diff --git a/.woodpecker/ci.yml b/.woodpecker/ci.yml
index 561112ac..8132fa8d 100644
--- a/.woodpecker/ci.yml
+++ b/.woodpecker/ci.yml
@@ -113,6 +113,40 @@ steps:
       # stub supplies the scale instead of the host's own checkout.
       - bash packages/mosaic/framework/tools/git/test-mosaic-worktree-large-repo.sh
 
+  # Canonical repo-structure declaration gate (T51 WP5c, spec §5.4 point 2):
+  # .mosaic/repo.json is the machine-readable structure SSOT consumed by git
+  # wrappers and the T32 gate seat; this is its repo-side CI enforcement.
+  # Path-conditional: runs when the declaration, the vendored validator, or this
+  # pipeline config changes (manual runs always include it). Fails the pipeline
+  # on any VALIDATION_ERROR and enforces the schema_version 2 authoring rule
+  # (--require-v2: edited/new declarations may not stay v1). The validator is
+  # vendored into the framework tree (spec §5.1 final home) — provenance in its
+  # header; the hostile-input suite (101 arms, hermetic) runs alongside so the
+  # gate's own instrument ships in the same commit as the gate.
+  structure-declaration:
+    image: *node_image
+    commands:
+      - apk add --no-cache bash git
+      # MOSAIC_HOST_ROOT is a runtime anchor (spec §1.2a: unset fails closed
+      # for managed validation). CI has no host, so the step provisions an
+      # EXPLICIT fixture root — honest configuration for the resolution path,
+      # never a guess about a real host; the per-host containment checks are
+      # runtime concerns and do not run against a fixture. Grammar, schema,
+      # refs, flow, remote normalization, and path grammar all prove here.
+      - mkdir -p /tmp/t51-ci-hostroot
+      - bash packages/mosaic/framework/tools/structure/validate-repo-json.sh .mosaic/repo.json --require-v2
+      - bash packages/mosaic/framework/tools/structure/test-validate-repo-json.sh
+    environment:
+      MOSAIC_HOST_ROOT: /tmp/t51-ci-hostroot
+    when:
+      - event: pull_request
+        path:
+          include:
+            - '.mosaic/repo.json'
+            - 'packages/mosaic/framework/tools/structure/**'
+            - '.woodpecker/ci.yml'
+      - event: manual
+
   # Canonical verify:release stage `upgrade-guard`.
   # Blocking gate (#791): a framework upgrade must never write or delete an
   # operator-owned path. The HARD GATE proves an unanticipated operator sentinel
diff --git a/packages/mosaic/framework/guides/TOOLS-REFERENCE.md b/packages/mosaic/framework/guides/TOOLS-REFERENCE.md
index f2763b75..4e8c427a 100644
--- a/packages/mosaic/framework/guides/TOOLS-REFERENCE.md
+++ b/packages/mosaic/framework/guides/TOOLS-REFERENCE.md
@@ -219,6 +219,23 @@ Multi-instance support: `-a <instance>` selects a named instance (e.g. `personal
 ~/.config/mosaic/tools/health/stack-health.sh -f json
 ```
 
+### Repo Structure Declaration (T51)
+
+```bash
+# Validate a .mosaic/repo.json declaration (schema v1/v2, host:/ grammar,
+# ref grammar, cross-field rules, remote normalization; spec §5)
+~/.config/mosaic/tools/structure/validate-repo-json.sh <repo>/.mosaic/repo.json
+
+# CI authoring rule: new/edited declarations must be schema_version 2
+~/.config/mosaic/tools/structure/validate-repo-json.sh <repo>/.mosaic/repo.json --require-v2
+
+# Display mode (warns and omits root-dependent checks when MOSAIC_HOST_ROOT unset)
+~/.config/mosaic/tools/structure/validate-repo-json.sh <repo>/.mosaic/repo.json --mode display
+
+# Hermetic hostile-input suite (101 arms)
+~/.config/mosaic/tools/structure/test-validate-repo-json.sh
+```
+
 ### Shared Credential Loader
 
 ```bash
diff --git a/packages/mosaic/framework/tools/structure/test-validate-repo-json.sh b/packages/mosaic/framework/tools/structure/test-validate-repo-json.sh
new file mode 100755
index 00000000..c9ef0336
--- /dev/null
+++ b/packages/mosaic/framework/tools/structure/test-validate-repo-json.sh
@@ -0,0 +1,289 @@
+#!/usr/bin/env bash
+# test-validate-repo-json.sh — hostile-input suite for the T51 declaration validator.
+# (Vendored with validate-repo-json.sh from mosaic-brain @ 515bcbab — see the
+# validator header for provenance.)
+#
+# Hermetic: all fixtures in a tracked mktemp sandbox removed by an EXIT trap
+# (pass and fail paths both — zero residue). No network, no real repos, no host
+# state mutated. MOSAIC_HOST_ROOT is set/unset per arm via env only.
+#
+# T51P2RW1: arms extended per review T51P2R1 (F1-F5): root gate for ordinary
+# v2 declarations (unset AND explicitly empty; display warns), git-grammar
+# branch arms (double slash, dot component, control byte), contract-escape
+# arms (list enums, invalid UTF-8, NaN — one VALIDATION_ERROR line, never a
+# traceback), remote normalization (.git/ ordering, port preservation), and
+# mirror-component fullmatch arms (trailing newline, control bytes).
+
+set -u
+
+HERE=$(cd "$(dirname "$0")" && pwd)
+V="$HERE/validate-repo-json.sh"
+
+PASS=0; FAIL=0; FAILED=""
+ok()  { PASS=$((PASS+1)); }
+bad() { FAIL=$((FAIL+1)); FAILED="$FAILED $1"; printf 'FAIL: %s\n' "$1" >&2; }
+
+SB=$(mktemp -d "${TMPDIR:-/tmp}/vrj-test.XXXXXX")
+trap 'rm -rf "$SB"' EXIT
+
+fx() { printf '%s' "$2" > "$SB/$1"; }
+
+run() { # [env KV=V ...] -- args...
+  local envs=()
+  while [ "$1" != "--" ]; do envs+=("$1"); shift; done; shift
+  OUT=$(env "${envs[@]:-_=_}" bash "$V" "$@" 2>&1 < /dev/null; echo "__RC__$?")
+  RC=${OUT##*__RC__}; OUT=${OUT%__RC__*}; OUT=${OUT%$'\n'}
+}
+expect_ok()   { local d="$1"; shift; run "$@"; if [ "$RC" = 0 ] && printf '%s' "$OUT" | grep -q '^OK'; then ok; else bad "$d (rc=$RC out=$(printf '%s' "$OUT" | head -1))"; fi; }
+expect_err() { # desc expected-substring [env... -- args...]
+  local d="$1" sub="$2"; shift 2
+  run "$@"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "VALIDATION_ERROR.*$sub"; then ok
+  else bad "$d (rc=$RC, wanted error ~$sub, got: ${OUT%%$'\n'*})"; fi
+}
+expect_err_notrace() { # like expect_err, plus no traceback anywhere in output
+  local d="$1" sub="$2"; shift 2
+  run "$@"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "VALIDATION_ERROR.*$sub" && ! printf '%s' "$OUT" | grep -q "Traceback"; then ok
+  else bad "$d (rc=$RC, wanted clean error ~$sub, got: ${OUT%%$'\n'*})"; fi
+}
+
+STACK='{"schema_version":2,"integration_trunk":"next","release_branch":"main","flow":"trunk-release","canonical_remote":"https://git.mosaicstack.dev/mosaicstack/stack","canonical_clone":"host:/src/mosaic-stack","worktree_root":"host:/src/mosaic-stack-worktrees","worktree_policy":"orchestrator-precreated","notes":"x"}'
+BRAIN='{"schema_version":2,"integration_trunk":"main","release_branch":"main","flow":"direct","canonical_remote":"https://git.example.invalid/acme/brain","canonical_clone":"host:/.mosaic","worktree_root":"host:/.mosaic-worktrees","worktree_policy":"orchestrator-precreated"}'
+ROOT="$SB/hostroot"; mkdir -p "$ROOT"
+
+echo "== (0) syntax + version =="
+bash -n "$V" && ok || bad "bash -n"
+run -- --version; [ "$RC" = 0 ] && case "$OUT" in validate-repo-json\ *) ok ;; *) bad "version output" ;; esac || bad "version rc"
+
+echo "== (1) spec examples: stack + brain OK (root set) =="
+fx stack.json "$STACK"; fx brain.json "$BRAIN"
+expect_ok a1 MOSAIC_HOST_ROOT=$ROOT -- "$SB/stack.json"
+expect_ok a2 MOSAIC_HOST_ROOT=$ROOT -- "$SB/brain.json"
+
+echo "== (2) malformed JSON (stable contract, no traceback) =="
+fx bad.json '{"schema_version": 2, '
+expect_err_notrace b1 "json:" -- "$SB/bad.json"
+fx arr.json '[1,2]'
+expect_err_notrace b2 "top level" -- "$SB/arr.json"
+printf '\xff\xfe{"schema_version":2}' > "$SB/utf8.json"
+expect_err_notrace b3 "UTF-8" MOSAIC_HOST_ROOT=$ROOT -- "$SB/utf8.json"
+fx nan.json '{"schema_version":NaN}'
+expect_err_notrace b4 "malformed JSON" MOSAIC_HOST_ROOT=$ROOT -- "$SB/nan.json"
+
+echo "== (3) unknown schema_version = ABSENT-loud =="
+fx v3.json "${STACK/schema_version\":2/schema_version\":3}"
+expect_err c1 "schema_version" MOSAIC_HOST_ROOT=$ROOT -- "$SB/v3.json"
+
+echo "== (4) v1 mode + authoring rule (v1 consumes no paths: no root needed) =="
+fx v1.json '{"integration_trunk":"next","release_branch":"main"}'
+expect_ok  d1 -- "$SB/v1.json"
+expect_err d2 "schema_version" -- --require-v2 "$SB/v1.json"
+fx v1x.json '{"integration_trunk":"next","release_branch":"main","notes":"no"}'
+expect_err d3 "x_extensions" -- "$SB/v1x.json"
+
+echo "== (5) unknown top-level key rejected; x_extensions home OK =="
+fx unk.json "${STACK%\}*},\"typo_key\":1}"
+expect_err e1 "typo_key" MOSAIC_HOST_ROOT=$ROOT -- "$SB/unk.json"
+fx ext.json "${STACK%\}*},\"x_extensions\":{\"future\":true}}"
+expect_ok e2 MOSAIC_HOST_ROOT=$ROOT -- "$SB/ext.json"
+
+echo "== (6) flow: required (no defaulting) + cross-field =="
+fx noflow.json "$(printf '%s' "$STACK" | python3 -c 'import json,sys; d=json.load(sys.stdin); del d["flow"]; print(json.dumps(d))')"
+expect_err f1 "flow" MOSAIC_HOST_ROOT=$ROOT -- "$SB/noflow.json"
+fx xdirect.json "${STACK/\"trunk-release\"/\"direct\"}"
+expect_err f2 "direct" MOSAIC_HOST_ROOT=$ROOT -- "$SB/xdirect.json"
+fx xtr.json "${BRAIN/\"direct\"/\"trunk-release\"}"
+expect_err f3 "trunk-release" MOSAIC_HOST_ROOT=$ROOT -- "$SB/xtr.json"
+
+echo "== (7) dot-segment / empty-segment / tilde escapes =="
+fx dots.json "${STACK/host:\/src\/mosaic-stack\"/host:/src/../secrets\"}"
+expect_err g1 "dot segment" MOSAIC_HOST_ROOT=$ROOT -- "$SB/dots.json"
+fx dot1.json "${STACK/host:\/src\/mosaic-stack\"/host:/src/./mosaic-stack\"}"
+expect_err g2 "dot segment" MOSAIC_HOST_ROOT=$ROOT -- "$SB/dot1.json"
+fx empty.json "${STACK/host:\/src\/mosaic-stack\"/host://src/mosaic-stack\"}"
+expect_err g3 "empty segment" MOSAIC_HOST_ROOT=$ROOT -- "$SB/empty.json"
+fx tild.json "${STACK/host:\/src\/mosaic-stack\"/~jw/src/mosaic-stack\"}"
+expect_err g4 "tilde" MOSAIC_HOST_ROOT=$ROOT -- "$SB/tild.json"
+fx tailslash.json "${STACK/host:\/src\/mosaic-stack\"/host:/src/mosaic-stack/\"}"
+expect_err g5 "empty segment" MOSAIC_HOST_ROOT=$ROOT -- "$SB/tailslash.json"
+fx noanchor.json "${STACK/host:\/src\/mosaic-stack\"//src/mosaic-stack\"}"
+expect_err g6 "host:/" MOSAIC_HOST_ROOT=$ROOT -- "$SB/noanchor.json"
+
+echo "== (8) branch-name grammar (delegated to git check-ref-format, F2) =="
+fx badbr.json "${STACK/\"next\"/\"bad..name\"}"
+expect_err h1 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/badbr.json"
+fx sp.json "${STACK/\"next\"/\"fea ture\"}"
+expect_err h2 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/sp.json"
+fx lock.json "${STACK/\"next\"/\"feature/x.lock\"}"
+expect_err h3 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/lock.json"
+fx slash.json "${STACK/\"next\"/\"feature/x\"}"
+expect_ok h4 MOSAIC_HOST_ROOT=$ROOT -- "$SB/slash.json"
+fx dslash.json "${STACK/\"next\"/\"feature//x\"}"
+expect_err h5 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/dslash.json"
+fx hidden.json "${STACK/\"next\"/\"feature/.hidden\"}"
+expect_err h6 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/hidden.json"
+fx ctrl.json "$(printf '%s' "$STACK" | python3 -c 'import json,sys; d=json.load(sys.stdin); d["integration_trunk"]="feature/\x01x"; print(json.dumps(d))')"
+expect_err h7 "branch name" MOSAIC_HOST_ROOT=$ROOT -- "$SB/ctrl.json"
+
+echo "== (8b) reflog shorthand rejected independent of ambient checkout history (B1) =="
+# Hermetic repo WITH checkout history: proves '@{-1}' (which git would expand to
+# 'main' from THIS repo's reflog) is still refused by the pre-delegation gate.
+HISTREPO="$SB/histrepo"; mkdir -p "$HISTREPO"
+(cd "$HISTREPO" && git init -q -b main . \
+  && git -c user.name=t -c user.email=t@t commit -q --allow-empty -m m \
+  && git checkout -q -b feature/x \
+  && git checkout -q main \
+  && git check-ref-format --branch "@{-1}" >/dev/null 2>&1 && echo "ambient-expandable" || echo "not-expandable") \
+  | grep -q ambient-expandable && ok || bad "fixture repo failed to make @{-1} expandable"
+fx atminus1.json "$(printf '%s' "$STACK" | python3 -c 'import json,sys; d=json.load(sys.stdin); d["integration_trunk"]="@{-1}"; print(json.dumps(d))')"
+# Run the validator from INSIDE the history repo via command substitution so the
+# assertion runs in the PARENT shell (T51P2R3 B1: the previous ( subshell ) form
+# mutated ok/bad counters only in a dead subshell — FAIL printed, suite rc 0).
+OUTX=$(cd "$HISTREPO" && MOSAIC_HOST_ROOT=$ROOT bash "$V" "$SB/atminus1.json" 2>&1 </dev/null; echo "__RC__$?")
+RCX=${OUTX##*__RC__}
+if [ "$RCX" = 1 ] && printf '%s' "$OUTX" | grep -q "VALIDATION_ERROR.*@{"; then ok
+else bad "@{-1} must be rejected inside a repo with checkout history (got rc=$RCX)"; fi
+fx atbrace.json "$(printf '%s' "$STACK" | python3 -c 'import json,sys; d=json.load(sys.stdin); d["integration_trunk"]="@{u}"; print(json.dumps(d))')"
+expect_err h9 "@{" MOSAIC_HOST_ROOT=$ROOT -- "$SB/atbrace.json"
+
+echo "== (9) canonical_remote: userinfo, list-type, normalization (F3/F4) =="
+fx user.json "${STACK/https:\/\/git.mosaicstack.dev/https:\/\/bot:s3cret@git.mosaicstack.dev}"
+expect_err_notrace i1 "userinfo" MOSAIC_HOST_ROOT=$ROOT -- "$SB/user.json"
+fx listflow.json "${STACK/\"trunk-release\"/[\"trunk-release\"]}"
+expect_err_notrace i2 "flow" MOSAIC_HOST_ROOT=$ROOT -- "$SB/listflow.json"
+fx listpol.json "${STACK/\"orchestrator-precreated\"/[\"tool-managed\"]}"
+expect_err_notrace i3 "worktree_policy" MOSAIC_HOST_ROOT=$ROOT -- "$SB/listpol.json"
+run -- --normalize-remote "HTTPS://Git.Example.Invalid/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.example.invalid/o/r" ] && ok || bad "norm .git/case ($OUT)"
+run -- --normalize-remote "https://git.mosaicstack.dev/mosaicstack/stack/"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.mosaicstack.dev/mosaicstack/stack" ] && ok || bad "norm trailing slash ($OUT)"
+run -- --normalize-remote "git.mosaicstack.dev/mosaicstack/stack"
+[ "$RC" = 1 ] && ok || bad "schemeless must fail"
+run -- --normalize-remote "HTTPS://Git.Example.Invalid/o/r.git/"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.example.invalid/o/r" ] && ok || bad "norm .git-then-slash ($OUT)"
+run -- --normalize-remote "https://Git.Example.Invalid:8443/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.example.invalid:8443/o/r" ] && ok || bad "port must be preserved ($OUT)"
+run -- --normalize-remote "https://[2001:db8::1]:8443/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://[2001:db8::1]:8443/o/r" ] && ok || bad "IPv6 must stay bracketed with port ($OUT)"
+run -- --normalize-remote "https://[2001:db8::1]/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://[2001:db8::1]/o/r" ] && ok || bad "IPv6 must stay bracketed ($OUT)"
+run -- --normalize-remote "https://Git.Example.Invalid:0/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.example.invalid:0/o/r" ] && ok || bad "explicit port 0 must be preserved ($OUT)"
+run -- --normalize-remote "https://[::1].evil.example/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "suffix after ] must be rejected (.evil.example)"
+run -- --normalize-remote "https://[::1]x:8443/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "suffix after ] must be rejected (x:8443)"
+run -- --normalize-remote "https://[::1]x/o/r.git"
+[ "$RC" = 1 ] && ok || bad "suffix after ] must be rejected (x)"
+echo "== (9b) IPvFuture bracketed authorities (R4-B1: guard keys off raw netloc) =="
+run -- --normalize-remote "https://[v1.fe80]/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://[v1.fe80]/o/r" ] && ok || bad "valid IPvFuture must keep brackets ($OUT)"
+run -- --normalize-remote "https://[vF.foo]:8443/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://[vf.foo]:8443/o/r" ] && ok || bad "valid IPvFuture+port must keep brackets ($OUT)"
+run -- --normalize-remote "https://[v1.fe80]evil/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "IPvFuture suffix must be rejected (evil)"
+run -- --normalize-remote "https://[v1.fe80].evil.example/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "IPvFuture suffix must be rejected (.evil.example)"
+run -- --normalize-remote "https://[vF.foo]x:8443/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "IPvFuture suffix must be rejected (x:8443)"
+echo "== (9d) non-bracketed authority grammar (R6-B1) =="
+for U in "https://:8443/o/r.git" "https://bad host/o/r.git" "https://bad^host/o/r.git" "https://bad\\host/o/r.git" "https://bad%zz/o/r.git" "https://bad%2/o/r.git" "https://bad%/o/r.git"; do
+  run -- --normalize-remote "$U"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "VALIDATION_ERROR canonical_remote"; then ok
+  else bad "non-bracketed authority must be rejected: $U (rc=$RC out=$OUT)"; fi
+done
+run -- --normalize-remote "https://git.mosaicstack.dev:9000/mosaicstack/stack"
+[ "$RC" = 0 ] && [ "$OUT" = "https://git.mosaicstack.dev:9000/mosaicstack/stack" ] && ok || bad "valid host:port unchanged ($OUT)"
+run -- --normalize-remote "https://192.168.1.10:8443/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://192.168.1.10:8443/o/r" ] && ok || bad "IPv4 reg-name stays valid ($OUT)"
+run -- --normalize-remote "https://bad%2Fx/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://bad%2fx/o/r" ] && ok || bad "complete %HH must stay legal, case-normalized ($OUT)"
+echo "== (9e) ASCII-only authority bytes (R7-B1) =="
+# isolated port arm (R8): VALID ASCII host + full-width-digit port ONLY —
+# unconfounded, so restoring Unicode-aware isdigit() goes red right here.
+run -- --normalize-remote "https://git.example.invalid:４４３/o/r.git"
+if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "ASCII digits only"; then ok
+else bad "full-width-digit port on a VALID host must be rejected with the port reason (rc=$RC out=$OUT)"; fi
+for U in "https://éxample.invalid/o/r.git" "https://例え.テスト/o/r.git" "https://full－width.invalid/o/r.git" "https://mosaic－stack.dev:４４３/o/r.git"; do
+  run -- --normalize-remote "$U"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "VALIDATION_ERROR canonical_remote"; then ok
+  else bad "non-ASCII authority must be rejected: $U (rc=$RC out=$OUT)"; fi
+done
+run -- --normalize-remote "https://xn--xample-9ua.invalid/o/r.git"
+[ "$RC" = 0 ] && [ "$OUT" = "https://xn--xample-9ua.invalid/o/r" ] && ok || bad "punycode xn-- host must stay legal ($OUT)"
+echo "== (9c) bracket-payload grammar + raw control bytes (R5-B1) =="
+for P in "v1. " "v1.a b" "v1.a^b" "v1.a\\b" "v1.%20" "not-an-ip" "::gg::1"; do
+  run -- --normalize-remote "https://[$P]/o/r.git"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "VALIDATION_ERROR canonical_remote"; then ok
+  else bad "bracket payload [$P] must be rejected (rc=$RC out=$OUT)"; fi
+done
+for CB in $'\t' $'\n' $'\r'; do
+  run -- --normalize-remote "https://[v1.a${CB}b]/o/r.git"
+  if [ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "control byte"; then ok
+  else bad "raw control byte must be rejected before urlsplit (rc=$RC out=$OUT)"; fi
+done
+run -- --normalize-remote "https://[v1.fe80%zone]/o/r.git"
+[ "$RC" = 1 ] && ok || bad "percent (not in RFC host grammar) must be rejected ($OUT)"
+run -- --normalize-remote "https://[fe80::1%eth0]/o/r.git"
+[ "$RC" = 1 ] && printf '%s' "$OUT" | grep -q "canonical_remote" && ok || bad "IPv6 zone-id (not RFC host grammar) must be rejected ($OUT)"
+
+echo "== (10) root gate: every v2 managed validation fails closed (F1) =="
+# NOTE (spec §4.5): host:/ paths resolve UNDER MOSAIC_HOST_ROOT by construction,
+# so tool-managed is not declarable today — the cross-check fails for every
+# host:/ root until the anchor scheme grows an outside-root form (J3 era).
+expect_err j1 "MOSAIC_HOST_ROOT" MOSAIC_HOST_ROOT= -- "$SB/stack.json"
+expect_err j2 "MOSAIC_HOST_ROOT" MOSAIC_HOST_ROOT= -- "$SB/brain.json"
+expect_err j3 "MOSAIC_HOST_ROOT" MOSAIC_HOST_ROOT= -- "$SB/stack.json"
+# rider (T51P2R2): genuine ABSENCE, not just explicitly empty — captured via
+# command substitution, asserted in the parent shell (no subshell-counter shape).
+OUTU=$(cd "$SB" && env -u MOSAIC_HOST_ROOT bash "$V" "$SB/stack.json" 2>&1 </dev/null; echo "__RC__$?")
+RCU=${OUTU##*__RC__}
+if [ "$RCU" = 1 ] && printf '%s' "$OUTU" | grep -q "VALIDATION_ERROR.*MOSAIC_HOST_ROOT"; then ok
+else bad "unset-by-absence root must fail closed in managed mode (rc=$RCU)"; fi
+run MOSAIC_HOST_ROOT= -- --mode display "$SB/stack.json"
+if [ "$RC" = 0 ] && printf '%s' "$OUT" | grep -q '^OK' && printf '%s' "$OUT" | grep -q "host root unset"; then ok
+else bad "display-mode unset must pass with the specified warning (rc=$RC)"; fi
+run MOSAIC_HOST_ROOT= -- --mode display "$SB/brain.json"
+if [ "$RC" = 0 ] && printf '%s' "$OUT" | grep -q "host root unset"; then ok
+else bad "display-mode warn missing for brain fixture"; fi
+TM='{"schema_version":2,"integration_trunk":"next","release_branch":"main","flow":"trunk-release","canonical_remote":"https://git.mosaicstack.dev/mosaicstack/stack","canonical_clone":"host:/src/mosaic-stack","worktree_root":"host:/src/mosaic-stack-worktrees","worktree_policy":"tool-managed"}'
+fx tm.json "$TM"; mkdir -p "$ROOT/src"
+expect_err j4 "inside MOSAIC_HOST_ROOT" MOSAIC_HOST_ROOT=$ROOT -- "$SB/tm.json"
+fx tm_noroot.json "$(printf '%s' "$TM" | python3 -c 'import json,sys; d=json.load(sys.stdin); del d["worktree_root"]; print(json.dumps(d))')"
+expect_err j5 "worktree_root" MOSAIC_HOST_ROOT=$ROOT -- "$SB/tm_noroot.json"
+echo "== (10b) symlink escape cannot fake outside-ness (B3 fix: lexical containment) =="
+OUTSIDE="$SB/outside-target"; mkdir -p "$OUTSIDE"
+ln -s "$OUTSIDE" "$ROOT/escape"
+TM_ESC="${TM/host:\/src\/mosaic-stack-worktrees/host:/escape/worktrees}"
+fx tmsym.json "$TM_ESC"
+expect_err j6 "inside MOSAIC_HOST_ROOT" MOSAIC_HOST_ROOT=$ROOT -- "$SB/tmsym.json"
+
+echo "== (11) mirror-path components: collision/delimiter/control fixtures (F5) =="
+run -- --mirror-path git.mosaicstack.dev mosaicstack stack
+[ "$RC" = 0 ] && [ "$OUT" = "projects/git.mosaicstack.dev/mosaicstack/stack/repo.json" ] && ok || bad "mirror path ok ($OUT)"
+expect_err k1 "mirror-component" -- --mirror-path "git.mosaicstack.dev" "a__b" "c"
+expect_err k2 "mirror-component" -- --mirror-path "git.mosaicstack.dev" "a" "b__c"
+expect_err k3 "mirror-component" -- --mirror-path "git.mosaicstack.dev/x" "a" "b"
+expect_err k4 "mirror-component" -- --mirror-path "git.mosaicstack.dev" "A" "B"
+expect_err k5 "mirror-component" -- --mirror-path "git.mosaicstack.dev" "" "stack"
+run -- --mirror-path git.mosaicstack.dev a b.c
+[ "$RC" = 0 ] && [ "$OUT" = "projects/git.mosaicstack.dev/a/b.c/repo.json" ] && ok || bad "distinct path ($OUT)"
+run -- --mirror-path $'git.example.invalid\n' owner repo
+[ "$RC" = 1 ] && ok || bad "trailing-newline host must be rejected (fullmatch)"
+run -- --mirror-path $'git.\texample' owner repo
+[ "$RC" = 1 ] && ok || bad "control-byte host must be rejected"
+
+echo "== (12) missing required keys =="
+for key in release_branch canonical_clone; do
+  fx miss.json "$(printf '%s' "$STACK" | python3 -c "import json,sys; d=json.load(sys.stdin); del d['$key']; print(json.dumps(d))")"
+  expect_err "l-$key" "$key" MOSAIC_HOST_ROOT=$ROOT -- "$SB/miss.json"
+done
+
+echo "== (13) absent file =="
+expect_err m1 "file" -- "$SB/nonexistent.json"
+
+echo
+echo "pass=$PASS fail=$FAIL"
+if [ "$FAIL" -gt 0 ]; then echo "FAILED:$FAILED"; exit 1; fi
+echo "ALL GREEN"
diff --git a/packages/mosaic/framework/tools/structure/validate-repo-json.sh b/packages/mosaic/framework/tools/structure/validate-repo-json.sh
new file mode 100755
index 00000000..eeaa871c
--- /dev/null
+++ b/packages/mosaic/framework/tools/structure/validate-repo-json.sh
@@ -0,0 +1,386 @@
+#!/usr/bin/env bash
+# validate-repo-json.sh — declaration validator for T51 repo structure declarations.
+#
+# Spec of record: docs/plans/2026-08-23_repo-structure-declaration.md @ 1896adc1
+# (R3). Implements the spec's validation surface: schema v1/v2 (§1.2), host:/
+# path grammar with canonical segment normalization — empty/./.. rejected
+# BEFORE resolution — and tilde rejection (§1.2a), mirror path component
+# validation (§3.1), cross-field rules (§5.2), remote normalization (§5.3).
+#
+# PROVENANCE (T51 WP5c vendoring): ported verbatim from the mosaic-brain tree —
+# tools/repo-structure-decl/validate-repo-json.sh @ brain main merge 515bcbab
+# (PR 28, wave-1 R9 PASS, 101-arm suite green). This file is now the framework
+# home per spec §5.1 ("shipped in the framework package"); the brain copy is the
+# development origin. Re-sync rule: changes land here via reviewed PR and are
+# back-ported to the brain tree (or the brain copy retires) — never fork silently.
+# Validator version at port: 1.1.0+t51spec-r3+t51p2rw1 (R2-R8 rework included).
+# No operator literal appears in this file; the host root is read from
+# MOSAIC_HOST_ROOT configuration only.
+#
+# Unset-root semantics (spec §1.2a, fail-closed; T51P2R1 F1): v2 declarations
+# always consume a path (canonical_clone is required), so in --mode managed an
+# unset OR EMPTY MOSAIC_HOST_ROOT is a VALIDATION_ERROR for every v2 file —
+# not only tool-managed. In --mode display it warns and omits root-dependent
+# resolution; grammar checks still run.
+#
+# Error contract (T51P2R1 F3): every malformed input — bad UTF-8, non-RFC JSON
+# constants (NaN/Infinity), wrong-typed enums, anything unexpected — yields
+# exactly one stable VALIDATION_ERROR line and exit 1. No traceback ever
+# escapes. Branch names are validated by delegating to `git check-ref-format
+# --branch` (F2), translated into this contract.
+#
+# Usage:
+#   validate-repo-json.sh <repo.json> [--mode managed|display] [--require-v2]
+#   validate-repo-json.sh --mirror-path <host> <owner> <repo>   # §3.1 component check
+#   validate-repo-json.sh --normalize-remote <url>              # §5.3, prints normalized
+#   validate-repo-json.sh --version
+#
+# Output: OK (exit 0) | VALIDATION_ERROR <key>: <reason> (exit 1) | warnings on stderr.
+set -euo pipefail
+
+VERSION="1.1.0+t51spec-r3+t51p2rw1"
+
+if [ "${1:-}" = "--version" ]; then echo "validate-repo-json $VERSION"; exit 0; fi
+
+exec python3 - "$@" <<'PYEOF'
+import json, os, re, subprocess, sys, urllib.parse
+
+def err(key, reason):
+    print(f"VALIDATION_ERROR {key}: {reason}")
+    sys.exit(1)
+
+def warn(msg):
+    print(f"warning: {msg}", file=sys.stderr)
+
+def main():
+    ARGS = sys.argv[1:]
+    MODE = "managed"
+    REQUIRE_V2 = False
+
+    # ---- subcommands first (they take no file argument) ----
+    def check_mirror_components(host, owner, repo):
+        # fullmatch: '$' must bind at true end (F5 — a trailing newline must
+        # NOT pass); the charset excludes control bytes outright.
+        comp_re = re.compile(r"[a-z0-9][a-z0-9.-]*")
+        for label, value in (("host", host), ("owner", owner), ("repo", repo)):
+            if not isinstance(value, str) or not value or not comp_re.fullmatch(value):
+                err("mirror-component", f"{label} {value!r} fails §3.1 charset ^[a-z0-9][a-z0-9.-]*$ (fullmatch, no '/', no delimiter, no control bytes)")
+        return f"projects/{host}/{owner}/{repo}/repo.json"
+
+    def normalize_remote(url):
+        # R5-B1 part 1: reject raw control bytes BEFORE urlsplit — urlsplit
+        # silently strips TAB/LF/CR, so different input bytes would normalize
+        # to a different host. The raw bytes ARE the input; nothing may rewrite them.
+        for ch in url:
+            if ord(ch) < 0x20 or ord(ch) == 0x7F:
+                err("canonical_remote", "control byte in URL rejected before parsing (urlsplit would strip it and change the host)")
+        try:
+            p = urllib.parse.urlsplit(url)
+        except ValueError as e:
+            # py3.12 urlsplit itself validates bracketed hosts (ipaddress) and
+            # raises for garbage authorities — translate, never traceback.
+            err("canonical_remote", f"invalid URL authority: {e}")
+        if not p.scheme or not p.netloc:
+            err("canonical_remote", f"not a URL with scheme+host: {url!r}")
+        if p.username or p.password or "@" in (p.netloc or ""):
+            err("canonical_remote", "userinfo in URL is rejected (§5.3)")
+        scheme = p.scheme.lower()
+        # T51P2R4 B1: bracketing is detected from the RAW netloc ('[' prefix),
+        # not from a ':' in the parsed hostname — IPvFuture literals ([v1.fe80])
+        # contain no colon and must not bypass the raw-authority proof.
+        bracketed = p.netloc.startswith("[")
+        if bracketed:
+            # Prove the RAW authority is exactly '[host]' + optional ':port'
+            # (case-normalized); any text after ']' is hostile/truncated input,
+            # rejected — never silently rewritten.
+            import re as _re
+            import ipaddress as _ip
+            m = _re.fullmatch(r"\[([^\]]*)\](?::([0-9]+))?", p.netloc)
+            if not m:
+                err("canonical_remote", f"malformed bracketed authority {p.netloc!r}: text after ']' is rejected (no silent truncation)")
+            payload = m.group(1)
+            # R5-B1 part 2: the bracket payload must be a REAL RFC literal —
+            # an IPv6 address (ipaddress parse) or an IPvFuture literal
+            # ("v" + HEXDIG+ + "." + unreserved / sub-delims / ":" only).
+            # Anything else inside brackets is rejected, closing the payload
+            # grammar as a class.
+            if _re.fullmatch(r"v[0-9A-Fa-f]+\.[A-Za-z0-9._~!$&'()*+,;=:-]*", payload):
+                pass  # IPvFuture (case-normalized below)
+            elif _re.fullmatch(r"[0-9A-Fa-f:.]+", payload):
+                # strict IPv6 lexical form (hex/colon/dot only — ipaddress alone
+                # would also accept scoped zone-ids like fe80::1%eth0, which are
+                # not valid URI host grammar unless %25-encoded)
+                try:
+                    _ip.IPv6Address(payload)
+                except ValueError:
+                    err("canonical_remote",
+                        f"bracket payload {payload!r} is not a valid IPv6 address")
+            else:
+                err("canonical_remote",
+                    f"bracket payload {payload!r} is neither a valid IPv6 address nor an IPvFuture literal (v+HEXDIG+.+unreserved/sub-delims/colon)")
+            host = f"[{payload.lower()}]"  # brackets preserved (IPv6 + IPvFuture)
+        else:
+            # R6-B1: the non-bracketed branch — urlsplit PARSES but does not
+            # VALIDATE reg-name, and netloc-nonempty is not host presence.
+            # Split the raw authority ourselves (host[:port]) and validate the
+            # raw host against real grammar: unreserved / sub-delims / complete
+            # %HH octets (reg-name), or IPv4 dotted-quad (reg-name's numeric
+            # case). Port must be all digits. No branch trusts urlsplit alone.
+            import re as _re
+            raw_host, sep, raw_port = p.netloc.rpartition(":")
+            if sep and _re.fullmatch(r"[0-9]+", raw_port):
+                pass  # host:port split
+            elif sep:
+                err("canonical_remote", f"invalid port {raw_port!r} in authority {p.netloc!r} (ASCII digits only)")
+            else:
+                raw_host, raw_port = p.netloc, None
+            if not raw_host:
+                err("canonical_remote", f"empty host in authority {p.netloc!r}")
+            # strict reg-name / IPv4 scan: unreserved + sub-delims, with '%'
+            # only inside complete %HH octets (IPv4 dotted-quad is a subset of
+            # this charset — digits and dots — so one scan covers both).
+            import re as _re
+            i = 0
+            ok_host = True
+            while i < len(raw_host):
+                c = raw_host[i]
+                if c == "%":
+                    if i + 2 >= len(raw_host) or not _re.fullmatch(r"[0-9A-Fa-f]{2}", raw_host[i+1:i+3]):
+                        ok_host = False; break
+                    i += 3
+                elif c in "!$&'()*+,;=-._~" or ("a" <= c <= "z") or ("A" <= c <= "Z") or ("0" <= c <= "9"):
+                    # R7-B1: EXPLICIT ASCII only — str.isalnum() is Unicode-aware
+                    # and admits non-ASCII letters/digits (é, full-width ０). Policy
+                    # is ASCII-only reg-name; punycode xn-- is the sanctioned
+                    # Unicode spelling and remains legal under this charset.
+                    i += 1
+                else:
+                    ok_host = False; break
+            if not ok_host:
+                err("canonical_remote", f"host {raw_host!r} is not valid reg-name/IPv4 grammar (unreserved/sub-delims/complete %HH only)")
+            host = raw_host.lower()
+        port = p.port  # None when absent; preserved whenever explicitly present (B2: incl. 0)
+        authority = host + (f":{port}" if port is not None else "")
+        path = p.path or "/"
+        # canonical trailing-slash + .git strip as ONE operation (F4): slash
+        # first, then .git, then any slash exposed by that strip.
+        path = path.rstrip("/")
+        if path.endswith(".git"):
+            path = path[:-4].rstrip("/")
+        return f"{scheme}://{authority}{path or ''}"
+
+    if "--mirror-path" in ARGS:
+        idx = ARGS.index("--mirror-path")
+        parts = ARGS[idx + 1:]
+        if len(parts) != 3:
+            err("usage", "--mirror-path takes <host> <owner> <repo>")
+        print(check_mirror_components(*parts))
+        sys.exit(0)
+    if "--normalize-remote" in ARGS:
+        idx = ARGS.index("--normalize-remote")
+        vals = ARGS[idx + 1:]
+        if len(vals) != 1:
+            err("usage", "--normalize-remote takes <url>")
+        print(normalize_remote(vals[0]))
+        sys.exit(0)
+
+    # ---- arg parsing ----
+    if not ARGS:
+        err("usage", "a repo.json path is required")
+    path = None
+    i = 0
+    while i < len(ARGS):
+        a = ARGS[i]
+        if a == "--mode":
+            i += 1
+            if i >= len(ARGS) or ARGS[i] not in ("managed", "display"):
+                err("usage", "--mode takes managed|display")
+            MODE = ARGS[i]
+        elif a == "--require-v2":
+            REQUIRE_V2 = True
+        elif a.startswith("--"):
+            err("usage", f"unknown option {a}")
+        else:
+            if path is not None:
+                err("usage", "multiple file arguments")
+            path = a
+        i += 1
+    if path is None:
+        err("usage", "a repo.json path is required")
+
+    # ---- load: strict UTF-8, strict RFC JSON (F3) ----
+    try:
+        with open(path, "rb") as fh:
+            raw_bytes = fh.read()
+    except OSError as e:
+        err("file", str(e))
+    try:
+        raw = raw_bytes.decode("utf-8")
+    except UnicodeDecodeError as e:
+        err("json", f"invalid UTF-8: {e}")
+    def _reject_constant(name):
+        raise ValueError(f"non-RFC JSON constant {name}")
+    try:
+        doc = json.loads(raw, parse_constant=_reject_constant)
+    except (json.JSONDecodeError, ValueError) as e:
+        err("json", f"malformed JSON: {e}")
+    if not isinstance(doc, dict):
+        err("json", "top level must be an object")
+
+    HOST_ROOT = os.environ.get("MOSAIC_HOST_ROOT", "")
+
+    V1_KEYS = {"integration_trunk", "release_branch"}
+    V2_REQUIRED = ["schema_version", "integration_trunk", "release_branch", "flow",
+                   "canonical_remote", "canonical_clone"]
+    V2_OPTIONAL = {"worktree_root", "worktree_policy", "notes", "x_extensions"}
+    ENUM_FLOW = {"direct", "trunk-release"}
+    ENUM_POLICY = {"tool-managed", "orchestrator-precreated"}
+
+    def check_branch(key, value):
+        # Delegate the full git branch grammar to git itself (F2). B1: reject
+        # reflog shorthand BEFORE delegation — `git check-ref-format --branch
+        # '@{-n}'` expands from the CALLER repo's checkout history, making
+        # validation cwd-dependent; a persistent declaration must never bind
+        # to ambient reflog state.
+        if not isinstance(value, str) or not value:
+            err(key, "must be a non-empty string")
+        if "@{" in value:
+            err(key, f"{value!r} contains '@{{' reflog/namespace shorthand — declarations must be literal branch names (B1)")
+        if value.startswith("refs/heads/"):  # check-ref-format --branch strips this; we do not allow it
+            err(key, "bare branch name expected, not a full ref")
+        try:
+            r = subprocess.run(["git", "check-ref-format", "--branch", value],
+                               capture_output=True)
+        except OSError as e:
+            err(key, f"cannot invoke git check-ref-format: {e}")
+        if r.returncode != 0:
+            err(key, f"{value!r} is not a valid git branch name (git check-ref-format, §5.2)")
+
+    def check_host_path(key, value):
+        # §1.2a: host:/-anchored; canonical segment normalization; empty/./.. rejected
+        # BEFORE resolution; tilde rejected outright.
+        if not isinstance(value, str) or not value:
+            err(key, "must be a non-empty string")
+        if "~" in value:
+            err(key, "tilde-anchored path rejected (§1.2a: ~ binds to caller HOME)")
+        if not value.startswith("host:/"):
+            err(key, "must be host:/-anchored (§1.2a)")
+        rest = value[len("host:/"):]
+        if rest == "":
+            err(key, "no segments after host:/")
+        segments = rest.split("/")
+        for seg in segments:
+            if seg == "":
+                err(key, f"empty segment in {value!r} (canonical normalization, §1.2a)")
+            if seg in (".", ".."):
+                err(key, f"dot segment {seg!r} rejected before resolution (§1.2a)")
+        return segments
+
+    # ---- version ----
+    sv = doc.get("schema_version")
+    if "schema_version" in doc:
+        if not isinstance(sv, int) or isinstance(sv, bool):
+            err("schema_version", "must be an integer")
+        if sv not in (1, 2):
+            err("schema_version", f"unknown schema_version {sv} — treated as ABSENT per keep-list K3; update tooling")
+        version = sv
+    else:
+        version = 1
+        warn("schema_version absent → v1 compatibility mode (two keys only)")
+
+    if REQUIRE_V2 and version != 2:
+        err("schema_version", "CI authoring rule: new or edited declarations must declare schema_version 2")
+
+    # ---- v1 ----
+    if version == 1:
+        for k in V1_KEYS:
+            check_branch(k, doc.get(k))
+        extra = set(doc) - V1_KEYS
+        if extra:
+            err("x_extensions", f"unknown top-level keys in v1: {sorted(extra)}")
+        print("OK (v1)")
+        sys.exit(0)
+
+    # ---- v2 required ----
+    for k in V2_REQUIRED:
+        if k not in doc:
+            err(k, "required for v2 (§1.2)")
+
+    check_branch("integration_trunk", doc["integration_trunk"])
+    check_branch("release_branch", doc["release_branch"])
+
+    # type-check BEFORE membership (F3: list-typed enums must not traceback)
+    if not isinstance(doc["flow"], str) or doc["flow"] not in ENUM_FLOW:
+        err("flow", f"must be one of {sorted(ENUM_FLOW)} (§1.2, required — no defaulting, R7)")
+
+    unknown = set(doc) - set(V2_REQUIRED) - V2_OPTIONAL
+    if unknown:
+        err("x_extensions", f"unknown top-level keys {sorted(unknown)} — place extensions inside x_extensions")
+
+    if "worktree_policy" in doc and (not isinstance(doc["worktree_policy"], str)
+                                     or doc["worktree_policy"] not in ENUM_POLICY):
+        err("worktree_policy", f"must be one of {sorted(ENUM_POLICY)}")
+    if "notes" in doc and not isinstance(doc["notes"], str):
+        err("notes", "must be a string")
+    if "x_extensions" in doc and not isinstance(doc["x_extensions"], dict):
+        err("x_extensions", "must be an object")
+    for strkey in ("canonical_remote", "canonical_clone", "worktree_root"):
+        if strkey in doc and not isinstance(doc[strkey], str):
+            err(strkey, "must be a string")
+
+    # ---- remote (§5.3) ----
+    if not isinstance(doc["canonical_remote"], str):
+        err("canonical_remote", "must be a string")
+    else:
+        normalize_remote(doc["canonical_remote"])
+
+    # ---- paths (§1.2a) ----
+    canonical_segments = check_host_path("canonical_clone", doc["canonical_clone"])
+    wt_segments = None
+    if "worktree_root" in doc:
+        wt_segments = check_host_path("worktree_root", doc["worktree_root"])
+
+    # ---- cross-field (§5.2) ----
+    trunk, rel, flow = doc["integration_trunk"], doc["release_branch"], doc["flow"]
+    if flow == "direct" and trunk != rel:
+        err("flow", "direct requires integration_trunk == release_branch (§5.2)")
+    if flow == "trunk-release" and trunk == rel:
+        err("flow", "trunk-release requires integration_trunk != release_branch (§5.2)")
+
+    # ---- root gate (§1.2a fail-closed; T51P2R1 F1) ----
+    # Every v2 declaration consumes a path (canonical_clone is required), so
+    # managed mode cannot proceed without a provable host anchor. Display mode
+    # warns and omits root-dependent resolution only.
+    if not HOST_ROOT:
+        if MODE == "managed":
+            err("MOSAIC_HOST_ROOT",
+                "unset or empty — managed validation of a v2 declaration consumes paths "
+                "(canonical_clone required); fail closed (§1.2a, DR3 X2b)")
+        else:
+            warn("host root unset; root-dependent resolution omitted (display mode, §1.2a)")
+
+    if doc.get("worktree_policy") == "tool-managed":
+        if "worktree_root" not in doc:
+            err("worktree_policy", "tool-managed requires worktree_root (containment provable, §4.5)")
+        if HOST_ROOT:
+            root_real = os.path.realpath(HOST_ROOT)
+            # B3 fix: containment is tested on the LEXICAL normalized path, not
+            # on realpath of the joined result — a child symlink under the host
+            # root can no longer fake outside-ness. host:/ segments are always
+            # lexically under the root, so tool-managed fails universally until
+            # the anchor scheme grows a real outside-root form (J3 charter).
+            resolved = os.path.normpath(os.path.join(root_real, *wt_segments))
+            if resolved == root_real or resolved.startswith(root_real + os.sep):
+                err("worktree_policy",
+                    f"tool-managed worktree_root resolves inside MOSAIC_HOST_ROOT (§4.5: outside-root requirement)")
+        # no-root case: managed mode already failed at the gate above; display warned
+
+    print("OK")
+
+try:
+    main()
+except SystemExit:
+    raise
+except Exception as e:  # F3: no traceback may ever escape the contract
+    err("internal", f"input rejected (unexpected condition: {type(e).__name__})")
+PYEOF