diff --git a/.github/workflows/governance-reusable.yml b/.github/workflows/governance-reusable.yml
index ed91053c..90c19ce7 100644
--- a/.github/workflows/governance-reusable.yml
+++ b/.github/workflows/governance-reusable.yml
@@ -1012,6 +1012,31 @@ jobs:
             exit 1
           fi
           echo "All actions are SHA-pinned"
+      # The step above proves a pin has the right SHAPE. It cannot prove the
+      # SHA EXISTS — a fabricated 40-hex string passes it. Measured 2026-07-28:
+      # 112 of 613 unique estate pins (18%) do not resolve, in 876 committed
+      # files. An unresolvable `uses:` yields NO check run, so those repos look
+      # green while the job never ran. This step closes that gap.
+      - name: Checkout standards for the pin-existence gate
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          repository: hyperpolymath/standards
+          ref: feat/pin-existence-check
+          path: .standards-checkout
+          persist-credentials: false
+          sparse-checkout: |
+            scripts
+          sparse-checkout-cone-mode: false
+      - name: Check action pins resolve upstream
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          # Copy out and delete the checkout before scanning, so the standards
+          # tree is never part of the caller's workspace (same idiom as the
+          # allowlist preflight above).
+          cp .standards-checkout/scripts/check-action-pins-resolve.sh "$RUNNER_TEMP/"
+          rm -rf .standards-checkout
+          bash "$RUNNER_TEMP/check-action-pins-resolve.sh" .
       - name: Check for duplicate workflows
         run: |
           if [ -f .github/workflows/codeql.yml ] && [ -f .github/workflows/codeql-analysis.yml ]; then
diff --git a/scripts/check-action-pins-resolve.sh b/scripts/check-action-pins-resolve.sh
new file mode 100755
index 00000000..9aec9509
--- /dev/null
+++ b/scripts/check-action-pins-resolve.sh
@@ -0,0 +1,154 @@
+#!/bin/bash
+# SPDX-License-Identifier: MPL-2.0
+set -uo pipefail
+
+# check-action-pins-resolve.sh — verify every SHA-pinned action actually EXISTS.
+#
+# ── Why this gate exists ────────────────────────────────────────────────────
+# The governance linter's "Check SHA-pinned actions" step verifies the *shape*
+# of a pin (`@` + 40 hex chars). It cannot tell a real commit from an invented
+# one, because a fabricated SHA is a perfectly well-formed 40-hex string.
+#
+# That gap is not theoretical. Measured across the estate on 2026-07-28:
+#
+#     613 unique (action, SHA) pins  →  112 (18%) DO NOT RESOLVE
+#       ·  80 = real action repo, SHA does not exist
+#       ·  32 = the action repository itself is gone (7 distinct actions)
+#     …present in 876 COMMITTED workflow files across ~310 repo roots.
+#
+# The failure mode is silent by design of the platform: Actions only resolves
+# a `uses:` ref at RUN time, and an unresolvable ref produces **no check run at
+# all** — not a red one. So `gh pr checks` shows nothing, the board looks
+# green, and the job never ran. A repo can be "fully green" with its security
+# scanning entirely absent. See dev-notes/estate-unresolvable-action-pins-*.md.
+#
+# This script closes that gap by asking GitHub whether each pin resolves.
+#
+# ── Failure semantics (deliberate, not handwaving) ──────────────────────────
+# HARD FAIL on a *determinate negative* — GitHub answered, and the answer was
+# "this does not exist":
+#   · 422/404 from the commits endpoint while the repo itself resolves
+#     → invented SHA.
+#   · 404 from the repo endpoint → dead/renamed/private action repo. (Note
+#     that Actions does NOT follow repo renames in `uses:`, so a rename is a
+#     genuine break, not a cosmetic one.)
+#
+# DO NOT FAIL on an *indeterminate* answer — rate limiting, 5xx, network loss.
+# Those say nothing about the pin. Failing on them would convert any GitHub
+# incident into an estate-wide red treadmill, the same trap the staleness gate
+# fell into (see check-workflow-staleness.sh). Instead they are counted and
+# reported LOUDLY as UNVERIFIED so the gap is visible rather than silently
+# green — a fail-open that announces itself is not a fake gate; a fail-open
+# that hides is.
+#
+# Rate limiting is not expected to bite: with GITHUB_TOKEN the limit is 1,000
+# requests/hour/repo, and the largest estate repo carries well under 100 unique
+# pins (only unique (repo,sha) pairs are queried, not every occurrence).
+#
+# USAGE:  check-action-pins-resolve.sh [path]     # default: current directory
+#         GH_TOKEN / GITHUB_TOKEN respected for auth.
+# EXIT:   0 = all pins resolve (or only indeterminate results)
+#         1 = at least one pin determinately does not exist
+
+TARGET="${1:-.}"
+WORKFLOW_DIR="$TARGET/.github/workflows"
+
+if [ ! -d "$WORKFLOW_DIR" ]; then
+  echo "No .github/workflows/ in $TARGET — nothing to check."
+  exit 0
+fi
+
+# ── Collect unique (repo, sha) pairs ────────────────────────────────────────
+# Handles `owner/repo@sha` and `owner/repo/sub/path@sha` (reusable workflows
+# and composite subpaths both pin at the repository level).
+# Skips local (`./`) and docker:// refs, which have no upstream commit.
+pairs="$(
+  grep -rhoE '\buses:[[:space:]]*[A-Za-z0-9_.-]+/[A-Za-z0-9_./-]+@[0-9a-f]{40}' \
+    "$WORKFLOW_DIR" 2>/dev/null \
+  | sed -E 's/.*uses:[[:space:]]*//' \
+  | awk -F'@' '{ split($1, p, "/"); print p[1] "/" p[2] "\t" $2 }' \
+  | sort -u
+)"
+
+if [ -z "$pairs" ]; then
+  echo "No SHA-pinned external actions found — nothing to check."
+  exit 0
+fi
+
+total=$(printf '%s\n' "$pairs" | wc -l | tr -d ' ')
+echo "Checking $total unique action pin(s) resolve upstream…"
+
+api() { # api <path> -> prints body, returns curl-visible HTTP code in $HTTP
+  local path="$1" auth=()
+  [ -n "${GH_TOKEN:-${GITHUB_TOKEN:-}}" ] && \
+    auth=(-H "Authorization: Bearer ${GH_TOKEN:-$GITHUB_TOKEN}")
+  HTTP="$(curl -sS -o /dev/null -w '%{http_code}' \
+    -H "Accept: application/vnd.github+json" \
+    -H "X-GitHub-Api-Version: 2022-11-28" \
+    "${auth[@]}" "https://api.github.com/$path" 2>/dev/null)" || HTTP="000"
+}
+
+bad=0
+unverified=0
+bad_list=""
+unver_list=""
+
+while IFS=$'\t' read -r repo sha; do
+  [ -z "$repo" ] && continue
+
+  api "repos/$repo/commits/$sha"
+  case "$HTTP" in
+    200)
+      : # resolves — good
+      ;;
+    404|422)
+      # Determinate negative from the commits endpoint. Disambiguate:
+      # is the SHA missing, or the whole repository?
+      api "repos/$repo"
+      if [ "$HTTP" = "200" ]; then
+        bad=$((bad + 1))
+        bad_list="${bad_list}  SHA-NOT-FOUND   $repo@$sha"$'\n'
+      elif [ "$HTTP" = "404" ]; then
+        bad=$((bad + 1))
+        bad_list="${bad_list}  REPO-NOT-FOUND  $repo (pinned @$sha)"$'\n'
+      else
+        # Could not confirm the repo either way — treat as indeterminate.
+        unverified=$((unverified + 1))
+        unver_list="${unver_list}  HTTP $HTTP on repos/$repo — $repo@$sha"$'\n'
+      fi
+      ;;
+    *)
+      # 403 (rate limit), 5xx, 000 (network) — says nothing about the pin.
+      unverified=$((unverified + 1))
+      unver_list="${unver_list}  HTTP $HTTP — $repo@$sha"$'\n'
+      ;;
+  esac
+done <<< "$pairs"
+
+echo
+
+if [ "$unverified" -gt 0 ]; then
+  echo "::warning::UNVERIFIED: $unverified of $total pin(s) could not be checked"
+  echo "  (rate limit, 5xx, or network — NOT evidence the pins are bad)"
+  printf '%s' "$unver_list"
+  echo "  These were NOT counted as failures. Re-run to confirm."
+  echo
+fi
+
+if [ "$bad" -gt 0 ]; then
+  echo "::error::$bad of $total action pin(s) DO NOT EXIST upstream."
+  printf '%s' "$bad_list"
+  echo
+  echo "An unresolvable 'uses:' produces NO check run — the job silently never"
+  echo "runs, so the board looks green while the gate is absent. Fix by:"
+  echo "  · SHA-NOT-FOUND  — repin to a real SHA. Resolve the intended tag with"
+  echo "                     git ls-remote <repo> refs/tags/vN   (use the ^{} value)"
+  echo "  · REPO-NOT-FOUND — the action is gone. Vendor the logic into this repo"
+  echo "                     and call it with 'run:' (see hyperpolymath/tangle#84),"
+  echo "                     or repoint at the live repository name."
+  exit 1
+fi
+
+verified=$((total - unverified))
+echo "All $verified verifiable action pin(s) resolve upstream."
+exit 0