stack/docs/compaction-refresh/reviews/GATE0-PROBE3-MOS-COATTEST-v-final.md

# GATE0 Probe-3 (#827) — Mos byte-scope-verify CO-ATTESTATION (v-final)

**Principal:** Mos (orchestrator, merge authority for the mosaic-stack governance lane).
**Committed under a DISTINCT git identity** (`mos-orchestrator@mosaic.local`) — deliberately NOT the
`ms-lead-reviewer@mosaic.local` lane signer — so this record stands as a *distinct-identity*
co-attestation, not a same-signer duplicate. See "Independence" below.

**Verify class:** independent provider-byte read (guarded `git show <full-40>:path | sha256sum` from a
read-only clone of `mosaicstack/stack`). Not a re-build, not a re-run — a byte/scope/hygiene audit of
the exact committed objects on the provider branch.

## Package under attestation

| Artifact | Ref |
|---|---|
| Branch | `feat/827-gate0-probe` |
| Harness commit-40 | `2d54a9dd14cb924701b2ae4ed72dae4df760c4e3` |
| Harness `p3_d4_focused_run.py` sha256 | `15a154df55273f51301763a984485fd63813f6d1f05d2728abb9fb8b9c040b1a` (27366 B) |
| §3-review-v6 commit-40 | `23c0caca9b5d44002e6184cd7f2b6c837e8795b2` |
| Review path | `docs/compaction-refresh/reviews/GATE0-PROBE3-NEW56-S3-REVIEW-v6.md` |

sha256 re-confirmed against the checked-out object at `HEAD:docs/compaction-refresh/probes/p3_d4_focused_run.py`
(git object id `8c68cd07…`) — matches the relayed value byte-for-byte.

## Findings — VERDICT: byte-scope + mechanism + hygiene **PASS**

**Anchors.** Harness sha256 matches (27366 B). review-v6 (`23c0caca`) parent == harness commit
`2d54a9dd`; review touches only the review `.md` (+82 lines, 1 file). Broker
(`p3_generation_broker.py`) delta vs `48484938…` = **exactly** the 16-line `action=="source-invalid"`
handler purge, byte-stable otherwise.

**NEW-6 (GATED_WI_ROOT off-by-one) — CLOSED.** `resolve_gated_wi_root()` selects the worktree by
`git worktree list --porcelain` enumeration, requires a UNIQUE match on `HEAD==GATED_WI_HEAD`
(`f400830738998db105107a2a4c69c7f2a2a6fd5d`) AND `branch==refs/heads/feat/830-compaction-revoke`,
then fail-closed re-validates (`is-inside-work-tree==true`, `rev-parse HEAD==GATED_WI_HEAD`);
`RuntimeError` on ambiguity/mismatch. The `HERE.parents[3].parent / "stack-cr-wi3-revoke"` off-by-one
and the hardcoded `/home/hermes/...` literal are **gone** — portable, zero hardcoded path.

**NEW-5 (TOCTOU / pinned-executed-bytes) — CLOSED via approach (i), as mandated.** The `git`-object
sha256 pin (`GATED_LAUNCHER_SHA256 = e950e422…`) is the trust anchor. Ordering/marker `.find()`
heuristics are downgraded to explicitly diagnostic-only ("never a substitute for the pin"). In
`launch_verified_pi()` the executed working-tree file is re-hashed against the pin **in the statement
immediately before `Popen`** (no interleaved yield/IO), and the launcher is executed **in place at the
pinned worktree path** — the higher-risk approach (ii) copy-to-fixture (previously at `7ff63cd5` /
`6164dc07`) is **reverted** (the only remaining `shutil.copy2` is the legitimate credential copy, not a
launcher copy). Residual sub-statement TOCTOU window on a local file inside a non-adversarial operator
fixture is within this probe's threat model; the gross precondition→much-later-exec gap homelab flagged
is closed.

**Hygiene — CLOSED.** `scrub_fixture_credentials(root)` removes the entire `.pi/agent` subtree in a
`finally` block (nested try/finally, after `pi.close()` + broker shutdown, before `return root`) and
`RuntimeError`s if the scrub fails — credentials are removed from retained evidence; logs retained.

**Invariants byte-stable (all INTACT):** assertion `lease_anchor_registered`; file-backed fidelity
checks (`generation_source=="state-file"`, `state_file_in_fixture_root`, `MUTATOR_UNVERIFIED`,
`STALE_GENERATION`); `-O`-safe (0 bare `assert`); allow-list env (0 `os.environ.copy`); single broker;
`--runs choices=(3,)`; ABSENT-sweep (`yolo`/`execRuntime`/`p3_bank`/`promote_p2`/`retry`) = 0.

**R1 mechanism-fidelity boundary — HELD.** D4 exercises #830's real file-backed revocation
(`generation-{sid}.state` / `MOSAIC_LEASE_GENERATION_FILE`, from `66b1e0a0`); `.state` stays inside the
fixture temp root; fixture-socket / child-write-escape / gettempdir isolation preconditions preserved;
launcher exec is in place at the pinned worktree (same surface as R1, not a new production/live
surface). **No path escapes the fixture root.** BEYOND-R1 tripwire NOT tripped.

## Independence

- **Substantive principal-independence** of review-v6 is satisfied by an orchestrator-dispatched,
  builder-distinct Opus SECREV (`ms-secrev-828`, byte-only, non-builder) — that IS the substance of
  Gate-16.
- The shared `ms-lead-reviewer` git signer on harness+review commits is **evidentiary, not
  substantive**. It is resolved by (1) this Mos co-attestation committed under a **distinct** identity
  (`mos-orchestrator`), and (2) a homelab third-principal verify under its own distinct identity —
  i.e. three distinct-identity principals of record.
- The shared signer is a tracked **fleet-infra tooling-gap** (durable fix = per-lane distinct signers),
  **not a blocker**.

## Scope of this record — byte-clear, NOT fire-authorization

Producing probe evidence **executes** the Gate0 mechanism; a byte-clear is not a fire-authorization.
This co-attestation clears the **bytes/scope/hygiene**. FIRE remains gated on: **homelab
third-principal verify** + **Mos transparency-to-Jason** (real-Pi consumes operator model creds inside
the isolated fixture, scrubbed post-run, never emitted) + **Mos explicit FIRE GO**. Until then: nothing
banked, WI-3 #830 held at `f4008307` (unmoved), C-hatch armed (if 3× isolation still no-fire /
wrong-value / isolation-FAIL → possible Case-C → STOP + escalate to Jason).

**Mos verdict: byte-scope + mechanism + hygiene PASS. Co-attestation of record — committed.**