diff --git a/scripts/apply-workflow-pins-remote.sh b/scripts/apply-workflow-pins-remote.sh
index a56a37bb3..c7dade0ac 100755
--- a/scripts/apply-workflow-pins-remote.sh
+++ b/scripts/apply-workflow-pins-remote.sh
@@ -63,6 +63,10 @@
 
 set -uo pipefail
 
+# The shared ancestry assertion (D5c): every pin bumper calls the same one.
+# shellcheck source-path=SCRIPTDIR source=lib/pin-ancestry.sh
+. "$(dirname "${BASH_SOURCE[0]}")/lib/pin-ancestry.sh"
+
 STANDARDS_REPO="hyperpolymath/standards"
 TARGET_SHA="${STANDARDS_TARGET_SHA:-}"
 DO_FIX=0
@@ -278,19 +282,7 @@ resolve_target() {
 # The comparison is done SERVER-side so a shallow/partial clone cannot produce
 # a false negative.
 validate_target() {
-  local sha="$1" status
-  case "$sha" in
-    *[!0-9a-fA-F]*|"") log "ERROR: target '$sha' is not hex."; return 1 ;;
-  esac
-  if [ "${#sha}" -ne 40 ]; then
-    log "ERROR: target must be a full 40-hex SHA, got ${#sha} chars."; return 1
-  fi
-  status=$(gh api "repos/${STANDARDS_REPO}/compare/${sha}...main" --jq '.status' 2>/dev/null)
-  case "$status" in
-    identical|ahead) return 0 ;;
-    "") log "ERROR: could not prove ${sha} is reachable from ${STANDARDS_REPO} main; refusing."; return 1 ;;
-    *)  log "ERROR: ${sha} is NOT reachable from main (compare status: ${status}); refusing."; return 1 ;;
-  esac
+  assert_pin_ancestry "$STANDARDS_REPO" "$1" main
 }
 
 # sha_is_commit <sha> — is this a real commit in standards?
diff --git a/scripts/lib/pin-ancestry.sh b/scripts/lib/pin-ancestry.sh
new file mode 100644
index 000000000..2091438de
--- /dev/null
+++ b/scripts/lib/pin-ancestry.sh
@@ -0,0 +1,90 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: MPL-2.0
+# SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+#
+# pin-ancestry.sh — the ONE ancestry assertion every pin bumper calls
+# (owner ruling D5c, standards#787; mandatory under D19a).
+#
+# Source it; do not execute it:
+#
+#   . "$(dirname "${BASH_SOURCE[0]}")/lib/pin-ancestry.sh"
+#   assert_pin_ancestry hyperpolymath/standards "$sha" || exit 1
+#
+# WHY: a pin can be a real commit that GitHub still refuses to run. A PR head
+# that is later squash-merged, or a commit on a deleted unmerged branch, is an
+# ancestor of nothing; `commits/<sha>` still answers 200, but a cross-repo
+# reusable-workflow ref at that SHA dies at graph resolution (standards#782:
+# 61 dead rows, 0 alive). Squash is the estate's only merge method (D19a), so
+# every bumper that may be handed a PR head must prove ancestry before writing.
+#
+# PREDICATE: compare/<sha>...<branch> on the server.
+#   identical | ahead   → <sha> is an ancestor of <branch>       → rc 0
+#   behind | diverged   → NOT an ancestor; never pin it          → rc 1
+#   not hex / not 40    → refuse; abbreviated SHAs are prefix
+#                         queries, not existence tests            → rc 1
+#   no/unknown answer   → indeterminate; callers must refuse too  → rc 2
+# The comparison is server-side on purpose: `git merge-base --is-ancestor` in a
+# shallow or partial clone reports false negatives (standards#782). A caller
+# may try a full local clone first, but only this function may say "no" from
+# incomplete local evidence.
+#
+# ENVIRONMENT
+#   STANDARDS_REACHABILITY_API_BASE  API root (default https://api.github.com);
+#                                    tests point it at an unreachable port.
+#   GITHUB_TOKEN / GH_TOKEN          bearer token; else `gh auth token` if gh
+#                                    is logged in; else unauthenticated.
+
+# pin_ancestry_log <msg> — write one diagnostic line to stderr.
+pin_ancestry_log() { printf '%s\n' "$*" >&2; }
+
+# pin_ancestry_token — print the bearer token to use, or nothing.
+pin_ancestry_token() {
+  if [ -n "${GITHUB_TOKEN:-}" ]; then printf '%s' "$GITHUB_TOKEN"
+  elif [ -n "${GH_TOKEN:-}" ]; then printf '%s' "$GH_TOKEN"
+  elif command -v gh >/dev/null 2>&1; then gh auth token 2>/dev/null || true
+  fi
+}
+
+# assert_pin_ancestry <owner/repo> <sha> [branch]
+# Succeed only if <sha> is a full 40-hex commit that is an ancestor of
+# <branch> (default: main) in <owner/repo>. Returns 0 ancestor, 1 not an
+# ancestor or malformed, 2 indeterminate. Logs the reason on any refusal.
+assert_pin_ancestry() {
+  local repo="$1" sha="$2" branch="${3:-main}"
+  if ! [[ "$sha" =~ ^[0-9a-fA-F]{40}$ ]]; then
+    pin_ancestry_log "ERROR: pin must be a full 40-character hex commit SHA, got '${sha}'; refusing."
+    return 1
+  fi
+
+  local api="${STANDARDS_REACHABILITY_API_BASE:-https://api.github.com}"
+  local token body status
+  local -a auth=()
+  token="$(pin_ancestry_token)"
+  [ -n "$token" ] && auth=(-H "Authorization: Bearer ${token}")
+
+  if ! body=$(curl -fsS --max-time 20 \
+      -H 'Accept: application/vnd.github+json' \
+      "${auth[@]}" \
+      "$api/repos/${repo}/compare/${sha}...${branch}" 2>/dev/null); then
+    pin_ancestry_log "ERROR: could not prove ${sha} is an ancestor of ${repo}@${branch} (compare request failed); refusing."
+    return 2
+  fi
+
+  # The top-level "status" precedes the commits/files arrays, whose entries
+  # carry their own "status" ("modified", …). GitHub pretty-prints this body
+  # today, but compact JSON puts them all on one line, where a greedy
+  # `sed 's/.*"status"…/'` takes the LAST one. bash =~ takes the leftmost in
+  # either layout, and needs no pipe that could SIGPIPE on a large body.
+  status=""
+  local re='"status"[[:space:]]*:[[:space:]]*"([a-z]+)"'
+  [[ "$body" =~ $re ]] && status="${BASH_REMATCH[1]}"
+  case "$status" in
+    identical|ahead) return 0 ;;
+    behind|diverged)
+      pin_ancestry_log "ERROR: ${sha} is NOT an ancestor of ${repo}@${branch} (compare status: ${status}). Repin to the merge commit, never a PR head; refusing."
+      return 1 ;;
+    *)
+      pin_ancestry_log "ERROR: could not prove ${sha} is an ancestor of ${repo}@${branch} (compare status: ${status:-none}); refusing."
+      return 2 ;;
+  esac
+}
diff --git a/scripts/propagate-workflow-pins.sh b/scripts/propagate-workflow-pins.sh
index c27f72794..e9d558298 100755
--- a/scripts/propagate-workflow-pins.sh
+++ b/scripts/propagate-workflow-pins.sh
@@ -45,6 +45,10 @@
 
 set -euo pipefail
 
+# The shared ancestry assertion (D5c): every pin bumper calls the same one.
+# shellcheck source-path=SCRIPTDIR source=lib/pin-ancestry.sh
+. "$(dirname "${BASH_SOURCE[0]}")/lib/pin-ancestry.sh"
+
 # ---------------------------------------------------------------------------
 # Configuration / argument parsing
 # ---------------------------------------------------------------------------
@@ -140,26 +144,10 @@ validate_target() {
 
   # Neither usable local main ref proved reachability. Local refs can be stale,
   # so only the authoritative server comparison may return a negative result.
-
-  local api="${STANDARDS_REACHABILITY_API_BASE:-https://api.github.com}"
-  local -a auth=()
-  [ -n "${GITHUB_TOKEN:-}" ] && auth=(-H "Authorization: Bearer ${GITHUB_TOKEN}")
-  local body status
-  body=$(curl -fsS --max-time 20 \
-    -H 'Accept: application/vnd.github+json' \
-    "${auth[@]}" \
-    "$api/repos/hyperpolymath/standards/compare/${TARGET_SHA}...main") || {
-      log "ERROR: could not prove target ${TARGET_SHA} is reachable from standards main; refusing propagation."
-      return 1
-    }
-  status=$(printf '%s' "$body" | sed -n 's/.*"status"[[:space:]]*:[[:space:]]*"\([a-z]*\)".*/\1/p' | head -n1)
-  case "$status" in
-    identical|ahead) return 0 ;;
-    *)
-      log "ERROR: target ${TARGET_SHA} is not reachable from standards main (compare status: ${status:-unknown}); refusing propagation."
-      return 1
-      ;;
-  esac
+  assert_pin_ancestry hyperpolymath/standards "$TARGET_SHA" main || {
+    log "ERROR: refusing propagation of ${TARGET_SHA}."
+    return 1
+  }
 }
 
 validate_target || exit 2
diff --git a/scripts/tests/pin-ancestry-test.sh b/scripts/tests/pin-ancestry-test.sh
new file mode 100644
index 000000000..fd62b910c
--- /dev/null
+++ b/scripts/tests/pin-ancestry-test.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: MPL-2.0
+# SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+set -euo pipefail
+
+# Unit test for scripts/lib/pin-ancestry.sh — the shared ancestry assertion
+# (D5c). The compare API is served from file:// fixtures, so every verdict,
+# including each refusal, is a planted positive control rather than an absence.
+
+TEST_DIR=$(mktemp -d)
+trap 'rm -rf "$TEST_DIR"' EXIT
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+# shellcheck source-path=SCRIPTDIR source=../lib/pin-ancestry.sh
+. "$SCRIPT_DIR/../lib/pin-ancestry.sh"
+
+export STANDARDS_REACHABILITY_API_BASE="file://$TEST_DIR"
+export GITHUB_TOKEN=test-token
+REPO=hyperpolymath/standards
+mkdir -p "$TEST_DIR/repos/$REPO/compare"
+
+pass=0 fail=0
+
+# plant <sha> <body> — serve <body> as compare/<sha>...main.
+plant() { printf '%s' "$2" > "$TEST_DIR/repos/$REPO/compare/$1...main"; }
+
+# expect <rc> <label> <sha> — assert assert_pin_ancestry returns <rc>.
+expect() {
+  local want="$1" label="$2" sha="$3" got=0
+  assert_pin_ancestry "$REPO" "$sha" main 2>/dev/null || got=$?
+  if [ "$got" -eq "$want" ]; then
+    echo "PASS: $label (rc=$got)"; pass=$((pass + 1))
+  else
+    echo "FAIL: $label (want rc=$want, got rc=$got)"; fail=$((fail + 1))
+  fi
+}
+
+# sha <n> — print <n> zero-padded to a 40-character fake commit SHA.
+sha() { printf '%040d' "$1"; }
+
+plant "$(sha 1)" '{"url":"x","status":"identical","ahead_by":0,"behind_by":0}'
+plant "$(sha 2)" '{"url":"x","status":"ahead","ahead_by":3,"behind_by":0,"commits":[{"status":"diverged"}]}'
+plant "$(sha 3)" '{"url":"x","status":"behind","ahead_by":0,"behind_by":2}'
+plant "$(sha 4)" '{"url":"x","status":"diverged","ahead_by":4,"behind_by":1}'
+plant "$(sha 5)" '{"message":"Not Found"}'
+
+expect 0 "identical is an ancestor"                  "$(sha 1)"
+expect 0 "ahead is an ancestor (first status wins)"  "$(sha 2)"
+expect 1 "behind is NOT an ancestor (PR head)"       "$(sha 3)"
+expect 1 "diverged is NOT an ancestor (orphan)"      "$(sha 4)"
+expect 2 "body without status is indeterminate"      "$(sha 5)"
+expect 2 "failed request is indeterminate"           "$(sha 6)"
+expect 1 "abbreviated SHA is refused before probing" "5b1d0022"
+expect 1 "non-hex SHA is refused"                    "$(sha 1 | tr 0 g)"
+
+# The refusal must name the remedy, or the operator repins a PR head again.
+msg=$(assert_pin_ancestry "$REPO" "$(sha 4)" main 2>&1 || true)
+if grep -q 'never a PR head' <<<"$msg"; then
+  echo "PASS: non-ancestor refusal names the remedy"; pass=$((pass + 1))
+else
+  echo "FAIL: non-ancestor refusal lacks the remedy: $msg"; fail=$((fail + 1))
+fi
+
+echo "----------------------------------------"
+echo "$pass/$((pass + fail)) test cases passed."
+[ "$fail" -eq 0 ]