator memory\n' > "$H/memory/note.md"
+  printf 'TOKEN=%s\n' "$SECRET" > "$H/tools/_lib/credentials.json"
+  echo 3 > "$H/.framework-version"
+  # Deliberately NO guides/E2E-DELIVERY.md so the sync copies it (framework file,
+  # bytes differ) — that copy is where the Part-5 corruption shim fires.
+}
+
+# A pre-v2 (legacy) keep-mode install: SOUL.md marks it recognized, and a bin/
+# tree with NO .framework-version makes installed_framework_version() report 1, so
+# the v1→v2 migration (which deletes bin/) runs. bin/ is unknown⇒operator, so the
+# durable snapshot captures it — the verify net must NOT heal the intended removal.
+seed_home_v1() {
+  local H="$1"
+  mkdir -p "$H/agents" "$H/tools/_lib" "$H/memory" "$H/bin"
+  printf '%s\n' "$SOUL_ORIG" > "$H/SOUL.md"
+  printf 'TOKEN=%s\n' "$SECRET" > "$H/tools/_lib/credentials.json"
+  printf '#!/bin/sh\necho legacy\n' > "$H/bin/tool.sh"; chmod +x "$H/bin/tool.sh"
+  # Deliberately NO .framework-version and NO guides/E2E-DELIVERY.md (see seed_home).
+}
+
+# A cp shim that, while the framework POISON file is being copied during sync,
+# swaps the operator credentials file for a symlink pointing at an attacker-
+# readable file OUTSIDE the target — simulating post-snapshot tampering (CWE-59).
+# The durable snapshot already holds the real credentials (it is taken before any
+# sync), so the verify net must restore a REAL file in place WITHOUT following the
+# link (which would write the snapshot's secret out through it). $EXFIL_TARGET is
+# expanded at shim-write time from the caller's environment.
+#
+# PORTABILITY (why this shim, not the real `cp`): the CWE-59 leak this exercises is
+# `cp` writing THROUGH a symlinked destination. GNU/BSD cp — what a real operator
+# runs `mosaic update` under — follows the dest symlink and leaks. busybox cp (the
+# Alpine CI image) REPLACES a symlinked dest instead of following it, so under the
+# CI harness the leak vector simply does not exist and the negative control could
+# never reproduce it. This shim therefore emulates the real-target GNU cp behavior
+# PORTABLY: when the destination is a symlink it writes the source bytes through the
+# link via redirection (which follows symlinks on every coreutils, busybox included);
+# otherwise it delegates to the host's real cp unchanged. Both the shipped-case and
+# the negative control run through this identical shim, so the ONLY difference
+# between them remains the SYMLINK-LEAF-GUARD — the control stays load-bearing and
+# non-tautological. It does NOT touch install.sh (approved) or the real assertions:
+# with the guard present the symlinked leaf is dropped BEFORE this cp runs, so the
+# dest is a real file and the delegate path is taken exactly as on a GNU host.
+make_symlink_leaf_shim() {
+  local dir="$1" home="$2"
+  cat > "$dir/cp" <<SHIM
+#!/usr/bin/env bash
+dest="\${@: -1}"
+src="\${@:(-2):1}"
+case "\$dest" in
+  */$POISON_REL)
+    rm -f "$home/tools/_lib/credentials.json"
+    ln -s "$EXFIL_TARGET" "$home/tools/_lib/credentials.json"
+    ;;
+esac
+# Coreutils-agnostic emulation of GNU cp's follow-through-dest-symlink behavior.
+if [[ -L "\$dest" && -f "\$src" ]]; then
+  cat "\$src" > "\$dest"
+  exit \$?
+fi
+exec env PATH="$ORIG_PATH" cp "\$@"
+SHIM
+  chmod +x "$dir/cp"
+}
+
+# A cp shim that, while the framework POISON file is being copied during sync,
+# also appends garbage to the operator SOUL.md — simulating a manifest bug that
+# writes outside the framework lane. The framework copy itself still succeeds
+# (real cp runs), so the sync completes 0 and the post-sync verify is what must
+# catch and undo the operator-file damage. The snapshot's own cp only ever
+# targets operator files (never guides/…), so it is never corrupted by this shim.
+make_corrupt_shim() {
+  local dir="$1" home="$2"
+  cat > "$dir/cp" <<SHIM
+#!/usr/bin/env bash
+dest="\${@: -1}"
+case "\$dest" in
+  */$POISON_REL) printf 'CORRUPTION-mid-sync\n' >> "$home/SOUL.md" 2>/dev/null || true ;;
+esac
+exec env PATH="$ORIG_PATH" cp "\$@"
+SHIM
+  chmod +x "$dir/cp"
+}
+
+# Run one keep-mode, sync-only upgrade with $XDG_STATE_HOME redirected to a
+# throwaway dir (so the real ~/.local/state is never touched). Optional args:
+#   $2 shim-maker (default none), $3 MOSAIC_BACKUP_RETENTION (default unset).
+# Echoes: "<exit>\t<out>\t<state-dir>\t<home>".
+#   $4 seeder (default seed_home) — swap in seed_home_v1 for the migration case.
+run_snap() {
+  local installer="$1" shim_maker="${2:-}" retention="${3:-}" seeder="${4:-seed_home}" H STATE OUT SHIM rc pathpre
+  H=$(mktemp -d); STATE=$(mktemp -d); OUT=$(mktemp); pathpre="$ORIG_PATH"
+  "$seeder" "$H"
+  if [[ -n "$shim_maker" ]]; then
+    SHIM=$(mktemp -d); "$shim_maker" "$SHIM" "$H"; pathpre="$SHIM:$ORIG_PATH"
+  fi
+  set +e
+  env PATH="$pathpre" XDG_STATE_HOME="$STATE" \
+      ${retention:+MOSAIC_BACKUP_RETENTION="$retention"} \
+      MOSAIC_HOME="$H" MOSAIC_INSTALL_MODE=keep MOSAIC_SYNC_ONLY=1 \
+      bash "$installer" >"$OUT" 2>&1
+  rc=$?
+  set -e 2>/dev/null || true
+  [[ -n "$shim_maker" ]] && rm -rf "$SHIM"
+  printf '%s\t%s\t%s\t%s\n' "$rc" "$OUT" "$STATE" "$H"
+}
+
+# Resolve the single pre-update-* snapshot dir under a state dir (newest if many).
+snap_dir() {
+  find "$1/mosaic/backups" -maxdepth 1 -type d -name 'pre-update-*' 2>/dev/null \
+    | LC_ALL=C sort -r | head -1
+}
+
+echo "── Part 1/2/3: durable snapshot scope, perms, no-leak ──────────────────"
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$INSTALL")
+SNAP="$(snap_dir "$STATE")"
+
+chk "upgrade succeeds"                        "[ '$rc' -eq 0 ]"
+chk "exactly one pre-update snapshot created" "[ \$(find '$STATE/mosaic/backups' -maxdepth 1 -type d -name 'pre-update-*' | wc -l) -eq 1 ]"
+chk "snapshot: SOUL.md captured"              "[ -f '$SNAP/SOUL.md' ]"
+chk "snapshot: operator subtree captured"     "[ -f '$SNAP/agents/coder0.conf' ]"
+chk "snapshot: memory captured"               "[ -f '$SNAP/memory/note.md' ]"
+chk "snapshot: credentials carve-out captured" "[ -f '$SNAP/tools/_lib/credentials.json' ]"
+chk "snapshot: SOUL.md bytes preserved"       "[ \"\$(cat '$SNAP/SOUL.md')\" = '$SOUL_ORIG' ]"
+chk "snapshot: framework file NOT captured"   "[ ! -e '$SNAP/CONSTITUTION.md' ] && [ ! -e '$SNAP/$POISON_REL' ]"
+
+# Part 2 — permissions (0700 dirs, 0600 files); never world-readable.
+chk "perms: backup root is 0700"              "[ \$(stat -c '%a' '$STATE/mosaic/backups') -eq 700 ]"
+chk "perms: snapshot dir is 0700"             "[ \$(stat -c '%a' '$SNAP') -eq 700 ]"
+chk "perms: nested dir is 0700"               "[ \$(stat -c '%a' '$SNAP/agents') -eq 700 ]"
+chk "perms: credentials backup is 0600"       "[ \$(stat -c '%a' '$SNAP/tools/_lib/credentials.json') -eq 600 ]"
+chk "perms: SOUL.md backup is 0600"           "[ \$(stat -c '%a' '$SNAP/SOUL.md') -eq 600 ]"
+
+# Part 3 — the secret is backed up but never emitted to stdout/stderr.
+chk "no-leak: secret IS in the backup file"   "grep -q '$SECRET' '$SNAP/tools/_lib/credentials.json'"
+chk "no-leak: secret NOT on stdout/stderr"    "! grep -q '$SECRET' '$OUT'"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+echo "── Part 4: retention prune (MOSAIC_BACKUP_RETENTION) ───────────────────"
+# Pre-seed four dated snapshots, then take one real snapshot with retention=2:
+# only the two newest (the fresh real one + the newest pre-seeded) must survive.
+IFS=$'\t' read -r rc OUT STATE H < <(
+  H=$(mktemp -d); STATE=$(mktemp -d); OUT=$(mktemp)
+  seed_home "$H"
+  mkdir -p "$STATE/mosaic/backups"
+  for ts in 20200101T000000Z 20210101T000000Z 20220101T000000Z 20230101T000000Z; do
+    mkdir -p "$STATE/mosaic/backups/pre-update-$ts"
+  done
+  set +e
+  env PATH="$ORIG_PATH" XDG_STATE_HOME="$STATE" MOSAIC_BACKUP_RETENTION=2 \
+      MOSAIC_HOME="$H" MOSAIC_INSTALL_MODE=keep MOSAIC_SYNC_ONLY=1 \
+      bash "$INSTALL" >"$OUT" 2>&1
+  rc=$?
+  set -e 2>/dev/null || true
+  printf '%s\t%s\t%s\t%s\n' "$rc" "$OUT" "$STATE" "$H"
+)
+chk "retention: upgrade succeeds"             "[ '$rc' -eq 0 ]"
+chk "retention: pruned to exactly 2 snapshots" "[ \$(find '$STATE/mosaic/backups' -maxdepth 1 -type d -name 'pre-update-*' | wc -l) -eq 2 ]"
+chk "retention: newest pre-seeded survives"   "[ -d '$STATE/mosaic/backups/pre-update-20230101T000000Z' ]"
+chk "retention: oldest pre-seeded pruned"     "[ ! -d '$STATE/mosaic/backups/pre-update-20200101T000000Z' ]"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+echo "── Part 5: post-sync verify restores an operator file (+ control) ──────"
+# Shipped installer: the cp shim corrupts SOUL.md mid-sync; verify must restore it.
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$INSTALL" make_corrupt_shim)
+chk "verify: upgrade still succeeds"          "[ '$rc' -eq 0 ]"
+chk "verify: SOUL.md restored to original"    "[ \"\$(cat '$H/SOUL.md')\" = '$SOUL_ORIG' ]"
+chk "verify: no corruption remains in SOUL.md" "! grep -q 'CORRUPTION-mid-sync' '$H/SOUL.md'"
+chk "verify: loud restore warning emitted"    "grep -qi 'restored from the pre-update snapshot' '$OUT'"
+chk "verify: secret still not leaked"         "! grep -q '$SECRET' '$OUT'"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+# Control: strip the verify call → the corruption must SURVIVE (net is load-bearing).
+sed '/# VERIFY-NET/d' "$INSTALL" > "$VERIFYCTRL"
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$VERIFYCTRL" make_corrupt_shim)
+chk "control: SOUL.md corruption survives"    "grep -q 'CORRUPTION-mid-sync' '$H/SOUL.md'"
+chk "control: no restore warning emitted"     "! grep -qi 'restored from the pre-update snapshot' '$OUT'"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+echo "── Part 6: verify net honors an intentional migration removal (+ control) ─"
+# BLOCKER regression: on a pre-v2 install, bin/ is operator-classified so the durable
+# snapshot captures it — but the v1→v2 migration deletes bin/ ON PURPOSE. The verify
+# net must SKIP that removal (is_migration_removed), or it heals bin/ back and the
+# migration is silently undone forever once the version is stamped.
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$INSTALL" "" "" seed_home_v1)
+chk "migration: upgrade succeeds"                 "[ '$rc' -eq 0 ]"
+chk "migration: legacy bin/ stays removed"        "[ ! -e '$H/bin' ]"
+chk "migration: operator SOUL.md untouched"       "[ \"\$(cat '$H/SOUL.md')\" = '$SOUL_ORIG' ]"
+chk "migration: version stamped to $FRAMEWORK_VERSION" "[ \"\$(cat '$H/.framework-version')\" = '$FRAMEWORK_VERSION' ]"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+# Control: strip the MIGRATION-SKIP-GUARD → the verify net restores bin/ from the
+# snapshot, silently undoing the migration (proves the guard is load-bearing).
+MIGCTRL="$(mk_control 'MIGRATION-SKIP-GUARD' migration-control)"
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$MIGCTRL" "" "" seed_home_v1)
+chk "control: bin/ wrongly restored by verify"    "[ -e '$H/bin/tool.sh' ]"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+echo "── Part 7: verify net never restores a secret through a symlink (+ control) ─"
+# HIGH (CWE-59) regression: an attacker who swaps an operator file for a symlink
+# AFTER the durable snapshot must not cause the verify net's restore to write the
+# snapshot's secret out THROUGH that link. The shipped net drops a symlinked leaf and
+# writes a real file in its place, leaving the external target untouched.
+EXFIL_DIR=$(mktemp -d); EXFIL_TARGET="$EXFIL_DIR/stolen"
+printf 'ATTACKER-PLACEHOLDER\n' > "$EXFIL_TARGET"
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$INSTALL" make_symlink_leaf_shim)
+chk "symlink-leaf: upgrade succeeds"              "[ '$rc' -eq 0 ]"
+chk "symlink-leaf: secret NOT written through link" "! grep -q '$SECRET' '$EXFIL_TARGET'"
+chk "symlink-leaf: credentials.json is a real file" "[ -f '$H/tools/_lib/credentials.json' ] && [ ! -L '$H/tools/_lib/credentials.json' ]"
+chk "symlink-leaf: credentials.json restored intact" "grep -q '$SECRET' '$H/tools/_lib/credentials.json'"
+chk "symlink-leaf: secret not leaked to stdout/stderr" "! grep -q '$SECRET' '$OUT'"
+rm -rf "$STATE" "$H" "$EXFIL_DIR"; rm -f "$OUT"
+
+# Control: strip the SYMLINK-LEAF-GUARD → cp follows the swapped-in link and writes
+# the snapshot secret out through it (proves the guard is load-bearing).
+EXFIL_DIR=$(mktemp -d); EXFIL_TARGET="$EXFIL_DIR/stolen"
+printf 'ATTACKER-PLACEHOLDER\n' > "$EXFIL_TARGET"
+LEAFCTRL="$(mk_control 'SYMLINK-LEAF-GUARD' symlinkleaf-control)"
+IFS=$'\t' read -r rc OUT STATE H < <(run_snap "$LEAFCTRL" make_symlink_leaf_shim)
+chk "control: secret leaked through the symlink"  "grep -q '$SECRET' '$EXFIL_TARGET'"
+rm -rf "$STATE" "$H" "$EXFIL_DIR"; rm -f "$OUT"
+
+echo "── Part 8: snapshot umask 077 does not leak into synced files (+ control) ──"
+# SHOULD-FIX regression: umask 077 is process-global. Scoped to the snapshot it keeps
+# backups 0600; leaked past it, every later cp/mkdir inherits 0600/0700. A freshly-
+# synced framework file must be 0644 (per the ambient 022 umask) while the backup of
+# a secret stays 0600.
+IFS=$'\t' read -r rc OUT STATE H < <(umask 022; run_snap "$INSTALL")
+SNAP="$(snap_dir "$STATE")"
+chk "umask: upgrade succeeds"                     "[ '$rc' -eq 0 ]"
+chk "umask: synced framework file is 0644"        "[ \$(stat -c '%a' '$H/$POISON_REL') -eq 644 ]"
+chk "umask: backup of a secret stays 0600"        "[ \$(stat -c '%a' '$SNAP/tools/_lib/credentials.json') -eq 600 ]"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+# Control: strip the UMASK-RESTORE-NORMAL line → umask 077 leaks past the snapshot,
+# so the newly-synced framework file is created 0600 (proves the restore matters).
+UMASKCTRL="$(mk_control 'UMASK-RESTORE-NORMAL' umask-control)"
+IFS=$'\t' read -r rc OUT STATE H < <(umask 022; run_snap "$UMASKCTRL")
+chk "control: leaked umask makes synced file 0600" "[ \$(stat -c '%a' '$H/$POISON_REL') -eq 600 ]"
+rm -rf "$STATE" "$H"; rm -f "$OUT"
+
+echo ""
+echo "RESULT: $pass passed, $fail failed"
+[ "$fail" -eq 0 ]
--- a/packages/mosaic/framework/tools/quality/scripts/test-upgrade-manifest-guard.sh
+++ b/packages/mosaic/framework/tools/quality/scripts/test-upgrade-manifest-guard.sh
@@ -29,6 +29,13 @@ INSTALL="$FW/install.sh"
 pass=0; fail=0
 chk() { if eval "$2"; then echo "  ✓ $1"; pass=$((pass + 1)); else echo "  ✗ $1"; fail=$((fail + 1)); fi; }

+# Redirect the #791 PR2 durable pre-update snapshot ($XDG_STATE_HOME/mosaic/backups)
+# into a throwaway so a keep-mode upgrade under test never writes into the real
+# ~/.local/state. This test asserts operator-surface fidelity, not backup content.
+export XDG_STATE_HOME
+XDG_STATE_HOME="$(mktemp -d)"
+trap 'rm -rf "$XDG_STATE_HOME"' EXIT
+
 SECRET='SUPER-SECRET-TOKEN-do-not-log-3f9a'

 # Seed a throwaway MOSAIC_HOME with an operator sentinel per ownership class.
@@ -206,7 +213,7 @@ fi
 #    does not silently depend on rsync being installed. (Provide the coreutils the
 #    installer needs on the stripped PATH.)
 FBIN=$(mktemp -d)
-for t in bash cp find mktemp rm mkdir chmod cmp sed grep cat dirname basename stat sha256sum awk tr; do
+for t in bash cp find mktemp rm mkdir chmod cmp sed grep cat dirname basename stat sha256sum awk tr date sort; do
  p=$(command -v "$t" 2>/dev/null) && ln -s "$p" "$FBIN/$t"
 done
 run_matrix "rsync-absent" env "PATH=$FBIN"
--- a/packages/mosaic/src/cli.ts
+++ b/packages/mosaic/src/cli.ts
@@ -17,6 +17,7 @@ import { registerConfigCommand } from './commands/config.js';
 import { registerFleetCommand } from './commands/fleet.js';
 import { registerMissionCommand } from './commands/mission.js';
 import { registerUninstallCommand } from './commands/uninstall.js';
+import { registerRestoreCommand } from './commands/restore.js';
 // prdy is registered via launch.ts
 import { registerLaunchCommands } from './commands/launch.js';
 import { registerAuthCommand } from './commands/auth.js';
@@ -406,6 +407,10 @@ registerStorageCommand(program);

 registerUninstallCommand(program);

+// ─── restore ─────────────────────────────────────────────────────────────────
+
+registerRestoreCommand(program);
+
 // ─── telemetry ───────────────────────────────────────────────────────────────

 registerTelemetryCommand(program);
--- a/packages/mosaic/src/commands/fleet-regen-command.spec.ts
+++ b/packages/mosaic/src/commands/fleet-regen-command.spec.ts
@@ -0,0 +1,885 @@
+import { chmod, mkdir, mkdtemp, open, readFile, rm, stat, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { Command } from 'commander';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { registerFleetCommand, type CommandResult, type FleetCommandDeps } from './fleet.js';
+import { executeFleetRegen, formatFleetRegenReport } from './fleet-regen-command.js';
+import {
+  acquirePrivateRosterMutationLock,
+  projectRosterV2AgentGeneratedEnv,
+} from '../fleet/fleet-reconciler.js';
+import { applyPreparedGeneratedAgentEnvironmentProjection } from '../fleet/generated-env-boundary.js';
+import { parseRosterV2 } from '../fleet/roster-v2.js';
+
+// A two-agent roster-v2 SSOT. `mosaic fleet regen` must rebuild each agent's
+// `fleet/agents/<name>.env.generated` projection from exactly this source and
+// nothing else — deterministically, without ever touching agent lifecycle.
+const rosterYaml = `
+version: 2
+generation: 7
+transport: tmux
+tmux:
+  socket_name: mosaic-fleet
+  holder_session: _holder
+defaults:
+  working_directory: /srv/mosaic
+  runtime: pi
+runtimes:
+  pi:
+    reset_command: /new
+agents:
+  - name: coder0
+    alias: Coder 0
+    class: code
+    runtime: pi
+    provider: openai
+    model: gpt-5.6-sol
+    reasoning: high
+    tool_policy: code
+    working_directory: /srv/mosaic
+    persistent_persona: false
+    reset_between_tasks: true
+    lifecycle:
+      enabled: true
+      desired_state: stopped
+    launch:
+      yolo: true
+  - name: coder1
+    alias: Coder 1
+    class: code
+    runtime: pi
+    provider: openai
+    model: gpt-5.6-sol
+    reasoning: medium
+    tool_policy: code
+    working_directory: /srv/other
+    persistent_persona: false
+    reset_between_tasks: true
+    lifecycle:
+      enabled: true
+      desired_state: stopped
+    launch:
+      yolo: true
+`;
+
+let cleanup: string | undefined;
+
+afterEach(async (): Promise<void> => {
+  vi.restoreAllMocks();
+  process.exitCode = undefined;
+  if (cleanup) await rm(cleanup, { recursive: true, force: true });
+  cleanup = undefined;
+});
+
+async function fleetHome(withRoster = rosterYaml): Promise<string> {
+  cleanup = await mkdtemp(join(tmpdir(), 'mosaic-fleet-regen-command-'));
+  for (const directory of ['fleet', 'fleet/agents', 'fleet/roles']) {
+    await mkdir(join(cleanup, directory), { recursive: true, mode: 0o700 });
+    await chmod(join(cleanup, directory), 0o700);
+  }
+  await chmod(cleanup, 0o700);
+  await writeFile(join(cleanup, 'fleet', 'roster.yaml'), withRoster, { mode: 0o600 });
+  // Semantic roster validation (matching `fleet reconcile`) resolves each agent
+  // class to a persona; seed the classes the fixtures reference.
+  for (const klass of ['code', 'merge-gate']) {
+    await writeFile(
+      join(cleanup, 'fleet', 'roles', `${klass}.md`),
+      `\`class: ${klass}\`\n\n# ${klass} persona\n`,
+      { mode: 0o600 },
+    );
+  }
+  return cleanup;
+}
+
+/**
+ * A runner spy that records EVERY invocation. `regen` is projection-only and
+ * must never issue a lifecycle/restart call, so a non-empty call log is a
+ * hard failure — this is the load-bearing "never restarts" gate.
+ */
+function recordingRunner(
+  calls: string[][],
+): (command: string, args: string[]) => Promise<CommandResult> {
+  return async (command: string, args: string[]): Promise<CommandResult> => {
+    calls.push([command, ...args]);
+    return { stdout: '', stderr: '', exitCode: 0 };
+  };
+}
+
+function program(mosaicHome: string, runner: FleetCommandDeps['runner']): Command {
+  const result = new Command();
+  result.exitOverride();
+  registerFleetCommand(result, { mosaicHome, runner });
+  return result;
+}
+
+function capture(): string[] {
+  const lines: string[] = [];
+  vi.spyOn(console, 'log').mockImplementation((value: string): void => {
+    lines.push(value);
+  });
+  return lines;
+}
+
+async function exists(path: string): Promise<boolean> {
+  try {
+    await stat(path);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+describe('projectRosterV2AgentGeneratedEnv', (): void => {
+  it('maps a roster-v2 agent to exactly the eight generated projection keys', (): void => {
+    const roster = parseRosterV2(rosterYaml, 'yaml');
+    const agent = roster.agents.find((candidate) => candidate.name === 'coder0');
+    expect(agent).toBeDefined();
+    const values = projectRosterV2AgentGeneratedEnv(roster, agent!);
+    expect(values).toEqual({
+      MOSAIC_AGENT_NAME: 'coder0',
+      MOSAIC_AGENT_CLASS: 'code',
+      MOSAIC_AGENT_RUNTIME: 'pi',
+      MOSAIC_AGENT_MODEL: 'gpt-5.6-sol',
+      MOSAIC_AGENT_REASONING: 'high',
+      MOSAIC_AGENT_TOOL_POLICY: 'code',
+      MOSAIC_AGENT_WORKDIR: '/srv/mosaic',
+      MOSAIC_TMUX_SOCKET: 'mosaic-fleet',
+    });
+  });
+});
+
+describe('mosaic fleet regen', (): void => {
+  it('is dry-run by default: reports the plan and writes nothing', async (): Promise<void> => {
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    const lines = capture();
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--json',
+    ]);
+
+    const result = JSON.parse(lines.pop() ?? '{}');
+    expect(result).toMatchObject({
+      mode: 'dry-run',
+      generation: 7,
+      agentCount: 2,
+      written: 0,
+    });
+    expect(result.agents.map((agent: { name: string }) => agent.name)).toEqual([
+      'coder0',
+      'coder1',
+    ]);
+    expect(
+      result.agents.every((agent: { disposition: string }) => agent.disposition === 'create'),
+    ).toBe(true);
+    // Nothing on disk.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder1.env.generated'))).toBe(false);
+    // No lifecycle/restart call — ever.
+    expect(calls).toEqual([]);
+  });
+
+  it('forwards configured persona roots (rolesDir/overrideDir) from reconcileDeps into regen', async (): Promise<void> => {
+    // Codex r6: regen must resolve personas the SAME way reconcile does. If the
+    // top-level registration drops reconcileDeps.rolesDir/overrideDir, a deployment
+    // with custom persona roots gets reconcile ACCEPTING a roster while regen
+    // REJECTS it (validating against the wrong default `<home>/fleet/roles`),
+    // blocking the recovery command. Pin the wiring: seed personas ONLY under the
+    // custom roots, leave the default roles dir empty, require regen to succeed.
+    const home = await mkdtemp(join(tmpdir(), 'mosaic-fleet-regen-wiring-'));
+    cleanup = home;
+    for (const directory of ['fleet', 'fleet/agents', 'fleet/roles']) {
+      await mkdir(join(home, directory), { recursive: true, mode: 0o700 });
+      await chmod(join(home, directory), 0o700);
+    }
+    await chmod(home, 0o700);
+    await writeFile(join(home, 'fleet', 'roster.yaml'), rosterYaml, { mode: 0o600 });
+    // Personas live ONLY under the configured roots — the default `fleet/roles`
+    // stays empty, so broken wiring fails persona resolution.
+    const customRoles = join(home, 'custom-roles');
+    const customOverride = join(home, 'custom-roles.local');
+    await mkdir(customRoles, { recursive: true, mode: 0o700 });
+    await mkdir(customOverride, { recursive: true, mode: 0o700 });
+    for (const klass of ['code', 'merge-gate']) {
+      await writeFile(join(customRoles, `${klass}.md`), `\`class: ${klass}\`\n\n# ${klass}\n`, {
+        mode: 0o600,
+      });
+    }
+
+    const command = new Command();
+    command.exitOverride();
+    registerFleetCommand(command, {
+      mosaicHome: home,
+      runner: recordingRunner([]),
+      reconcileDeps: { rolesDir: customRoles, overrideDir: customOverride },
+    });
+    const lines = capture();
+
+    await command.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--json']);
+
+    // Regen validated against the CONFIGURED roots → success. Broken wiring
+    // resolves against the empty default and fails (exitCode 1, no JSON result).
+    expect(process.exitCode).not.toBe(1);
+    const result = JSON.parse(lines.pop() ?? '{}');
+    expect(result).toMatchObject({ mode: 'dry-run', agentCount: 2 });
+  });
+
+  it('--write rebuilds each generated projection from the roster SSOT', async (): Promise<void> => {
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    const lines = capture();
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    const result = JSON.parse(lines.pop() ?? '{}');
+    expect(result).toMatchObject({ mode: 'write', written: 2, agentCount: 2 });
+
+    const coder0 = await readFile(join(home, 'fleet', 'agents', 'coder0.env.generated'), 'utf8');
+    expect(coder0).toContain('MOSAIC_AGENT_NAME=coder0');
+    expect(coder0).toContain('MOSAIC_AGENT_WORKDIR=/srv/mosaic');
+    expect(coder0).toContain('MOSAIC_TMUX_SOCKET=mosaic-fleet');
+    const coder1 = await readFile(join(home, 'fleet', 'agents', 'coder1.env.generated'), 'utf8');
+    expect(coder1).toContain('MOSAIC_AGENT_NAME=coder1');
+    expect(coder1).toContain('MOSAIC_AGENT_WORKDIR=/srv/other');
+    // No lifecycle/restart call — ever.
+    expect(calls).toEqual([]);
+  });
+
+  it('is deterministic and idempotent across repeated --write runs', async (): Promise<void> => {
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    const path = join(home, 'fleet', 'agents', 'coder0.env.generated');
+
+    const cli = program(home, recordingRunner(calls));
+    capture();
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--write', '--json']);
+    const first = await readFile(path, 'utf8');
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--write', '--json']);
+    const second = await readFile(path, 'utf8');
+
+    expect(second).toBe(first);
+    expect(calls).toEqual([]);
+  });
+
+  it('reports rebuild disposition once the generated projection already exists', async (): Promise<void> => {
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    const cli = program(home, recordingRunner(calls));
+    const lines = capture();
+
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--write', '--json']);
+    lines.length = 0;
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--json']);
+
+    const result = JSON.parse(lines.pop() ?? '{}');
+    expect(
+      result.agents.every((agent: { disposition: string }) => agent.disposition === 'rebuild'),
+    ).toBe(true);
+    expect(result.written).toBe(0);
+    expect(calls).toEqual([]);
+  });
+
+  it('never issues a lifecycle/restart call in either mode', async (): Promise<void> => {
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    const cli = program(home, recordingRunner(calls));
+    capture();
+
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--json']);
+    await cli.parseAsync(['node', 'mosaic', 'fleet', 'regen', '--write', '--json']);
+
+    // Structural guarantee: regen has no path to systemctl/tmux at all.
+    expect(calls).toEqual([]);
+    const systemctlCalls = calls.filter(([command]) => command === 'systemctl');
+    expect(systemctlCalls).toEqual([]);
+  });
+
+  it('human-readable --write output prints the do-not-restart recovery runbook', async (): Promise<void> => {
+    const home = await fleetHome();
+    const lines = capture();
+
+    await program(home, recordingRunner([])).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+    ]);
+
+    const output = lines.join('\n');
+    expect(output).toMatch(/do not restart|before.*restart/i);
+    expect(output).toMatch(/env\.generated/);
+    // The unit has no EnvironmentFile= directive, so the runbook must NOT tell the
+    // operator to verify one — verify the launcher/generated file instead.
+    expect(output).not.toContain('EnvironmentFile');
+    expect(output).toMatch(/systemctl --user cat mosaic-agent@<name>/);
+    expect(output).toMatch(/restart/i);
+  });
+
+  it('emits paths and counts only — never the projected env body (secrev)', async (): Promise<void> => {
+    const home = await fleetHome();
+    const lines = capture();
+
+    await program(home, recordingRunner([])).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+    ]);
+
+    const output = lines.join('\n');
+    // Relative paths + counts are fine; the rendered projection body (KEY=value
+    // lines) must never be echoed to stdout.
+    expect(output).toContain('coder0.env.generated');
+    expect(output).not.toMatch(/^MOSAIC_AGENT_\w+=/m);
+    expect(output).not.toContain('MOSAIC_TMUX_SOCKET=mosaic-fleet');
+  });
+
+  it('is projection-only: leaves legacy .env untouched and writes no .env.local/.env.quarantine', async (): Promise<void> => {
+    // Recovery contract: regen rebuilds ONLY <name>.env.generated. It must never
+    // relocate, quarantine, or unlink the operator-owned legacy .env surface — the
+    // full reconciler apply path does, so regen must NOT use it.
+    const home = await fleetHome();
+    const legacyPath = join(home, 'fleet', 'agents', 'coder0.env');
+    await writeFile(legacyPath, 'MOSAIC_RUNTIME_BIN=/usr/bin/pi\n', { mode: 0o600 });
+    capture();
+
+    await program(home, recordingRunner([])).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    // Generated projection rebuilt...
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(true);
+    // ...but the operator's legacy .env is preserved verbatim, and no local/quarantine
+    // files were fabricated from it.
+    expect(await readFile(legacyPath, 'utf8')).toBe('MOSAIC_RUNTIME_BIN=/usr/bin/pi\n');
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.local'))).toBe(false);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.quarantine'))).toBe(false);
+  });
+
+  it('prepares every agent before writing any: a later prepare failure leaves nothing written', async (): Promise<void> => {
+    // Fail-closed across agents. Pre-seed coder1's projection with world-readable
+    // perms so its prepare rejects; coder0 (valid) must NOT be written because
+    // preparation is fully completed before the first apply.
+    const home = await fleetHome();
+    const coder1Path = join(home, 'fleet', 'agents', 'coder1.env.generated');
+    await writeFile(coder1Path, 'MOSAIC_AGENT_NAME=coder1\n', { mode: 0o644 });
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    // coder0 is valid but must remain unwritten — no partial rebuild.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('fails closed on a semantically invalid roster (protected-class tool-policy mismatch)', async (): Promise<void> => {
+    // A hand-edited roster giving a protected class a weaker tool_policy is rejected
+    // by reconcile/plan/verify; regen must enforce the SAME gate, not silently
+    // project the downgraded policy into <name>.env.generated.
+    const home = await fleetHome(`
+version: 2
+generation: 3
+transport: tmux
+tmux:
+  socket_name: mosaic-fleet
+  holder_session: _holder
+defaults:
+  working_directory: /srv/mosaic
+  runtime: pi
+runtimes:
+  pi:
+    reset_command: /new
+agents:
+  - name: gate0
+    alias: Gate 0
+    class: merge-gate
+    runtime: pi
+    provider: openai
+    model: gpt-5.6-sol
+    reasoning: high
+    tool_policy: code
+    working_directory: /srv/mosaic
+    persistent_persona: false
+    reset_between_tasks: true
+    lifecycle:
+      enabled: true
+      desired_state: stopped
+    launch:
+      yolo: true
+`);
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    expect(await exists(join(home, 'fleet', 'agents', 'gate0.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('refuses to write while a concurrent reconcile holds the mutation lock', async (): Promise<void> => {
+    // regen --write mutates the same projections as reconcile; it must take the
+    // shared reconcile lock so a concurrent reconcile cannot race a stale write.
+    const home = await fleetHome();
+    await writeFile(join(home, 'fleet', 'roster.yaml.reconcile.lock'), 'held\n', { mode: 0o600 });
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('fails closed (non-zero, no writes) on an invalid roster', async (): Promise<void> => {
+    // roster-v2 requires >= 1 agent; a malformed roster must abort regen without
+    // writing any projection and without touching lifecycle.
+    const home = await fleetHome('version: 2\ngeneration: 1\n');
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('rejects a non-canonical --roster path (canonical roster only)', async (): Promise<void> => {
+    // `fleet` exposes a global `--roster`; reconcile rejects a non-canonical value
+    // rather than silently reading the canonical roster. regen must enforce the
+    // SAME guard so `--roster /elsewhere regen --write` cannot mislead the operator.
+    const home = await fleetHome();
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      '--roster',
+      join(home, 'other', 'roster.yaml'),
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    expect(errors.join('')).toMatch(/canonical roster/i);
+    // No projection written against a misdirected roster path.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('reports an incomplete rebuild when a later projection write fails mid-loop', async (): Promise<void> => {
+    // Fault injected on the SECOND apply: coder0 is already replaced on disk, so a
+    // bare throw would hide the partial state. regen must surface which agents were
+    // rebuilt and which failed, with a verify-before-restart recovery instruction.
+    const home = await fleetHome();
+    const realApply = applyPreparedGeneratedAgentEnvironmentProjection;
+    const result = await executeFleetRegen(
+      {
+        runner: recordingRunner([]),
+        mosaicHome: home,
+        applyProjection: async (prepared): Promise<string> => {
+          if (prepared.generatedPath.endsWith('coder1.env.generated')) {
+            throw new Error('simulated ENOSPC on coder1');
+          }
+          return realApply(prepared);
+        },
+      },
+      { write: true },
+    );
+
+    // coder0 written before the fault; coder1 not.
+    expect(result.written).toBe(1);
+    expect(result.incomplete).toMatchObject({
+      code: 'projection-apply-failed',
+      failedAgent: 'coder1',
+      writtenAgents: ['coder0'],
+    });
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(true);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder1.env.generated'))).toBe(false);
+
+    // The human report names the failed agent and directs a verify-before-restart
+    // recovery — not a bare success runbook.
+    const report = formatFleetRegenReport(result).join('\n');
+    expect(report).toMatch(/incomplete/i);
+    expect(report).toContain('coder1');
+    expect(report).toMatch(/verify|re-run|retry/i);
+    // Structural: still no lifecycle call anywhere on the failure path.
+    expect(report).not.toMatch(/systemctl --user restart mosaic-agent@<name>\n.*\n/);
+  });
+
+  it('refuses to write while agent CRUD holds the roster mutation lock', async (): Promise<void> => {
+    // regen --write overwrites the SAME generated projections that `fleet agent
+    // create/update/delete` writes under roster.yaml.mutation.lock. If regen only
+    // took the reconcile lock it could read a stale roster and overwrite a just-
+    // committed projection. It must also take the mutation lock and fail closed.
+    const home = await fleetHome();
+    await writeFile(join(home, 'fleet', 'roster.yaml.mutation.lock'), 'crud\n', { mode: 0o600 });
+    const calls: string[][] = [];
+    capture();
+    const errors: string[] = [];
+    vi.spyOn(process.stderr, 'write').mockImplementation((chunk: string | Uint8Array): boolean => {
+      errors.push(String(chunk));
+      return true;
+    });
+
+    await program(home, recordingRunner(calls)).parseAsync([
+      'node',
+      'mosaic',
+      'fleet',
+      'regen',
+      '--write',
+      '--json',
+    ]);
+
+    expect(process.exitCode).toBe(1);
+    expect(errors.join('')).toMatch(/regen failed/i);
+    // No projection written against a roster a concurrent CRUD is mutating.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder1.env.generated'))).toBe(false);
+    expect(calls).toEqual([]);
+  });
+
+  it('surfaces BOTH the roster failure and a stale-lock warning on a double fault', async (): Promise<void> => {
+    // Worst case with nothing written: run() fails (invalid roster, fail-closed) AND
+    // a lock release faults. The operator must be told the lock may be stale, not
+    // just the roster error — otherwise the next regen/reconcile is silently blocked.
+    const home = await fleetHome('version: 2\ngeneration: 1\n');
+    const throwingRelease =
+      () => async (): Promise<() => Promise<void>> => async (): Promise<void> => {
+        throw new Error('simulated lock unlink fault');
+      };
+    await expect(
+      executeFleetRegen(
+        {
+          runner: recordingRunner([]),
+          mosaicHome: home,
+          acquireReconcileLock: throwingRelease,
+          acquireRosterMutationLock: throwingRelease,
+        },
+        { write: true },
+      ),
+    ).rejects.toThrow(/stale|inspect|lock/i);
+    // Nothing written on the fail-closed path.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+  });
+
+  it('propagates a roster-mutation-lock release failure instead of silently keeping a stale lock', async (): Promise<void> => {
+    // Finding L: the real `acquirePrivateRosterMutationLock` release must SURFACE an
+    // unlink failure, not swallow it. If it swallowed (like the CRUD-internal
+    // acquireMutationLock does), a stale roster.yaml.mutation.lock left after a
+    // successful regen would block later regen/agent CRUD while the command reported
+    // success — and the finding-J stale-lock warning would never fire for this lock.
+    const home = await fleetHome();
+    const release = await acquirePrivateRosterMutationLock(home)();
+    // Simulate the lock vanishing (or being unremovable) before release runs: the
+    // underlying unlink then faults. A swallowing release would resolve and hide it.
+    await rm(join(home, 'fleet', 'roster.yaml.mutation.lock'));
+    await expect(release()).rejects.toThrow();
+  });
+
+  it('preserves a complete rebuild result and flags lock-cleanup when release faults', async (): Promise<void> => {
+    // A lock-release fault after a successful write must NOT discard the fact that
+    // projections are now live — the operator needs to know the write happened and
+    // that the shared lock may be stale, not just a bare "regen failed".
+    const home = await fleetHome();
+    const result = await executeFleetRegen(
+      {
+        runner: recordingRunner([]),
+        mosaicHome: home,
+        acquireReconcileLock:
+          () => async (): Promise<() => Promise<void>> => async (): Promise<void> => {
+            throw new Error('simulated lock unlink fault');
+          },
+      },
+      { write: true },
+    );
+
+    expect(result.written).toBe(2);
+    expect(result.incomplete).toBeUndefined();
+    expect(result.cleanup).toMatchObject({
+      code: 'lock-cleanup-failed',
+      action: 'inspect-lock-before-retry',
+    });
+    // Projections are genuinely on disk despite the release fault.
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(true);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder1.env.generated'))).toBe(true);
+
+    const report = formatFleetRegenReport(result).join('\n');
+    expect(report).toMatch(/lock/i);
+    expect(report).toMatch(/stale|inspect|clear/i);
+  });
+
+  it('preserves a partial rebuild AND flags lock-cleanup when both apply and release fault', async (): Promise<void> => {
+    // Worst case: a mid-loop apply fault leaves a partial rebuild, then the release
+    // also faults. Both recovery states must survive so the operator sees exactly
+    // which projections are live and that the lock may be stale.
+    const home = await fleetHome();
+    const realApply = applyPreparedGeneratedAgentEnvironmentProjection;
+    const result = await executeFleetRegen(
+      {
+        runner: recordingRunner([]),
+        mosaicHome: home,
+        applyProjection: async (prepared): Promise<string> => {
+          if (prepared.generatedPath.endsWith('coder1.env.generated')) {
+            throw new Error('simulated ENOSPC on coder1');
+          }
+          return realApply(prepared);
+        },
+        acquireReconcileLock:
+          () => async (): Promise<() => Promise<void>> => async (): Promise<void> => {
+            throw new Error('simulated lock unlink fault');
+          },
+      },
+      { write: true },
+    );
+
+    expect(result.written).toBe(1);
+    expect(result.incomplete).toMatchObject({
+      code: 'projection-apply-failed',
+      failedAgent: 'coder1',
+      writtenAgents: ['coder0'],
+    });
+    expect(result.cleanup).toMatchObject({ code: 'lock-cleanup-failed' });
+
+    const report = formatFleetRegenReport(result).join('\n');
+    expect(report).toMatch(/incomplete/i);
+    expect(report).toMatch(/lock/i);
+  });
+
+  it('fails closed without deleting a REPLACEMENT roster-mutation lock it no longer owns', async (): Promise<void> => {
+    // Finding M1 (replacement-lock race): after this invocation created its lock, the
+    // lock is cleared and re-created by ANOTHER writer (new inode + foreign token)
+    // BEFORE release runs. The release must PROVE ownership (device/inode + token) and
+    // refuse to unlink the stranger's live lock — otherwise it deletes it, a third
+    // writer enters concurrently, and mutual exclusion over the projections is broken.
+    // This pins the REACHABLE window (replaced-before-release); the residual sub-window
+    // between the final check and the path `unlink` is unreachable within the `wx`
+    // writer protocol and matches the merged reconcile lock — see the acquirer doc.
+    const home = await fleetHome();
+    const lockPath = join(home, 'fleet', 'roster.yaml.mutation.lock');
+    const release = await acquirePrivateRosterMutationLock(home)();
+    // A different writer replaces the lock: same path, NEW inode, foreign content.
+    await rm(lockPath);
+    await writeFile(lockPath, 'a-different-writer\n', { mode: 0o600 });
+    const replacement = await stat(lockPath);
+    // Release must reject (it no longer owns this file) AND the diagnosis must name
+    // the MUTATION lock specifically — a fault on this shared helper must not be
+    // mislabeled as the reconcile lock (that would defeat the M3 accurate-diagnosis
+    // goal, since regen serializes on both) ...
+    await expect(release()).rejects.toThrow(/roster\.yaml\.mutation\.lock/);
+    // ... and MUST NOT have unlinked the stranger's live lock.
+    expect(await exists(lockPath)).toBe(true);
+    expect((await stat(lockPath)).ino).toBe(replacement.ino);
+  });
+
+  it('does not strand the lock file when initialization fails after creating it', async (): Promise<void> => {
+    // Codex r4: if writeFile/stat/close/ownership-check throws AFTER the `wx` create
+    // succeeds, the just-created lock must NOT be left behind — a stranded lock would
+    // permanently block future regen AND agent CRUD (both contend on this path). The
+    // cleanup is dev/ino-guarded (never deletes a replacement) and best-effort (never
+    // masks the initialization error).
+    const home = await fleetHome();
+    const lockPath = join(home, 'fleet', 'roster.yaml.mutation.lock');
+    // Inject an opener that performs the REAL `wx` create (so the lock file really
+    // lands on disk) but whose handle faults on the token write — a post-create
+    // initialization failure.
+    const faultingOpen = (async (
+      path: Parameters<typeof open>[0],
+      flags: Parameters<typeof open>[1],
+      mode: Parameters<typeof open>[2],
+    ) => {
+      const handle = await open(path, flags, mode);
+      return new Proxy(handle, {
+        get(target, prop, receiver): unknown {
+          if (prop === 'writeFile') {
+            return async (): Promise<never> => {
+              throw new Error('simulated ENOSPC on token write');
+            };
+          }
+          const value = Reflect.get(target, prop, receiver);
+          return typeof value === 'function' ? value.bind(target) : value;
+        },
+      });
+    }) as typeof open;
+
+    await expect(acquirePrivateRosterMutationLock(home, faultingOpen)()).rejects.toThrow(
+      /mutation\.lock/,
+    );
+    // The lock we created must have been cleaned up — not stranded on disk.
+    expect(await exists(lockPath)).toBe(false);
+  });
+
+  it('does not strand the lock file when the post-create stat itself fails', async (): Promise<void> => {
+    // Codex r5: the narrower sub-path where `handle.stat()` ITSELF rejects right after
+    // the `wx` create succeeds (transient EIO/EBADF). device/inode identity is then
+    // unavailable, so the init-failure cleanup cannot prove ownership by dev/ino — yet
+    // the lock must STILL not be stranded, because a stranded lock permanently blocks
+    // future regen AND agent CRUD. The token we persisted before the failure uniquely
+    // identifies OUR lock, so cleanup falls back to matching it.
+    const home = await fleetHome();
+    const lockPath = join(home, 'fleet', 'roster.yaml.mutation.lock');
+    // Real `wx` create (lock really lands on disk); the token write SUCCEEDS so our
+    // random token is persisted, then `stat()` faults — a post-create init failure.
+    const faultingStatOpen = (async (
+      path: Parameters<typeof open>[0],
+      flags: Parameters<typeof open>[1],
+      mode: Parameters<typeof open>[2],
+    ) => {
+      const handle = await open(path, flags, mode);
+      return new Proxy(handle, {
+        get(target, prop, receiver): unknown {
+          if (prop === 'stat') {
+            return async (): Promise<never> => {
+              throw new Error('simulated EIO on post-create stat');
+            };
+          }
+          const value = Reflect.get(target, prop, receiver);
+          return typeof value === 'function' ? value.bind(target) : value;
+        },
+      });
+    }) as typeof open;
+
+    await expect(acquirePrivateRosterMutationLock(home, faultingStatOpen)()).rejects.toThrow(
+      /mutation\.lock/,
+    );
+    // Even without dev/ino, the token-based fallback must have removed our lock.
+    expect(await exists(lockPath)).toBe(false);
+  });
+
+  it('surfaces a lock-cleanup fault when unwinding after a later lock acquire fails', async (): Promise<void> => {
+    // Finding M2: the FIRST lock is taken, the SECOND lock's acquire throws, and
+    // unwinding the first lock's release ALSO faults. The acquire-unwind path must
+    // surface both — the already-taken lock may now be stale and silently block the
+    // next regen/reconcile — not just re-throw the acquire error and drop the fault.
+    const home = await fleetHome();
+    await expect(
+      executeFleetRegen(
+        {
+          runner: recordingRunner([]),
+          mosaicHome: home,
+          // Mutation lock (acquired first) succeeds, but its RELEASE faults on unwind.
+          acquireRosterMutationLock:
+            () => async (): Promise<() => Promise<void>> => async (): Promise<void> => {
+              throw new Error('simulated mutation-lock unlink fault');
+            },
+          // Reconcile lock (acquired second) ACQUIRE fails, triggering the unwind.
+          acquireReconcileLock: () => async (): Promise<() => Promise<void>> => {
+            throw new Error('simulated reconcile acquire failure');
+          },
+        },
+        { write: true },
+      ),
+    ).rejects.toThrow(/stale|inspect|lock/i);
+    expect(await exists(join(home, 'fleet', 'agents', 'coder0.env.generated'))).toBe(false);
+  });
+
+  it('names BOTH fleet locks in the stale-lock warning (cleanup can come from either)', async (): Promise<void> => {
+    // Finding M3: finding L made the roster MUTATION lock's release fault reachable, so
+    // the cleanup marker can now originate from either lock. The human report must not
+    // claim only the reconcile lock is stale — it must name both candidate lock files
+    // so the operator inspects the right one.
+    const home = await fleetHome();
+    const result = await executeFleetRegen(
+      {
+        runner: recordingRunner([]),
+        mosaicHome: home,
+        // Fault the MUTATION lock's release specifically (reconcile lock is real).
+        acquireRosterMutationLock:
+          () => async (): Promise<() => Promise<void>> => async (): Promise<void> => {
+            throw new Error('simulated mutation-lock unlink fault');
+          },
+      },
+      { write: true },
+    );
+
+    expect(result.written).toBe(2);
+    expect(result.cleanup).toMatchObject({ code: 'lock-cleanup-failed' });
+    const report = formatFleetRegenReport(result).join('\n');
+    expect(report).toContain('roster.yaml.mutation.lock');
+    expect(report).toContain('roster.yaml.reconcile.lock');
+  });
+});
--- a/packages/mosaic/src/commands/fleet-regen-command.ts
+++ b/packages/mosaic/src/commands/fleet-regen-command.ts
@@ -0,0 +1,441 @@
+import { readFile, stat } from 'node:fs/promises';
+import { homedir } from 'node:os';
+import { join, relative, resolve } from 'node:path';
+import type { Command } from 'commander';
+import type { CommandRunner } from './fleet.js';
+import {
+  applyPreparedGeneratedAgentEnvironmentProjection,
+  prepareGeneratedAgentEnvironmentProjection,
+  type PreparedGeneratedAgentEnvironmentProjection,
+} from '../fleet/generated-env-boundary.js';
+import {
+  acquirePrivateReconcileLock,
+  acquirePrivateRosterMutationLock,
+  projectRosterV2AgentGeneratedEnv,
+} from '../fleet/fleet-reconciler.js';
+import {
+  parseRosterV2,
+  validateRosterV2Semantics,
+  type FleetRosterV2,
+} from '../fleet/roster-v2.js';
+
+/**
+ * `mosaic fleet regen` — recovery-framed regeneration of the roster-derived
+ * agent env projections (`fleet/agents/<name>.env.generated`) from the roster
+ * SSOT. It is the recovery layer of #791: when a wiped/diverged operator surface
+ * has cost an agent its generated projection, regen rebuilds it deterministically
+ * from `roster.yaml` so the launcher can source the intended identity again.
+ *
+ * It is intentionally a THIN projection-only wrapper over the same mapping the
+ * reconciler apply path uses ({@link projectRosterV2AgentGeneratedEnv}), and it
+ * has NO code path to systemd lifecycle: regen never starts, stops, or restarts
+ * an agent. Recovery order forbids restart-before-verify, so the operator must
+ * verify each rebuilt projection resolves before restarting anything.
+ */
+export interface FleetRegenAgentPlan {
+  readonly name: string;
+  /** Generated-projection path, relative to the Mosaic home (never absolute in output). */
+  readonly path: string;
+  /** `create` when the projection is currently absent, `rebuild` when it already exists. */
+  readonly disposition: 'create' | 'rebuild';
+}
+
+/**
+ * Present ONLY when a `--write` projection apply failed partway through the
+ * fleet. Every prior agent was already validated and prepared, so `writtenAgents`
+ * are byte-complete on disk; `failedAgent` is where the write faulted. The
+ * command surfaces this instead of a bare throw so the operator can finish the
+ * rebuild and verify every projection before restarting any unit.
+ */
+export interface FleetRegenIncomplete {
+  readonly code: 'projection-apply-failed';
+  /** The agent whose generated-projection write faulted. */
+  readonly failedAgent: string;
+  /** Agents whose projections were fully written before the fault (in apply order). */
+  readonly writtenAgents: readonly string[];
+}
+
+export interface FleetRegenResult {
+  readonly mode: 'dry-run' | 'write';
+  /** Roster path, relative to the Mosaic home. */
+  readonly rosterPath: string;
+  readonly generation: number;
+  readonly agentCount: number;
+  /** Count of projections written to disk (always 0 in dry-run). */
+  readonly written: number;
+  readonly agents: readonly FleetRegenAgentPlan[];
+  /** Set ONLY when a `--write` apply faulted mid-fleet — a partial rebuild. */
+  readonly incomplete?: FleetRegenIncomplete;
+  /**
+   * Set ONLY when a shared fleet lock could not be released after a write completed
+   * (or partially completed) — EITHER `roster.yaml.mutation.lock` or
+   * `roster.yaml.reconcile.lock`, since regen holds both. The projections in this
+   * result are real; the lock file may be stale and must be inspected before the
+   * next mutation. Independent of {@link incomplete} — both can be present at once.
+   */
+  readonly cleanup?: {
+    readonly code: 'lock-cleanup-failed';
+    readonly action: 'inspect-lock-before-retry';
+  };
+}
+
+export interface FleetRegenDeps {
+  /**
+   * Present ONLY so an accidental future lifecycle wiring is caught by the
+   * "never restarts" test — regen is contractually forbidden to invoke it.
+   */
+  readonly runner: CommandRunner;
+  readonly mosaicHome?: string;
+  /** Persona roots for semantic roster validation (defaults mirror the reconciler). */
+  readonly rolesDir?: string;
+  readonly overrideDir?: string;
+  /** Test seam: canonical roster reader. Defaults to parse + semantic validation. */
+  readonly readRoster?: (rosterPath: string) => Promise<FleetRosterV2>;
+  /**
+   * Test seam: generated-only projection validator. Defaults to the audited
+   * boundary helper that validates ONLY `<name>.env.generated`.
+   */
+  readonly prepareProjection?: typeof prepareGeneratedAgentEnvironmentProjection;
+  /** Test seam: generated-only projection writer. Defaults to the audited boundary helper. */
+  readonly applyProjection?: typeof applyPreparedGeneratedAgentEnvironmentProjection;
+  /**
+   * Test seam: acquire the shared reconcile lock before a `--write`. Defaults to
+   * the reconciler's private lock so regen and reconcile are mutually exclusive
+   * and a concurrent reconcile cannot race a stale projection write.
+   */
+  readonly acquireReconcileLock?: (mosaicHome: string) => () => Promise<() => Promise<void>>;
+  /**
+   * Test seam: acquire the shared roster mutation lock before a `--write`.
+   * Defaults to the reconciler's hardened, ownership-proving acquirer for the
+   * same `fleet/roster.yaml.mutation.lock` path that CRUD contends on, so regen
+   * serializes against agent create/update/delete — both rewrite the same
+   * generated projections, so without this a concurrent CRUD could race a stale
+   * projection write.
+   */
+  readonly acquireRosterMutationLock?: (mosaicHome: string) => () => Promise<() => Promise<void>>;
+  /** Test seam: existence probe for create/rebuild disposition. */
+  readonly fileExists?: (path: string) => Promise<boolean>;
+}
+
+interface FleetRegenOptions {
+  readonly write?: boolean;
+}
+
+function defaultMosaicHome(deps: FleetRegenDeps): string {
+  return deps.mosaicHome ?? process.env['MOSAIC_HOME'] ?? join(homedir(), '.config', 'mosaic');
+}
+
+async function defaultFileExists(path: string): Promise<boolean> {
+  try {
+    await stat(path);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Projection-only regeneration core. Deterministic and side-effect-free in
+ * dry-run; in `--write` it rebuilds each generated projection through the
+ * audited generated-only boundary writer and NOTHING else — it never writes
+ * `.env.local`/`.env.quarantine`, never unlinks the legacy `.env`, and never
+ * touches `deps.runner`.
+ *
+ * Fail-closed by construction: the roster is validated (structural + semantic)
+ * and EVERY agent's projection is prepared before ANY is written, so a single
+ * invalid agent (or a semantically rejected roster) leaves the whole surface
+ * untouched. In `--write` mode the read-prepare-apply sequence runs under the
+ * shared reconcile lock so a concurrent reconcile cannot race a stale write.
+ */
+export async function executeFleetRegen(
+  deps: FleetRegenDeps,
+  options: FleetRegenOptions,
+): Promise<FleetRegenResult> {
+  const mosaicHome = defaultMosaicHome(deps);
+  const agentEnvDir = join(mosaicHome, 'fleet', 'agents');
+  const rosterPath = join(mosaicHome, 'fleet', 'roster.yaml');
+  const readRoster = deps.readRoster ?? defaultReadRoster(deps, mosaicHome);
+  const prepare = deps.prepareProjection ?? prepareGeneratedAgentEnvironmentProjection;
+  const apply = deps.applyProjection ?? applyPreparedGeneratedAgentEnvironmentProjection;
+  const fileExists = deps.fileExists ?? defaultFileExists;
+  const acquireReconcileLock = deps.acquireReconcileLock ?? acquirePrivateReconcileLock;
+  const acquireRosterMutationLock =
+    deps.acquireRosterMutationLock ?? acquirePrivateRosterMutationLock;
+  const write = options.write === true;
+
+  const run = async (): Promise<FleetRegenResult> => {
+    const roster = await readRoster(rosterPath);
+
+    // Prepare (validate) every agent BEFORE any write, so a later failure never
+    // leaves an earlier projection partially rebuilt.
+    const prepared: (PreparedGeneratedAgentEnvironmentProjection & {
+      readonly name: string;
+      readonly disposition: 'create' | 'rebuild';
+    })[] = [];
+    for (const agent of roster.agents) {
+      const projection = await prepare({
+        mosaicHome,
+        agentEnvDir,
+        agentName: agent.name,
+        generated: projectRosterV2AgentGeneratedEnv(roster, agent),
+      });
+      const disposition = (await fileExists(projection.generatedPath)) ? 'rebuild' : 'create';
+      prepared.push({ ...projection, name: agent.name, disposition });
+    }
+
+    // Every projection is already validated (prepared); an apply here can only
+    // fault on the underlying I/O (ENOSPC, EIO, a race outside the lock). If a
+    // later write faults, earlier agents are already replaced on disk, so we
+    // stop and report the partial state rather than throwing a bare error that
+    // hides which projections are now live.
+    let written = 0;
+    let incomplete: FleetRegenIncomplete | undefined;
+    if (write) {
+      for (const projection of prepared) {
+        try {
+          await apply(projection);
+        } catch {
+          incomplete = {
+            code: 'projection-apply-failed',
+            failedAgent: projection.name,
+            writtenAgents: prepared.slice(0, written).map((entry) => entry.name),
+          };
+          break;
+        }
+        written += 1;
+      }
+    }
+
+    return {
+      mode: write ? 'write' : 'dry-run',
+      rosterPath: relative(mosaicHome, rosterPath),
+      generation: roster.generation,
+      agentCount: roster.agents.length,
+      written,
+      agents: prepared.map((projection) => ({
+        name: projection.name,
+        path: relative(mosaicHome, projection.generatedPath),
+        disposition: projection.disposition,
+      })),
+      ...(incomplete ? { incomplete } : {}),
+    };
+  };
+
+  // Dry-run is preview-only and never mutates, so it stays lock-free (usable even
+  // while a reconcile or CRUD holds a lock). A `--write` must serialize against
+  // BOTH roster writers of the same generated projections: agent CRUD (roster
+  // mutation lock) and reconcile (reconcile lock). Acquire in a fixed order — the
+  // locks are non-blocking (they throw on contention rather than wait), so no
+  // deadlock is possible — and fail closed if either is already held.
+  if (!write) return run();
+
+  const releases: (() => Promise<void>)[] = [];
+  try {
+    releases.push(await acquireRosterMutationLock(mosaicHome)());
+    releases.push(await acquireReconcileLock(mosaicHome)());
+  } catch (error: unknown) {
+    // A later acquire failed; unwind any lock already taken (reverse order). If
+    // that unwind ALSO faults, surface both — the already-taken lock may now be
+    // stale and silently block the next regen/reconcile — rather than dropping it.
+    const releaseFault = await releaseFleetLocks(releases);
+    throw augmentWithLockCleanupFault(error, releaseFault);
+  }
+
+  let result: FleetRegenResult | undefined;
+  let primaryError: unknown;
+  try {
+    result = await run();
+  } catch (error: unknown) {
+    primaryError = error;
+  }
+
+  // Release both locks (reverse acquire order), continuing past a fault so neither
+  // leaks. A release fault must never discard a completed (or partial) rebuild —
+  // the projections are already on disk — and must not be silently hidden even
+  // when the run itself failed with nothing written.
+  const releaseFault = await releaseFleetLocks(releases);
+  if (result) {
+    return releaseFault === undefined
+      ? result
+      : {
+          ...result,
+          cleanup: { code: 'lock-cleanup-failed', action: 'inspect-lock-before-retry' },
+        };
+  }
+  // result is undefined ⇒ run() threw ⇒ primaryError is set.
+  throw augmentWithLockCleanupFault(primaryError, releaseFault);
+}
+
+/**
+ * Releases held locks in reverse acquire order, continuing past a fault so a
+ * single failed release never leaks the others. Returns the first fault seen (or
+ * undefined when every release succeeded).
+ */
+async function releaseFleetLocks(releases: readonly (() => Promise<void>)[]): Promise<unknown> {
+  let firstFault: unknown;
+  for (const release of [...releases].reverse()) {
+    try {
+      await release();
+    } catch (error: unknown) {
+      if (firstFault === undefined) firstFault = error;
+    }
+  }
+  return firstFault;
+}
+
+/**
+ * When a run failed with nothing written AND lock cleanup also faulted, surface
+ * both: the operator needs to know the lock may be stale (else the next
+ * regen/reconcile is silently blocked), not just the original run error.
+ */
+function augmentWithLockCleanupFault(primaryError: unknown, releaseFault: unknown): unknown {
+  if (releaseFault === undefined) return primaryError;
+  const base = primaryError instanceof Error ? primaryError.message : String(primaryError);
+  return new Error(
+    `${base} (additionally, a fleet lock could not be released — ` +
+      'fleet/roster.yaml.mutation.lock or roster.yaml.reconcile.lock may be stale; ' +
+      'inspect and clear it before retry)',
+  );
+}
+
+function defaultReadRoster(
+  deps: FleetRegenDeps,
+  mosaicHome: string,
+): (rosterPath: string) => Promise<FleetRosterV2> {
+  return async (rosterPath: string): Promise<FleetRosterV2> => {
+    const roster = parseRosterV2(await readFile(rosterPath, 'utf8'), 'yaml');
+    // Enforce the SAME semantic gate as reconcile/plan/verify (persona resolution
+    // + protected-class tool-policy match) so regen cannot project a roster the
+    // rest of the fleet surface would reject.
+    await validateRosterV2Semantics(roster, {
+      rolesDir: deps.rolesDir ?? join(mosaicHome, 'fleet', 'roles'),
+      overrideDir: deps.overrideDir ?? join(mosaicHome, 'fleet', 'roles.local'),
+    });
+    return roster;
+  };
+}
+
+/**
+ * Human-readable report — paths and counts ONLY. The rendered projection body
+ * (KEY=value lines) is never echoed (secrev). In `--write` mode it prints the
+ * recovery runbook: verify each projection resolves BEFORE any restart.
+ */
+export function formatFleetRegenReport(result: FleetRegenResult): string[] {
+  const lines: string[] = [];
+  const header =
+    result.mode === 'dry-run'
+      ? 'mosaic fleet regen — dry run (no changes written)'
+      : 'mosaic fleet regen — wrote roster-derived projections';
+  lines.push(header);
+  lines.push(
+    `roster: ${result.rosterPath} (generation ${result.generation}, ${result.agentCount} agent(s))`,
+  );
+  for (const agent of result.agents) {
+    lines.push(`  ${agent.name}  ${agent.path}  [${agent.disposition}]`);
+  }
+
+  if (result.mode === 'dry-run') {
+    lines.push(
+      `Plan: would write ${result.agentCount} generated projection(s). ` +
+        `Re-run with --write to apply. regen never restarts agents.`,
+    );
+    return lines;
+  }
+
+  if (result.incomplete) {
+    const { failedAgent, writtenAgents } = result.incomplete;
+    lines.push(
+      `INCOMPLETE: rebuild stopped at agent ${failedAgent} — its projection write faulted.`,
+    );
+    lines.push(
+      `Rebuilt before the fault: ${writtenAgents.length} projection(s)` +
+        (writtenAgents.length > 0 ? ` (${writtenAgents.join(', ')})` : '') +
+        `; ${result.agentCount - writtenAgents.length} not written.`,
+    );
+    lines.push(
+      'Recover: re-run `mosaic fleet regen --write` to finish, then VERIFY every ' +
+        "agent's fleet/agents/<name>.env.generated resolves BEFORE restarting any " +
+        'unit. regen never restarts agents.',
+    );
+  } else {
+    lines.push(`Wrote ${result.written} generated projection(s).`);
+    lines.push('Next steps — DO NOT restart agents before verifying:');
+    lines.push(
+      "  1. Confirm each agent's fleet/agents/<name>.env.generated exists and carries " +
+        'the intended MOSAIC_AGENT_* values.',
+    );
+    // The unit sets NO EnvironmentFile= — it launches from a minimal env and the
+    // launcher (start-agent-session.sh) sources .env.generated itself. Verify the
+    // launcher path, not a nonexistent EnvironmentFile property.
+    lines.push(
+      '  2. Confirm the unit launches from it: `systemctl --user cat mosaic-agent@<name>` ' +
+        'shows ExecStart running start-agent-session.sh, which reads .env.generated.',
+    );
+    lines.push(
+      '  3. Only then restart one unit at a time: systemctl --user restart mosaic-agent@<name>.',
+    );
+  }
+
+  if (result.cleanup) {
+    lines.push(
+      'WARNING: a fleet lock could not be released — ' +
+        'fleet/roster.yaml.mutation.lock or fleet/roster.yaml.reconcile.lock may be ' +
+        'stale; inspect and clear it before the next reconcile or regen.',
+    );
+  }
+  return lines;
+}
+
+function resolveRegenMosaicHome(fleetCommand: Command, deps: FleetRegenDeps): string {
+  const options = fleetCommand.optsWithGlobals<{ mosaicHome?: string }>();
+  return options.mosaicHome ?? defaultMosaicHome(deps);
+}
+
+/**
+ * regen reads the roster SSOT at `<mosaicHome>/fleet/roster.yaml` and ignores the
+ * `fleet --roster <path>` global. If an operator passes a non-canonical `--roster`
+ * they must NOT be silently served the canonical file — mirror reconcile's guard
+ * and reject, so `mosaic fleet --roster /elsewhere regen --write` fails closed.
+ */
+function assertRegenCanonicalRoster(fleetCommand: Command, mosaicHome: string): void {
+  const options = fleetCommand.optsWithGlobals<{ roster?: string }>();
+  const canonical = join(mosaicHome, 'fleet', 'roster.yaml');
+  if (options.roster !== undefined && resolve(options.roster) !== resolve(canonical)) {
+    throw new Error(
+      'Roster-v2 regeneration requires the canonical roster path (fleet/roster.yaml).',
+    );
+  }
+}
+
+/** Registers the recovery-framed `mosaic fleet regen` command. */
+export function registerFleetRegenCommand(fleetCommand: Command, deps: FleetRegenDeps): void {
+  fleetCommand
+    .command('regen')
+    .description(
+      'Regenerate roster-derived agent env projections (dry-run default, --write to apply; never restarts)',
+    )
+    .option('--write', 'Write the regenerated projections to disk (default: dry-run)')
+    .option('--json', 'Emit the plan/result as JSON')
+    .action(async (opts: { write?: boolean; json?: boolean }): Promise<void> => {
+      const mosaicHome = resolveRegenMosaicHome(fleetCommand, deps);
+      try {
+        assertRegenCanonicalRoster(fleetCommand, mosaicHome);
+        const result = await executeFleetRegen(
+          { ...deps, mosaicHome },
+          { write: opts.write === true },
+        );
+        if (opts.json === true) {
+          console.log(JSON.stringify(result));
+        } else {
+          for (const line of formatFleetRegenReport(result)) console.log(line);
+        }
+        // A partial rebuild or a stale-lock cleanup state is a non-zero outcome:
+        // the operator must finish/verify (and clear the lock) before restarting.
+        if (result.incomplete || result.cleanup) process.exitCode = 1;
+      } catch (error: unknown) {
+        process.exitCode = 1;
+        const message = error instanceof Error ? error.message : String(error);
+        process.stderr.write(`mosaic fleet regen failed: ${message}\n`);
+      }
+    });
+}
--- a/packages/mosaic/src/commands/fleet.spec.ts
+++ b/packages/mosaic/src/commands/fleet.spec.ts
@@ -97,6 +97,7 @@ describe('registerFleetCommand', () => {
      'provision',
      'ps',
      'reconcile',
+      'regen',
      'remove',
      'restart',
      'start',
--- a/packages/mosaic/src/commands/fleet.ts
+++ b/packages/mosaic/src/commands/fleet.ts
@@ -44,6 +44,7 @@ import {
  registerFleetReconcilerCommands,
  type FleetReconcilerCommandDeps,
 } from './fleet-reconciler-command.js';
+import { registerFleetRegenCommand } from './fleet-regen-command.js';
 import { resolveCommsBlock } from '../fleet/comms-onboarding.js';
 import {
  applyPreparedAgentEnvironmentProjection,
@@ -2063,6 +2064,18 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
    reconcileDeps: deps.reconcileDeps,
  });

+  // Recovery (#791 PR3): rebuild roster-derived env projections from the roster
+  // SSOT. Projection-only and preview-first — never issues a lifecycle restart.
+  registerFleetRegenCommand(cmd, {
+    runner,
+    mosaicHome: deps.mosaicHome,
+    // Resolve personas the SAME way reconcile does: forward any configured roots
+    // so a custom-persona-root deployment cannot have reconcile accept a roster
+    // that regen then rejects against the default `<mosaicHome>/fleet/roles`.
+    rolesDir: deps.reconcileDeps?.rolesDir,
+    overrideDir: deps.reconcileDeps?.overrideDir,
+  });
+
  return cmd;
 }

--- a/packages/mosaic/src/commands/restore.spec.ts
+++ b/packages/mosaic/src/commands/restore.spec.ts
@@ -0,0 +1,466 @@
+/**
+ * Tests for `mosaic restore` (#791 PR2 Task 12).
+ *
+ * The durable pre-update snapshot (install.sh: make_durable_snapshot) writes the
+ * operator-owned surface to $XDG_STATE_HOME/mosaic/backups/pre-update-<ts>/ with
+ * 0700 dirs / 0600 files. `mosaic restore` is the recovery counterpart:
+ *   • --list (default)   enumerate snapshots by timestamp — dry-run, never mutates.
+ *   • --from <ts>        restore that snapshot over MOSAIC_HOME, confirmation-gated.
+ * It reports counts and relative paths ONLY — a snapshot may contain secrets
+ * (credentials.json), so no file content is ever printed (secrev invariant).
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import {
+  mkdtempSync,
+  rmSync,
+  mkdirSync,
+  writeFileSync,
+  readFileSync,
+  statSync,
+  lstatSync,
+  symlinkSync,
+} from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir, homedir } from 'node:os';
+import { Command } from 'commander';
+import {
+  resolveBackupRoot,
+  listSnapshots,
+  resolveSnapshotDir,
+  planRestore,
+  applyRestore,
+  runRestore,
+  registerRestoreCommand,
+} from './restore.js';
+
+const SECRET = 'SUPER-SECRET-TOKEN-do-not-log-restore';
+
+function seedSnapshot(root: string, ts: string, files: Record<string, string>): string {
+  const dir = join(root, `pre-update-${ts}`);
+  for (const [rel, content] of Object.entries(files)) {
+    const abs = join(dir, rel);
+    mkdirSync(join(abs, '..'), { recursive: true });
+    writeFileSync(abs, content);
+  }
+  return dir;
+}
+
+describe('mosaic restore (#791 PR2)', () => {
+  let tmp: string;
+  let backups: string;
+
+  beforeEach(() => {
+    tmp = mkdtempSync(join(tmpdir(), 'mosaic-restore-'));
+    backups = join(tmp, 'state', 'mosaic', 'backups');
+    mkdirSync(backups, { recursive: true });
+    vi.clearAllMocks();
+  });
+
+  afterEach(() => {
+    rmSync(tmp, { recursive: true, force: true });
+  });
+
+  describe('resolveBackupRoot', () => {
+    it('honors XDG_STATE_HOME', () => {
+      expect(resolveBackupRoot({ XDG_STATE_HOME: '/x/state' })).toBe('/x/state/mosaic/backups');
+    });
+    it('falls back to ~/.local/state', () => {
+      expect(resolveBackupRoot({})).toBe(join(homedir(), '.local', 'state', 'mosaic', 'backups'));
+    });
+  });
+
+  describe('listSnapshots', () => {
+    it('returns [] when the backup root does not exist', () => {
+      expect(listSnapshots(join(tmp, 'nope'))).toEqual([]);
+    });
+
+    it('lists pre-update snapshots newest-first with file counts, ignoring other dirs', () => {
+      seedSnapshot(backups, '20240101T000000Z', { 'SOUL.md': 'a' });
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'b', 'agents/x.conf': 'c' });
+      mkdirSync(join(backups, 'unrelated-dir'), { recursive: true });
+
+      const snaps = listSnapshots(backups);
+      expect(snaps.map((s) => s.timestamp)).toEqual(['20260101T000000Z', '20240101T000000Z']);
+      expect(snaps[0]!.fileCount).toBe(2);
+      expect(snaps[1]!.fileCount).toBe(1);
+    });
+  });
+
+  describe('resolveSnapshotDir', () => {
+    it('resolves by bare timestamp and by full pre-update-<ts> name', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'a' });
+      expect(resolveSnapshotDir(backups, '20260101T000000Z')).toBe(dir);
+      expect(resolveSnapshotDir(backups, 'pre-update-20260101T000000Z')).toBe(dir);
+    });
+    it('returns undefined for an unknown timestamp', () => {
+      expect(resolveSnapshotDir(backups, '19990101T000000Z')).toBeUndefined();
+    });
+  });
+
+  describe('planRestore', () => {
+    it('walks nested dirs and returns every relative file path', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', {
+        'SOUL.md': 'a',
+        'agents/x.conf': 'b',
+        'tools/_lib/credentials.json': 'c',
+      });
+      expect(planRestore(dir).sort()).toEqual(
+        ['SOUL.md', 'agents/x.conf', 'tools/_lib/credentials.json'].sort(),
+      );
+    });
+  });
+
+  describe('applyRestore', () => {
+    it('restores byte-exact content, creates parent dirs, and sets 0600', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', {
+        'SOUL.md': 'original-soul',
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      // A diverged operator file that restore must overwrite.
+      writeFileSync(join(home, 'SOUL.md'), 'CORRUPTED');
+
+      const n = applyRestore(dir, home, planRestore(dir));
+      expect(n).toBe(2);
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('original-soul');
+      expect(readFileSync(join(home, 'tools/_lib/credentials.json'), 'utf8')).toBe(
+        `TOKEN=${SECRET}\n`,
+      );
+      expect(statSync(join(home, 'SOUL.md')).mode & 0o777).toBe(0o600);
+      expect(statSync(join(home, 'tools/_lib/credentials.json')).mode & 0o777).toBe(0o600);
+    });
+  });
+
+  describe('runRestore', () => {
+    it('--list prints timestamps and counts, mutating nothing', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'a', 'agents/x.conf': 'b' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+
+      const code = await runRestore({
+        list: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(0);
+      const out = log.mock.calls.flat().join('\n');
+      expect(out).toContain('20260101T000000Z');
+      expect(out).toMatch(/2\b/); // the file count is surfaced
+      log.mockRestore();
+    });
+
+    it('--from restores the snapshot over MOSAIC_HOME byte-exact (yes bypasses prompt)', async () => {
+      seedSnapshot(backups, '20260101T000000Z', {
+        'SOUL.md': 'restored-soul',
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'STALE');
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        yes: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(0);
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('restored-soul');
+      expect(readFileSync(join(home, 'tools/_lib/credentials.json'), 'utf8')).toBe(
+        `TOKEN=${SECRET}\n`,
+      );
+    });
+
+    it('--from with an unknown timestamp fails without mutating', async () => {
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'KEEP');
+      const err = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+      const code = await runRestore({
+        from: '19990101T000000Z',
+        yes: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(1);
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('KEEP');
+      err.mockRestore();
+    });
+
+    it('--dry-run with --from reports the plan but mutates nothing', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'snap' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'UNCHANGED');
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        dryRun: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(0);
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('UNCHANGED');
+      log.mockRestore();
+    });
+
+    it('--from prompts and applies the restore when the operator confirms', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'confirmed-soul' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'STALE');
+      vi.spyOn(console, 'log').mockImplementation(() => {});
+      const confirm = vi.fn().mockResolvedValue(true);
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+        confirm,
+      });
+
+      expect(code).toBe(0);
+      expect(confirm).toHaveBeenCalledOnce();
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('confirmed-soul');
+    });
+
+    it('--from aborts without mutating when the operator declines', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'snap' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'KEEP');
+      vi.spyOn(console, 'log').mockImplementation(() => {});
+      const confirm = vi.fn().mockResolvedValue(false);
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+        confirm,
+      });
+
+      expect(code).toBe(0);
+      expect(confirm).toHaveBeenCalledOnce();
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('KEEP');
+    });
+
+    it('MOSAIC_ASSUME_YES=1 bypasses the confirmation prompt', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'env-yes' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'STALE');
+      vi.spyOn(console, 'log').mockImplementation(() => {});
+      const confirm = vi.fn().mockResolvedValue(false);
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state'), MOSAIC_ASSUME_YES: '1' },
+        confirm,
+      });
+
+      expect(code).toBe(0);
+      expect(confirm).not.toHaveBeenCalled();
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('env-yes');
+    });
+
+    it('--list reports gracefully when no snapshots exist', async () => {
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+
+      const code = await runRestore({
+        list: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'empty-state') },
+      });
+
+      expect(code).toBe(0);
+      expect(log.mock.calls.flat().join('\n')).toMatch(/No pre-update snapshots/);
+    });
+
+    it('never prints a secret value found inside a backed-up file', async () => {
+      seedSnapshot(backups, '20260101T000000Z', {
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+      const err = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+      await runRestore({
+        list: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+      await runRestore({
+        from: '20260101T000000Z',
+        yes: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      const all = [...log.mock.calls, ...err.mock.calls].flat().join('\n');
+      expect(all).not.toContain(SECRET);
+      log.mockRestore();
+      err.mockRestore();
+    });
+  });
+
+  // Regression coverage for the codex code+security review of PR2 (#791):
+  // CWE-22 traversal via --from, and CWE-59 symlink write-through in applyRestore.
+  describe('security hardening', () => {
+    it.each([
+      '../../etc',
+      'pre-update-/../../tmp/poison',
+      'pre-update-../evil',
+      '20260101T000000Z/../../../tmp',
+      'not-a-timestamp',
+      '2026-01-01',
+    ])('resolveSnapshotDir rejects traversal / malformed selector %j', (bad) => {
+      // Even if a matching directory exists on disk, a non-timestamp selector
+      // must not resolve — the only accepted shape is <8>T<6>Z[-n].
+      expect(resolveSnapshotDir(backups, bad)).toBeUndefined();
+    });
+
+    it('runRestore --from a traversal selector fails closed without copying', async () => {
+      // Plant a real dir one level ABOVE the backup root. The naive resolver
+      // `join(root, from)` with `from='../poison'` would reach it (backups is
+      // .../mosaic/backups, so `../poison` == .../mosaic/poison) and import it.
+      const outside = join(tmp, 'state', 'mosaic', 'poison');
+      mkdirSync(outside, { recursive: true });
+      writeFileSync(join(outside, 'x'), 'attacker');
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      const err = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+      const code = await runRestore({
+        from: '../poison',
+        yes: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(1);
+      expect(statSync(home).isDirectory()).toBe(true);
+      // Nothing from `outside` was imported.
+      expect(() => statSync(join(home, 'x'))).toThrow();
+      err.mockRestore();
+    });
+
+    it('applyRestore refuses to write a secret through a symlinked leaf (CWE-59)', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', {
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(join(home, 'tools', '_lib'), { recursive: true });
+      // Attacker points the operator credentials file at a file they can read.
+      const exfil = join(tmp, 'exfil-target');
+      writeFileSync(exfil, 'original-attacker-content');
+      symlinkSync(exfil, join(home, 'tools', '_lib', 'credentials.json'));
+
+      expect(() => applyRestore(dir, home, planRestore(dir))).toThrow();
+      // The secret was NOT written through the link into the attacker's file.
+      expect(readFileSync(exfil, 'utf8')).toBe('original-attacker-content');
+    });
+
+    it('applyRestore refuses to write through a symlinked ancestor (CWE-59)', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', {
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(join(home, 'tools'), { recursive: true });
+      // Attacker replaces the `tools/_lib` ancestor with a symlink out of the root.
+      const exfilDir = join(tmp, 'exfil-dir');
+      mkdirSync(exfilDir, { recursive: true });
+      symlinkSync(exfilDir, join(home, 'tools', '_lib'));
+
+      expect(() => applyRestore(dir, home, planRestore(dir))).toThrow();
+      // Nothing was written into the attacker-controlled directory.
+      expect(() => statSync(join(exfilDir, 'credentials.json'))).toThrow();
+    });
+
+    it('runRestore surfaces a symlink violation as exit 1 without leaking the secret', async () => {
+      seedSnapshot(backups, '20260101T000000Z', {
+        'tools/_lib/credentials.json': `TOKEN=${SECRET}\n`,
+      });
+      const home = join(tmp, 'home');
+      mkdirSync(join(home, 'tools', '_lib'), { recursive: true });
+      const exfil = join(tmp, 'exfil-target');
+      writeFileSync(exfil, 'attacker');
+      symlinkSync(exfil, join(home, 'tools', '_lib', 'credentials.json'));
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+      const err = vi.spyOn(console, 'error').mockImplementation(() => {});
+
+      const code = await runRestore({
+        from: '20260101T000000Z',
+        yes: true,
+        mosaicHome: home,
+        env: { XDG_STATE_HOME: join(tmp, 'state') },
+      });
+
+      expect(code).toBe(1);
+      expect(readFileSync(exfil, 'utf8')).toBe('attacker');
+      const all = [...log.mock.calls, ...err.mock.calls].flat().join('\n');
+      expect(all).not.toContain(SECRET);
+      log.mockRestore();
+      err.mockRestore();
+    });
+
+    it('applyRestore replaces a diverged regular file in place with 0600 (not a symlink)', () => {
+      const dir = seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'restored' });
+      const home = join(tmp, 'home');
+      mkdirSync(home, { recursive: true });
+      writeFileSync(join(home, 'SOUL.md'), 'stale');
+
+      const n = applyRestore(dir, home, planRestore(dir));
+      expect(n).toBe(1);
+      expect(lstatSync(join(home, 'SOUL.md')).isSymbolicLink()).toBe(false);
+      expect(readFileSync(join(home, 'SOUL.md'), 'utf8')).toBe('restored');
+      expect(statSync(join(home, 'SOUL.md')).mode & 0o777).toBe(0o600);
+    });
+  });
+
+  describe('registerRestoreCommand', () => {
+    it('registers `restore` with the expected flags', () => {
+      const program = new Command();
+      program.exitOverride();
+      registerRestoreCommand(program);
+      const cmd = program.commands.find((c) => c.name() === 'restore');
+      expect(cmd).toBeDefined();
+      const longs = cmd!.options.map((o) => o.long);
+      expect(longs).toEqual(
+        expect.arrayContaining(['--list', '--from', '--dry-run', '--yes', '--mosaic-home']),
+      );
+    });
+
+    it('runs the list action end-to-end via the parsed command', async () => {
+      seedSnapshot(backups, '20260101T000000Z', { 'SOUL.md': 'a' });
+      const prevXdg = process.env['XDG_STATE_HOME'];
+      process.env['XDG_STATE_HOME'] = join(tmp, 'state');
+      const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+      try {
+        const program = new Command();
+        program.exitOverride();
+        registerRestoreCommand(program);
+        await program.parseAsync(['restore', '--list', '--mosaic-home', join(tmp, 'home')], {
+          from: 'user',
+        });
+        expect(log.mock.calls.flat().join('\n')).toContain('20260101T000000Z');
+      } finally {
+        log.mockRestore();
+        if (prevXdg === undefined) delete process.env['XDG_STATE_HOME'];
+        else process.env['XDG_STATE_HOME'] = prevXdg;
+      }
+    });
+  });
+});
--- a/packages/mosaic/src/commands/restore.ts
+++ b/packages/mosaic/src/commands/restore.ts
@@ -0,0 +1,313 @@
+/**
+ * restore.ts — top-level `mosaic restore` command (#791 PR2 Task 12)
+ *
+ * Recovery counterpart to the durable pre-update snapshot taken by install.sh
+ * (make_durable_snapshot). Before a keep-mode upgrade mutates anything, the
+ * installer copies the operator-owned surface to
+ *   $XDG_STATE_HOME/mosaic/backups/pre-update-<UTC-ts>/   (0700 dirs / 0600 files)
+ * This command lets the operator inspect and roll back to those snapshots:
+ *
+ *   mosaic restore                 # == --list: enumerate snapshots (dry-run)
+ *   mosaic restore --list
+ *   mosaic restore --from <ts>     # restore that snapshot over MOSAIC_HOME
+ *   mosaic restore --from <ts> --dry-run
+ *
+ * SECREV INVARIANT: a snapshot may contain secrets (e.g. tools/_lib/credentials.json).
+ * This command reports counts and RELATIVE PATHS only — it never reads a backed-up
+ * file into any logged string. Restored files are written back 0600 (owner-only),
+ * matching the snapshot's own private posture. The path convention here mirrors
+ * install.sh `backup_root()`; keep the two in sync (no shared code across the boundary).
+ */
+
+import {
+  existsSync,
+  readdirSync,
+  statSync,
+  lstatSync,
+  readFileSync,
+  openSync,
+  writeSync,
+  fchmodSync,
+  closeSync,
+  constants,
+} from 'node:fs';
+import { createInterface } from 'node:readline';
+import { homedir } from 'node:os';
+import { join, dirname, relative } from 'node:path';
+import type { Command } from 'commander';
+import { DEFAULT_MOSAIC_HOME } from '../constants.js';
+import { assertCanonicalContainment, ensureManagedDirectory } from '../fleet/secure-file.js';
+
+// ─── types ───────────────────────────────────────────────────────────────────
+
+export interface SnapshotInfo {
+  /** The UTC stamp after the `pre-update-` prefix, e.g. "20260716T232225Z". */
+  readonly timestamp: string;
+  /** Absolute path to the snapshot directory. */
+  readonly dir: string;
+  /** Number of files captured in the snapshot. */
+  readonly fileCount: number;
+}
+
+export interface RestoreOptions {
+  list?: boolean;
+  from?: string;
+  dryRun?: boolean;
+  yes?: boolean;
+  mosaicHome: string;
+  /** Environment source (injectable for tests); defaults to process.env. */
+  env?: NodeJS.ProcessEnv;
+  /**
+   * Confirmation gate (injectable for tests); defaults to an interactive
+   * readline prompt. Returns true to proceed with the overwrite.
+   */
+  confirm?: (question: string) => Promise<boolean>;
+}
+
+const SNAPSHOT_PREFIX = 'pre-update-';
+
+/**
+ * The exact shape install.sh `make_durable_snapshot()` stamps: `<8>T<6>Z` UTC,
+ * with an optional `-<n>` same-second collision suffix. `--from` is matched
+ * against this — nothing containing a path separator or `..` can pass, so a
+ * selector can never escape the backup root (CWE-22).
+ */
+const SNAPSHOT_TS_RE = /^\d{8}T\d{6}Z(?:-\d+)?$/;
+
+// ─── pure helpers ─────────────────────────────────────────────────────────────
+
+/** Resolve the durable-snapshot root, mirroring install.sh `backup_root()`. */
+export function resolveBackupRoot(env: NodeJS.ProcessEnv = process.env): string {
+  const stateHome = env['XDG_STATE_HOME'] || join(homedir(), '.local', 'state');
+  return join(stateHome, 'mosaic', 'backups');
+}
+
+/** Recursively collect every file under `dir` as a path relative to `dir`. */
+export function planRestore(dir: string): string[] {
+  const out: string[] = [];
+  const walk = (cur: string): void => {
+    for (const entry of readdirSync(cur, { withFileTypes: true })) {
+      const abs = join(cur, entry.name);
+      if (entry.isDirectory()) {
+        walk(abs);
+      } else if (entry.isFile()) {
+        out.push(relative(dir, abs));
+      }
+    }
+  };
+  if (existsSync(dir)) walk(dir);
+  return out;
+}
+
+/** Enumerate snapshots newest-first (the `pre-update-<ts>` names sort chronologically). */
+export function listSnapshots(root: string): SnapshotInfo[] {
+  if (!existsSync(root)) return [];
+  let entries: string[];
+  try {
+    entries = readdirSync(root);
+  } catch {
+    return [];
+  }
+  return entries
+    .filter((name) => name.startsWith(SNAPSHOT_PREFIX))
+    .map((name) => join(root, name))
+    .filter((dir) => {
+      try {
+        return statSync(dir).isDirectory();
+      } catch {
+        return false;
+      }
+    })
+    .sort()
+    .reverse()
+    .map((dir) => ({
+      timestamp: dir.split('/').at(-1)!.slice(SNAPSHOT_PREFIX.length),
+      dir,
+      fileCount: planRestore(dir).length,
+    }));
+}
+
+/**
+ * Resolve a snapshot dir from a `--from` selector. Accepts ONLY a strict
+ * generated identifier — a bare `<ts>` or the full `pre-update-<ts>` name — and
+ * builds exactly `join(root, 'pre-update-' + ts)`. A selector containing `/`,
+ * `..`, or anything but the timestamp shape is rejected (returns undefined), so
+ * `--from` can never traverse outside the backup root (CWE-22). The resolved dir
+ * must be a real, non-symlink directory (lstat, not stat), so a symlinked
+ * snapshot entry can't redirect the restore either.
+ */
+export function resolveSnapshotDir(root: string, from: string): string | undefined {
+  const ts = from.startsWith(SNAPSHOT_PREFIX) ? from.slice(SNAPSHOT_PREFIX.length) : from;
+  if (!SNAPSHOT_TS_RE.test(ts)) return undefined;
+  const dir = join(root, `${SNAPSHOT_PREFIX}${ts}`);
+  try {
+    if (lstatSync(dir).isDirectory()) return dir;
+  } catch {
+    /* absent or inaccessible */
+  }
+  return undefined;
+}
+
+/**
+ * Copy each `relPaths` entry from the snapshot back into `mosaicHome`, forcing
+ * 0600 on the restored file (owner-only — the operator surface may hold secrets).
+ * Returns the number of files restored. Never reads a file's content into a
+ * logged string.
+ *
+ * SYMLINK-SAFE (CWE-59): a snapshot may hold secrets, so we must never let a
+ * tampered destination redirect the write. Every destination path is contained
+ * within `mosaicHome` (assertCanonicalContainment) and every ancestor is proven
+ * to be a real, non-symlink directory (ensureManagedDirectory) before we write.
+ * The leaf itself is opened O_NOFOLLOW, so if it was swapped for a symlink the
+ * open fails closed (ELOOP) rather than writing the secret through the link.
+ */
+export function applyRestore(
+  snapDir: string,
+  mosaicHome: string,
+  relPaths: readonly string[],
+): number {
+  let restored = 0;
+  for (const rel of relPaths) {
+    const src = join(snapDir, rel);
+    const dst = join(mosaicHome, rel);
+    // Fail closed if the target path escapes the managed root or any ancestor is
+    // a symlink; create missing ancestors as private (0700) real directories.
+    assertCanonicalContainment(mosaicHome, dst);
+    ensureManagedDirectory(mosaicHome, dirname(dst));
+    // O_NOFOLLOW: refuse to follow a symlink at the leaf (secret exfil guard).
+    // O_CREAT|O_TRUNC: create a fresh 0600 file, or overwrite a diverged real one.
+    const fd = openSync(
+      dst,
+      constants.O_WRONLY | constants.O_CREAT | constants.O_TRUNC | constants.O_NOFOLLOW,
+      0o600,
+    );
+    try {
+      fchmodSync(fd, 0o600); // enforce 0600 even when the file pre-existed
+      writeSync(fd, readFileSync(src));
+    } finally {
+      closeSync(fd);
+    }
+    restored += 1;
+  }
+  return restored;
+}
+
+// ─── orchestration ────────────────────────────────────────────────────────────
+
+async function promptConfirm(question: string): Promise<boolean> {
+  const rl = createInterface({ input: process.stdin, output: process.stdout });
+  try {
+    return await new Promise<boolean>((resolve) => {
+      rl.question(`${question} [y/N] `, (ans) => resolve(ans.trim().toLowerCase() === 'y'));
+    });
+  } finally {
+    rl.close();
+  }
+}
+
+/**
+ * Run `mosaic restore`. Returns a process exit code (0 ok, 1 error) rather than
+ * calling process.exit, so it stays unit-testable.
+ */
+export async function runRestore(opts: RestoreOptions): Promise<number> {
+  const env = opts.env ?? process.env;
+  const root = resolveBackupRoot(env);
+
+  // Default action (and explicit --list): enumerate, never mutate.
+  if (opts.list || !opts.from) {
+    const snaps = listSnapshots(root);
+    if (snaps.length === 0) {
+      console.log(`No pre-update snapshots found under ${root}.`);
+      return 0;
+    }
+    console.log(`Pre-update snapshots under ${root} (newest first):\n`);
+    for (const s of snaps) {
+      console.log(`  ${s.timestamp}  —  ${s.fileCount} file(s)`);
+    }
+    console.log(`\nRestore one with:  mosaic restore --from <timestamp>`);
+    return 0;
+  }
+
+  // --from <ts>: restore over the operator surface.
+  const snapDir = resolveSnapshotDir(root, opts.from);
+  if (!snapDir) {
+    console.error(`No snapshot matching '${opts.from}' under ${root}.`);
+    console.error(`Run 'mosaic restore --list' to see available timestamps.`);
+    return 1;
+  }
+
+  const relPaths = planRestore(snapDir);
+  const ts = snapDir.split('/').at(-1)!.slice(SNAPSHOT_PREFIX.length);
+
+  if (opts.dryRun) {
+    console.log(
+      `[dry-run] Would restore ${relPaths.length} file(s) from snapshot ${ts} into ${opts.mosaicHome}:`,
+    );
+    for (const rel of relPaths) console.log(`  ${rel}`);
+    console.log('[dry-run] No changes made.');
+    return 0;
+  }
+
+  const assumeYes = opts.yes || env['MOSAIC_ASSUME_YES'] === '1';
+  if (!assumeYes) {
+    console.log(
+      `About to restore ${relPaths.length} operator file(s) from snapshot ${ts} into ${opts.mosaicHome}.`,
+    );
+    console.log('This OVERWRITES those files with their pre-update contents.');
+    const ok = await (opts.confirm ?? promptConfirm)('Proceed?');
+    if (!ok) {
+      console.log('Restore cancelled. No changes made.');
+      return 0;
+    }
+  }
+
+  let n: number;
+  try {
+    n = applyRestore(snapDir, opts.mosaicHome, relPaths);
+  } catch (err) {
+    // A containment/symlink violation is a fail-closed security stop, not a
+    // routine error — surface it without leaking file contents and abort.
+    console.error(
+      `Restore aborted: a destination path under ${opts.mosaicHome} is unsafe to write ` +
+        `(symlink or escapes the managed root). No files were restored. (${(err as Error).message})`,
+    );
+    return 1;
+  }
+  console.log(`Restored ${n} operator file(s) from snapshot ${ts} into ${opts.mosaicHome}.`);
+  return 0;
+}
+
+// ─── commander registration ───────────────────────────────────────────────────
+
+export function registerRestoreCommand(program: Command): void {
+  program
+    .command('restore')
+    .description('List or restore durable pre-update snapshots of your operator config (#791)')
+    .option('--list', 'List available snapshots by timestamp (default action)')
+    .option('--from <timestamp>', 'Restore the snapshot with this timestamp over MOSAIC_HOME')
+    .option('--dry-run', 'With --from: show what would be restored without changing anything')
+    .option('--yes, -y', 'Skip the confirmation prompt (also: MOSAIC_ASSUME_YES=1)')
+    .option(
+      '--mosaic-home <path>',
+      'Override MOSAIC_HOME directory',
+      process.env['MOSAIC_HOME'] ?? DEFAULT_MOSAIC_HOME,
+    )
+    .action(
+      async (opts: {
+        list?: boolean;
+        from?: string;
+        dryRun?: boolean;
+        yes?: boolean;
+        mosaicHome: string;
+      }) => {
+        const code = await runRestore({
+          list: opts.list,
+          from: opts.from,
+          dryRun: opts.dryRun,
+          yes: opts.yes,
+          mosaicHome: opts.mosaicHome,
+        });
+        if (code !== 0) process.exit(code);
+      },
+    );
+}
--- a/packages/mosaic/src/fleet/fleet-agent-crud.ts
+++ b/packages/mosaic/src/fleet/fleet-agent-crud.ts
@@ -381,6 +381,22 @@ function isMissingFile(error: unknown): boolean {
  return typeof error === 'object' && error !== null && 'code' in error && error.code === 'ENOENT';
 }

+/**
+ * Non-blocking acquire of the roster mutation lock: an exclusive `wx` create that
+ * throws `concurrent-mutation` (never waits) if the lock is already held. The
+ * on-disk format is an empty private file. CRUD releases in a plain `finally` with
+ * no result-preservation logic, so a release fault here must not override the
+ * mutation result — the release swallows unlink failures by design.
+ *
+ * The recovery-framed `fleet regen` contends on this exact same lock file, but it
+ * does so through the hardened, ownership-proving acquirer in the reconciler
+ * (`acquirePrivateRosterMutationLock`), not this one: whoever wins the `wx` create
+ * owns the file (the loser always gets `concurrent-mutation`), so the reconciler's
+ * ownership token is only ever written and read back by the same regen invocation,
+ * never by this empty-file writer. The two acquirers stay mutually compatible at
+ * the `wx`-contention layer while regen additionally proves ownership before it
+ * unlinks — a guarantee CRUD does not need because it releases unconditionally.
+ */
 async function acquireMutationLock(lockPath: string): Promise<() => Promise<void>> {
  try {
    const handle = await open(lockPath, 'wx', 0o600);
--- a/packages/mosaic/src/fleet/fleet-reconciler.ts
+++ b/packages/mosaic/src/fleet/fleet-reconciler.ts
@@ -584,6 +584,29 @@ function defaultValidateRoster(
  };
 }

+/**
+ * The single roster-v2 → generated-projection mapping. Both the reconciler's
+ * apply path ({@link defaultPrepareProjections}) and the recovery-framed
+ * `mosaic fleet regen` command derive their generated env from THIS one
+ * function, so the two paths can never drift (the #791 single-SSOT invariant).
+ * Pure: no IO, no clock — a deterministic function of the roster alone.
+ */
+export function projectRosterV2AgentGeneratedEnv(
+  roster: FleetRosterV2,
+  agent: FleetRosterV2Agent,
+): Readonly<Record<string, string>> {
+  return {
+    MOSAIC_AGENT_NAME: agent.name,
+    MOSAIC_AGENT_CLASS: agent.className,
+    MOSAIC_AGENT_RUNTIME: agent.runtime,
+    MOSAIC_AGENT_MODEL: agent.model,
+    MOSAIC_AGENT_REASONING: agent.reasoning,
+    MOSAIC_AGENT_TOOL_POLICY: agent.toolPolicy,
+    MOSAIC_AGENT_WORKDIR: agent.workingDirectory,
+    MOSAIC_TMUX_SOCKET: roster.tmux.socketName,
+  };
+}
+
 function defaultPrepareProjections(
  request: FleetReconcileRequest,
 ): (roster: FleetRosterV2) => Promise<readonly PreparedAgentEnvironmentProjection[]> {
@@ -596,16 +619,7 @@ function defaultPrepareProjections(
            mosaicHome,
            agentEnvDir: join(mosaicHome, 'fleet', 'agents'),
            agentName: agent.name,
-            generated: {
-              MOSAIC_AGENT_NAME: agent.name,
-              MOSAIC_AGENT_CLASS: agent.className,
-              MOSAIC_AGENT_RUNTIME: agent.runtime,
-              MOSAIC_AGENT_MODEL: agent.model,
-              MOSAIC_AGENT_REASONING: agent.reasoning,
-              MOSAIC_AGENT_TOOL_POLICY: agent.toolPolicy,
-              MOSAIC_AGENT_WORKDIR: agent.workingDirectory,
-              MOSAIC_TMUX_SOCKET: roster.tmux.socketName,
-            },
+            generated: projectRosterV2AgentGeneratedEnv(roster, agent),
          }),
      ),
    );
@@ -620,62 +634,193 @@ function mosaicHomeFor(deps: FleetReconcileDeps): string {
  return deps.mosaicHome ?? join(homedir(), '.config', 'mosaic');
 }

-/** Acquires a private lock only after proving the canonical managed path. */
+/** Acquires the private reconcile lock only after proving the canonical managed path. */
 export function acquirePrivateReconcileLock(
  mosaicHome: string,
  openLock: typeof open = open,
+): () => Promise<() => Promise<void>> {
+  return acquirePrivateManagedRosterLock(
+    mosaicHome,
+    'roster.yaml.reconcile.lock',
+    'Another roster reconciliation is in progress.',
+    openLock,
+  );
+}
+
+/**
+ * Acquires the private roster MUTATION lock (`roster.yaml.mutation.lock`) with the
+ * exact same ownership-proof discipline as the reconcile lock. Exposed so the
+ * recovery-framed `mosaic fleet regen` can serialize its projection rewrites
+ * against agent CRUD — which holds this very lock while it rewrites the roster and
+ * the same derived projections — reusing this hardened acquirer rather than
+ * reimplementing it.
+ *
+ * The release PROVES ownership (device/inode + token) before unlinking, so a lock
+ * that was already cleared and replaced by the time release runs is never deleted
+ * out from under the new owner (it fails closed as `lock-cleanup-failed` instead),
+ * and that cleanup fault is propagated to the caller rather than silently dropped.
+ * CRUD contends on the identical path with a plain empty-file `wx` create; the two
+ * never co-own it (whoever wins `wx` owns it; the loser gets `concurrent-mutation`),
+ * so the token this writes is only ever read back by the same regen invocation that
+ * wrote it.
+ *
+ * SCOPE of the guarantee (matches the merged reconcile lock exactly — see
+ * {@link acquirePrivateManagedRosterLock}): the ownership proof closes the
+ * *reachable* window — a stale-lock reaper or operator cleared our lock and another
+ * writer took it BEFORE our release began — not the residual sub-instruction window
+ * between the final check and the path-based `unlink`. Closing that residual fully
+ * requires an fd-held advisory lock adopted by every fleet writer (CRUD, reconcile,
+ * regen), which is a cross-cutting mechanism change out of scope for this
+ * projection-only recovery command; it is unreachable within the `wx` writer
+ * protocol regardless (no Mosaic writer removes a lock it does not own, so only
+ * external interference can vacate our inode mid-release).
+ */
+export function acquirePrivateRosterMutationLock(
+  mosaicHome: string,
+  openLock: typeof open = open,
+): () => Promise<() => Promise<void>> {
+  return acquirePrivateManagedRosterLock(
+    mosaicHome,
+    'roster.yaml.mutation.lock',
+    'Another roster mutation is in progress.',
+    openLock,
+  );
+}
+
+/**
+ * Shared body for the ownership-proving managed roster locks. Acquires `lockLeaf`
+ * under `<mosaicHome>/fleet` with a private `wx` create, writes an ownership token,
+ * and returns a release that re-proves device/inode + token before unlinking so it
+ * can never remove a lock it no longer owns. Non-blocking: throws `concurrent-mutation`
+ * (with `busyMessage`) on contention rather than waiting.
+ */
+function acquirePrivateManagedRosterLock(
+  mosaicHome: string,
+  lockLeaf: string,
+  busyMessage: string,
+  openLock: typeof open,
 ): () => Promise<() => Promise<void>> {
  const fleetDir = join(mosaicHome, 'fleet');
-  const lockPath = join(fleetDir, 'roster.yaml.reconcile.lock');
+  const lockPath = join(fleetDir, lockLeaf);
+  // The message-producing helpers below serve BOTH managed locks, so every fault
+  // names the actual lock file (`fleet/<leaf>`) — an operator needs to know WHICH
+  // lock is stale, not a hardcoded "reconciliation lock".
+  const lockLabel = `fleet/${lockLeaf}`;
  return async (): Promise<() => Promise<void>> => {
    await assertPrivateManagedDirectory(mosaicHome);
    await assertPrivateManagedDirectory(fleetDir);
-    await assertSafeLockLeafIfPresent(lockPath);
+    await assertSafeLockLeafIfPresent(lockPath, lockLabel);

    let handle: FileHandle;
    try {
      handle = await openLock(lockPath, 'wx', 0o600);
    } catch (error: unknown) {
      if (isCode(error, 'EEXIST')) {
-        await assertSafeLockLeafIfPresent(lockPath);
-        throw new FleetReconcileError(
-          'concurrent-mutation',
-          'Another roster reconciliation is in progress.',
-        );
+        await assertSafeLockLeafIfPresent(lockPath, lockLabel);
+        throw new FleetReconcileError('concurrent-mutation', busyMessage);
      }
-      throw new FleetReconcileError('lock-io-failed', 'The reconciliation lock cannot be created.');
+      throw new FleetReconcileError('lock-io-failed', `The ${lockLabel} lock cannot be created.`);
    }

+    // Identity of the lock WE exclusively created (the `wx` create guaranteed it is
+    // ours). Captured up front so both the release closure and the init-failure
+    // cleanup below can prove ownership by device/inode before touching the file.
+    const created = await handle.stat().catch((): undefined => undefined);
+
    const token = randomUUID();
+    let tokenPersisted = false;
    try {
      await handle.writeFile(`${token}\n`, 'utf8');
-      const opened = await handle.stat();
+      tokenPersisted = true;
      await handle.close();
-      await assertLockOwnership(lockPath, opened.dev, opened.ino, token, 'unsafe-lock');
+      if (!created)
+        throw new FleetReconcileError(
+          'lock-io-failed',
+          `The ${lockLabel} lock cannot be initialized.`,
+        );
+      await assertLockOwnership(
+        lockPath,
+        created.dev,
+        created.ino,
+        token,
+        'unsafe-lock',
+        lockLabel,
+      );
      return async (): Promise<void> => {
        try {
-          await assertLockOwnership(lockPath, opened.dev, opened.ino, token, 'lock-cleanup-failed');
-          await assertLockOwnership(lockPath, opened.dev, opened.ino, token, 'lock-cleanup-failed');
+          await assertLockOwnership(
+            lockPath,
+            created.dev,
+            created.ino,
+            token,
+            'lock-cleanup-failed',
+            lockLabel,
+          );
          await unlink(lockPath);
        } catch (error: unknown) {
          if (error instanceof FleetReconcileError) throw error;
-          throw new FleetReconcileError(
-            'lock-cleanup-failed',
-            'The reconciliation lock cleanup failed.',
-          );
+          throw new FleetReconcileError('lock-cleanup-failed', `The ${lockLabel} cleanup failed.`);
        }
      };
    } catch (error: unknown) {
      await handle.close().catch((): void => {});
+      // Best-effort: remove the lock THIS invocation created so a transient init
+      // fault does not strand a lock that would block future regen and agent CRUD.
+      // dev/ino-guarded so it can never delete a replacement; swallowed so cleanup
+      // failure never masks the initialization error being surfaced. The token is
+      // passed only once persisted, so the fallback path (when the post-create stat
+      // failed and dev/ino is unavailable) can still prove ownership by content.
+      await removeOwnedLockLeafBestEffort(lockPath, created, tokenPersisted ? token : undefined);
      if (error instanceof FleetReconcileError) throw error;
      throw new FleetReconcileError(
        'lock-io-failed',
-        'The reconciliation lock cannot be initialized.',
+        `The ${lockLabel} lock cannot be initialized.`,
      );
    }
  };
 }

+/**
+ * Best-effort removal of a managed lock leaf THIS process created, used only on the
+ * initialization-failure path. Two independent ownership proofs, so a lock that was
+ * already replaced is never deleted:
+ *
+ *   - primary: the captured device/inode of the file we exclusively `wx`-created; or
+ *   - fallback: when the post-create stat itself failed (dev/ino unavailable), the
+ *     random `ownershipToken` we persisted — only OUR lock carries it, so a CRUD
+ *     (empty) or a differently-tokened replacement is never removed.
+ *
+ * Every fault is swallowed because the caller is already surfacing the initialization
+ * error and a leftover lock is recoverable via the documented runbook. If BOTH proofs
+ * are unavailable (stat failed AND the token write never landed) the lock is left in
+ * place rather than risk deleting another writer's file — a doubly-degenerate case
+ * requiring two independent fs faults on a just-created fd.
+ */
+async function removeOwnedLockLeafBestEffort(
+  lockPath: string,
+  created: { dev: number; ino: number } | undefined,
+  ownershipToken: string | undefined,
+): Promise<void> {
+  try {
+    const current = await lstat(lockPath);
+    if (!current.isFile() || current.isSymbolicLink()) return;
+    if (created) {
+      if (current.dev === created.dev && current.ino === created.ino) {
+        await unlink(lockPath);
+      }
+      return;
+    }
+    if (ownershipToken !== undefined) {
+      const contents = await readFile(lockPath, 'utf8');
+      if (contents.trim() === ownershipToken) {
+        await unlink(lockPath);
+      }
+    }
+  } catch {
+    // Swallowed: leftover lock is recoverable; do not mask the init error.
+  }
+}
+
 async function assertPrivateManagedDirectory(path: string): Promise<void> {
  try {
    const metadata = await lstat(path);
@@ -691,16 +836,16 @@ async function assertPrivateManagedDirectory(path: string): Promise<void> {
  }
 }

-async function assertSafeLockLeafIfPresent(lockPath: string): Promise<void> {
+async function assertSafeLockLeafIfPresent(lockPath: string, lockLabel: string): Promise<void> {
  try {
    const metadata = await lstat(lockPath);
    if (!metadata.isFile() || metadata.isSymbolicLink() || (metadata.mode & 0o077) !== 0) {
-      throw new FleetReconcileError('unsafe-lock', 'The reconciliation lock path is unsafe.');
+      throw new FleetReconcileError('unsafe-lock', `The ${lockLabel} lock path is unsafe.`);
    }
  } catch (error: unknown) {
    if (isCode(error, 'ENOENT')) return;
    if (error instanceof FleetReconcileError) throw error;
-    throw new FleetReconcileError('unsafe-lock', 'The reconciliation lock path is unavailable.');
+    throw new FleetReconcileError('unsafe-lock', `The ${lockLabel} lock path is unavailable.`);
  }
 }

@@ -710,6 +855,7 @@ async function assertLockOwnership(
  inode: number,
  token: string,
  failureCode: 'unsafe-lock' | 'lock-cleanup-failed',
+  lockLabel: string,
 ): Promise<void> {
  try {
    const metadata = await lstat(lockPath);
@@ -720,24 +866,21 @@ async function assertLockOwnership(
      metadata.dev !== device ||
      metadata.ino !== inode
    ) {
-      throw new FleetReconcileError(failureCode, 'The reconciliation lock ownership changed.');
+      throw new FleetReconcileError(failureCode, `The ${lockLabel} ownership changed.`);
    }
    const handle = await open(lockPath, constants.O_RDONLY | constants.O_NOFOLLOW);
    try {
      const opened = await handle.stat();
      const contents = await handle.readFile({ encoding: 'utf8' });
      if (opened.dev !== device || opened.ino !== inode || contents !== `${token}\n`) {
-        throw new FleetReconcileError(failureCode, 'The reconciliation lock ownership changed.');
+        throw new FleetReconcileError(failureCode, `The ${lockLabel} ownership changed.`);
      }
    } finally {
      await handle.close();
    }
  } catch (error: unknown) {
    if (error instanceof FleetReconcileError) throw error;
-    throw new FleetReconcileError(
-      failureCode,
-      'The reconciliation lock ownership cannot be proven.',
-    );
+    throw new FleetReconcileError(failureCode, `The ${lockLabel} ownership cannot be proven.`);
  }
 }

--- a/packages/mosaic/src/fleet/generated-env-boundary.ts
+++ b/packages/mosaic/src/fleet/generated-env-boundary.ts
@@ -204,6 +204,53 @@ export async function prepareAgentEnvironmentProjection(
  };
 }

+/** A generated-only projection validated without touching any legacy/local/quarantine file. */
+export interface PreparedGeneratedAgentEnvironmentProjection {
+  readonly mosaicHome: string;
+  readonly agentEnvDir: string;
+  readonly generatedPath: string;
+  readonly generated: string;
+}
+
+/**
+ * Validates ONLY the roster-derived generated projection for a recovery rebuild.
+ * Unlike {@link prepareAgentEnvironmentProjection}, this never reads, classifies,
+ * relocates, or quarantines the legacy `.env` / `.env.local` operator surface — it
+ * exists so `mosaic fleet regen` has no code path that can mutate anything except
+ * `<name>.env.generated`. The existing generated file, if present, must already be
+ * a private regular file.
+ */
+export async function prepareGeneratedAgentEnvironmentProjection(
+  options: AgentEnvironmentProjectionOptions,
+): Promise<PreparedGeneratedAgentEnvironmentProjection> {
+  if (!AGENT_NAME.test(options.agentName)) {
+    throw new AgentEnvBoundaryError('unsafe-agent-name', 'MOSAIC_AGENT_NAME', options.agentName);
+  }
+  await validatePrivateProjectionDirectory(options.mosaicHome, options.agentEnvDir);
+  const generatedPath = join(options.agentEnvDir, `${options.agentName}.env.generated`);
+  const generated = renderGeneratedAgentEnvironment(options.generated);
+  await assertPrivateRegularFileIfPresent(generatedPath);
+  return {
+    mosaicHome: options.mosaicHome,
+    agentEnvDir: options.agentEnvDir,
+    generatedPath,
+    generated,
+  };
+}
+
+/**
+ * Applies a generated-only projection: writes ONLY `<name>.env.generated` atomically
+ * and touches nothing else. It never writes `.env.local`/`.env.quarantine` and never
+ * unlinks the legacy `.env` — the projection-only recovery guarantee is structural.
+ */
+export async function applyPreparedGeneratedAgentEnvironmentProjection(
+  prepared: PreparedGeneratedAgentEnvironmentProjection,
+): Promise<string> {
+  await ensurePrivateProjectionDirectory(prepared.mosaicHome, prepared.agentEnvDir);
+  await writePrivateAtomically(prepared.generatedPath, prepared.generated);
+  return prepared.generatedPath;
+}
+
 /**
 * Validates only the exact generated projection eligible for a roster delete.
 * Local overrides, legacy input, and quarantine records are operator-retained and
Author	SHA1	Message	Date
jason.woltje	9ddc6fbda8	feat(fleet): mosaic fleet regen — regenerate roster-derived projections (PR3 of #791 ) (#813 ) All checks were successful ci/woodpecker/push/publish Pipeline was successful Details ci/woodpecker/push/ci Pipeline was successful Details	2026-07-17 03:17:55 +00:00
jason.woltje	31607a4af6	feat(mosaic): durable pre-update snapshot + verify net + restore CLI (#791 PR2) (#811 ) All checks were successful ci/woodpecker/push/publish Pipeline was successful Details ci/woodpecker/push/ci Pipeline was successful Details	2026-07-17 00:43:18 +00:00