diff --git a/.github/workflows/actions.lock b/.github/workflows/actions.lock
index ae1d2d0de..5827fa86d 100644
--- a/.github/workflows/actions.lock
+++ b/.github/workflows/actions.lock
@@ -70,6 +70,8 @@ workflows:
     '.github/workflows/lockfile-drift-detect.yml':
         - 'actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1'
         - 'actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a'
+    '.github/workflows/lock-selfcheck.yml':
+        - 'actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1'
     '.github/workflows/makefile-blocker.yml':
         - 'actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1'
     '.github/workflows/mirror-reusable.yml':
diff --git a/.github/workflows/lock-selfcheck.yml b/.github/workflows/lock-selfcheck.yml
new file mode 100644
index 000000000..f7729c8d8
--- /dev/null
+++ b/.github/workflows/lock-selfcheck.yml
@@ -0,0 +1,61 @@
+# This workflow is managed by gh actions-lock.
+# SPDX-License-Identifier: MPL-2.0
+# lock-selfcheck.yml — is THIS commit safe for a caller to pin to?
+#
+# `uses: hyperpolymath/standards/.github/workflows/x.yml@<sha>` is resolved at
+# workflow STARTUP, and GitHub validates the callee against the CALLEE repo's
+# own `.github/workflows/actions.lock` AS IT EXISTS AT THAT SHA. If a reusable
+# here names an action ref that the lock at that SHA does not key, then every
+# caller in the estate pinned at that SHA dies before a single job exists.
+#
+# That death is close to invisible: conclusion `failure` (NOT
+# `startup_failure`), ZERO jobs, run `name` equal to the run `path`, and
+# neither REST nor GraphQL carries a reason — only the run page states one. A
+# required context whose workflow dies this way never reports at all, so the
+# gate reads as ABSENT rather than failing and the branch looks clean.
+#
+# Capability is therefore NOT monotonic in time, and it OSCILLATES: measured
+# across the 84 commits da2c748a..317101e0, 43 were POISON and 41
+# SELF-CONSISTENT, alternating, because Dependabot bumps a `uses:` inside a
+# reusable without regenerating the lock in the same commit. "Pin to HEAD" is
+# the wrong reflex. This gate exists so that a poisoned commit cannot reach
+# main unannounced, and so `main` is a safe thing to pin to by construction.
+#
+# SCOPE: this proves INTERNAL CONSISTENCY of the commit under test. It does
+# NOT prove the commit is reachable at the remote — that is a separate probe.
+name: Lock Self-Check
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+# Deliberately NO `paths:` filter. This check is intended to be REQUIRED, and
+# a required check that filters itself off does not report as passing — it
+# does not report at all, which leaves the PR blocked on a context that can
+# never arrive.
+
+permissions:
+  contents: read
+
+jobs:
+  selfcheck:
+    name: Lockfile self-consistency
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout the commit under test
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      - name: Check this commit validates against its own actions.lock
+        run: |
+          set -euo pipefail
+          sha="$(git rev-parse HEAD)"
+          # State the population. A gate that does not say what it examined
+          # cannot report that it examined the wrong thing.
+          echo "Commit under test: $sha"
+          echo "Event: ${{ github.event_name }}"
+          # On `pull_request` this is the MERGE PREVIEW, i.e. the content that
+          # would land on main, which is exactly the content callers will pin.
+          bash scripts/lock-selfcheck.sh "$sha"
diff --git a/scripts/lock-selfcheck.sh b/scripts/lock-selfcheck.sh
new file mode 100755
index 000000000..9380078b8
--- /dev/null
+++ b/scripts/lock-selfcheck.sh
@@ -0,0 +1,221 @@
+#!/usr/bin/env bash
+# lock-selfcheck.sh — is a given `standards` commit SAFE TO PIN A CALLER TO?
+#
+# WHY THIS EXISTS
+# ---------------
+# `uses: org/repo/.github/workflows/x.yml@<sha>` is resolved at workflow
+# STARTUP, and GitHub validates the callee against **the callee repo's own
+# `.github/workflows/actions.lock` AS IT EXISTS AT THAT SHA**. If the reusable
+# names an action ref the lock at that same SHA does not key, every caller dies
+# before any job exists — and the failure is near-invisible:
+#
+#   * conclusion is `failure`, NOT `startup_failure`
+#   * the run has ZERO jobs
+#   * the run's `name` equals its `path`
+#   * neither REST nor GraphQL carries a reason — only the run PAGE does
+#
+# Dependabot causes this routinely: it bumps a `uses:` inside a workflow and
+# never regenerates the lockfile. So **capability is not monotonic in time** —
+# the newest `standards` commit can be strictly less usable than an older one,
+# and "bump every caller to HEAD" is exactly the wrong reflex.
+#
+# Before a pin sweep repairs N callers onto a target SHA, that SHA must pass
+# this check. Otherwise the sweep converts N silently-ungated repos into N
+# loudly-broken ones.
+#
+# USAGE
+#   lock-selfcheck.sh <sha> [<sha>...]        # uses $STANDARDS_DIR or default
+#   STANDARDS_DIR=/path/to/standards lock-selfcheck.sh <sha>
+#
+# Reads git objects ONLY (`git show`); never checks out, fetches, or writes.
+# Safe against a checkout shared with a live concurrent writer.
+#
+# EXIT: 0 if every SHA examined self-validates; 1 if any does not; 2 on misuse.
+#
+# ⚠ SCOPE LIMIT, STATED SO IT IS NOT MISREAD: this proves a commit is
+# INTERNALLY CONSISTENT. It does NOT prove the commit is REACHABLE at the
+# remote. A local object store can hold commits GitHub has since orphaned
+# (a garbage-collected commit is the general case). Reachability is a
+# SEPARATE probe and must be made against the remote.
+#
+# SPDX-License-Identifier: MPL-2.0
+
+set -uo pipefail
+
+STANDARDS_DIR="${STANDARDS_DIR:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+LOCK_PATH='.github/workflows/actions.lock'
+
+if [ "$#" -eq 0 ]; then
+  echo "usage: $0 <sha> [<sha>...]" >&2
+  exit 2
+fi
+
+if ! git -C "$STANDARDS_DIR" rev-parse --git-dir >/dev/null 2>&1; then
+  echo "FATAL: not a git repository: $STANDARDS_DIR" >&2
+  exit 2
+fi
+
+TMP="${TMPDIR:-/tmp}/lock-selfcheck.$$"
+mkdir -p "$TMP"
+trap 'rm -rf "$TMP"' EXIT
+
+# Normalise a `uses:` value to the key shape the lockfile uses:
+#   github/codeql-action/upload-sarif@SHA  ->  github/codeql-action@SHA
+#   actions/checkout@SHA                   ->  actions/checkout@SHA
+# Local refs (./...) and docker:// refs are not lockfile-keyed and are skipped
+# by the caller, not here.
+normalise_ref() {
+  printf '%s\n' "$1" | awk -F'@' '{
+    n = split($1, p, "/")
+    owner_repo = (n >= 2) ? p[1] "/" p[2] : $1
+    print owner_repo "@" $2
+  }'
+}
+
+overall_rc=0
+
+for SHA in "$@"; do
+  echo "=============================================================="
+  if ! git -C "$STANDARDS_DIR" cat-file -e "${SHA}^{commit}" 2>/dev/null; then
+    echo "SHA ${SHA}: ABSENT from local object store — cannot check"
+    overall_rc=1
+    continue
+  fi
+  SHORT=$(git -C "$STANDARDS_DIR" rev-parse --short=12 "$SHA")
+  WHEN=$(git -C "$STANDARDS_DIR" show -s --format='%ci' "$SHA")
+  SUBJ=$(git -C "$STANDARDS_DIR" show -s --format='%s' "$SHA")
+  echo "SHA ${SHORT}  ${WHEN}"
+  echo "     ${SUBJ}"
+
+  if ! git -C "$STANDARDS_DIR" show "${SHA}:${LOCK_PATH}" > "$TMP/lock" 2>/dev/null; then
+    # THIRD STATE, and not the same as POISON. With no lockfile there is
+    # nothing for GitHub to validate the callee against, so a caller pinned
+    # here does NOT die at startup -- it simply gets no immutable-action
+    # guarantee. Predates the lock regime (which begins ~2026-08-03).
+    # Reporting this as unsafe-to-pin would be a false alarm, so it does not
+    # set the failure exit code.
+    echo "  VERDICT: NO-LOCK — ${LOCK_PATH} does not exist at this SHA."
+    echo "           NOT startup-fatal: with no lock there is nothing to"
+    echo "           validate against. But callers pinned here get no"
+    echo "           action-pinning guarantee either. Pre-dates the lock regime."
+    continue
+  fi
+
+  # Parse the lock into "<workflow-path>\t<owner/repo@sha>" pairs.
+  awk '
+    /^workflows:/ { in_wf = 1; next }
+    !in_wf { next }
+    # A workflow key line:  \x27.github/workflows/foo.yml\x27:
+    /^[[:space:]]+\x27[^\x27]+\x27:/ {
+      line = $0
+      sub(/^[[:space:]]+\x27/, "", line)
+      sub(/\x27:.*$/, "", line)
+      cur = line
+      next
+    }
+    # An entry line:  - \x27owner/repo@sha\x27
+    /^[[:space:]]*-[[:space:]]*\x27[^\x27]+\x27/ {
+      line = $0
+      sub(/^[[:space:]]*-[[:space:]]*\x27/, "", line)
+      sub(/\x27.*$/, "", line)
+      if (cur != "") print cur "\t" line
+    }
+  ' "$TMP/lock" | sort -u > "$TMP/locked"
+
+  locked_pairs=$(wc -l < "$TMP/locked")
+  locked_wfs=$(cut -f1 "$TMP/locked" | sort -u | wc -l)
+
+  # Enumerate the workflow files present at this SHA.
+  git -C "$STANDARDS_DIR" ls-tree -r --name-only "$SHA" -- .github/workflows \
+    | /usr/bin/grep -E '\.ya?ml$' | sort > "$TMP/wfs"
+  n_wfs=$(wc -l < "$TMP/wfs")
+
+  # The lockfile LOWERCASES the owner/repo it keys ('swatinem/rust-cache')
+  # while the workflow writes the upstream casing ('Swatinem/rust-cache').
+  # Same action, same SHA. Comparing case-sensitively invents defects, so
+  # fold both sides before matching. (This cost one false 'POISON' on
+  # rust-ci-reusable.yml before it was caught.)
+  tr 'A-Z' 'a-z' < "$TMP/locked" > "$TMP/locked.lc"
+
+  : > "$TMP/missing"
+  : > "$TMP/unkeyed_wf"
+  : > "$TMP/scanned"
+
+  while IFS= read -r wf; do
+    [ -n "$wf" ] || continue
+    git -C "$STANDARDS_DIR" show "${SHA}:${wf}" 2>/dev/null > "$TMP/wfbody" || continue
+    # Extract every `uses:` value, strip inline comments and quotes.
+    /usr/bin/grep -hoE '^[[:space:]]*(-[[:space:]]*)?uses:[[:space:]]*[^[:space:]#]+' "$TMP/wfbody" \
+      | sed -E 's/.*uses:[[:space:]]*//; s/^["\x27]//; s/["\x27]$//' \
+      | while IFS= read -r ref; do
+          [ -n "$ref" ] || continue
+          case "$ref" in
+            ./*|docker://*) continue ;;   # local / docker: not lockfile-keyed
+          esac
+          case "$ref" in
+            *@*) : ;;
+            *) continue ;;                # unpinned (no @) — a different defect
+          esac
+          # A reusable-workflow call carries a .yml before the @; the lockfile
+          # keys ACTIONS, not reusable calls. Report those separately.
+          before="${ref%@*}"
+          case "$before" in
+            *.yml|*.yaml) printf '%s\t%s\tREUSABLE-CALL\n' "$wf" "$ref" >> "$TMP/scanned"; continue ;;
+          esac
+          key=$(normalise_ref "$ref")
+          printf '%s\t%s\tACTION\n' "$wf" "$key" >> "$TMP/scanned"
+          key_lc=$(printf '%s' "$key" | tr 'A-Z' 'a-z')
+          wf_lc=$(printf '%s' "$wf" | tr 'A-Z' 'a-z')
+          # Two DIFFERENT questions, deliberately not conflated:
+          #   (1) this workflow has NO entry in the lock at all  -> UNKEYED
+          #   (2) this workflow IS keyed, but not for this ref   -> MISSING
+          # Only (2) is proven to kill a caller at startup. (1) is reported
+          # separately because the lock also carries explicit ': []' entries,
+          # which proves absence and "uses nothing" are distinct states the
+          # generator can express -- so absence means the generator never saw
+          # the file, not that the file is forbidden actions.
+          if ! /usr/bin/grep -qF "$(printf '%s\t' "$wf_lc")" "$TMP/locked.lc"; then
+            printf '%s\t%s\n' "$wf" "$key" >> "$TMP/unkeyed_wf"
+          elif ! /usr/bin/grep -qF "$(printf '%s\t%s' "$wf_lc" "$key_lc")" "$TMP/locked.lc"; then
+            printf '%s\t%s\n' "$wf" "$key" >> "$TMP/missing"
+          fi
+        done
+  done < "$TMP/wfs"
+
+  n_actions=$(/usr/bin/grep -c 'ACTION$' "$TMP/scanned" 2>/dev/null || echo 0)
+  n_reusable=$(/usr/bin/grep -c 'REUSABLE-CALL$' "$TMP/scanned" 2>/dev/null || echo 0)
+  n_missing=$(sort -u "$TMP/missing" 2>/dev/null | wc -l)
+  n_unkeyed=$(sort -u "$TMP/unkeyed_wf" 2>/dev/null | wc -l)
+  n_unkeyed_wf=$(cut -f1 "$TMP/unkeyed_wf" 2>/dev/null | sort -u | wc -l)
+
+  echo "  workflows at this SHA: ${n_wfs}   lockfile keys: ${locked_wfs} workflow(s), ${locked_pairs} pair(s)"
+  echo "  action refs used: ${n_actions}   reusable-workflow calls (not lock-keyed): ${n_reusable}"
+  if [ "$n_unkeyed" -gt 0 ]; then
+    echo "  note: ${n_unkeyed_wf} workflow(s) have NO lock entry at all (${n_unkeyed} ref(s)) —"
+    echo "        a separate question from the verdict below, and not counted in it:"
+    cut -f1 "$TMP/unkeyed_wf" | sort -u | sed 's/^/          /'
+  fi
+
+  if [ "$n_missing" -eq 0 ]; then
+    echo "  VERDICT: SELF-CONSISTENT — every action ref used is keyed in this SHA's own lock."
+    echo "           (Reachability at the remote is NOT proven by this check.)"
+  else
+    echo "  VERDICT: POISON — ${n_missing} action ref(s) used but NOT keyed in this SHA's own lock."
+    echo "           Any caller pinned here dies at startup with 0 jobs and no stated reason."
+    sort -u "$TMP/missing" | while IFS=$'\t' read -r wf key; do
+      echo "             ${wf}"
+      echo "               uses  ${key}"
+      locked_for=$(/usr/bin/grep -F "$(printf '%s\t' "$wf")" "$TMP/locked" | cut -f2 | sort -u | tr '\n' ' ')
+      echo "               lock keys for this workflow: ${locked_for:-<none>}"
+    done
+    overall_rc=1
+  fi
+done
+
+echo "=============================================================="
+if [ "$overall_rc" -eq 0 ]; then
+  echo "ALL EXAMINED SHAs SELF-VALIDATE (examined: $#)"
+else
+  echo "AT LEAST ONE SHA IS UNSAFE TO PIN TO (examined: $#)"
+fi
+exit "$overall_rc"