diff --git a/lib/merge.py b/lib/merge.py
index d2eeb32..b9ff910 100644
--- a/lib/merge.py
+++ b/lib/merge.py
@@ -560,9 +560,9 @@ async def _merge_no_ff_external(branch: str) -> tuple[bool, str]:
         merge_sha = merge_sha.strip().split("\n")[0]
 
         # Push to synthetic audit ref _merged/{branch} (does not touch contributor's
-        # gh-pr-N/* branch). Force-with-lease against expected nothing — the audit
-        # ref is per-PR and shouldn't pre-exist on a fresh merge. If it does (rare:
-        # a prior failed attempt), force-push is fine since the ref is bot-owned.
+        # gh-pr-N/* branch). Plain --force: the audit ref is bot-owned and per-PR;
+        # if a prior aborted attempt left a stale ref, overwriting it is the
+        # intended behavior, and there's no concurrent writer to lease against.
         rc, out = await _git(
             "push", "--force", "origin", f"HEAD:refs/heads/{audit_ref}",
             cwd=worktree_path, timeout=30,
@@ -969,6 +969,14 @@ async def _merge_domain_queue(conn, domain: str) -> tuple[int, int]:
                 audit_ref = m_ref.group(1) if m_ref else None
                 m_pr = re.search(r"external PR #(\d+)", pick_msg)
                 gh_pr_num = m_pr.group(1) if m_pr else None
+                # Surface drift between dispatch and _merge_no_ff_external if the
+                # success-message contract changes. Merge already succeeded; this
+                # is signal-only, not a gate on the close path.
+                if not (m and m_ref and m_pr):
+                    logger.warning(
+                        "PR #%d sentinel parse incomplete: M=%s, audit_ref=%s, gh_pr=%s, msg=%r",
+                        pr_num, bool(m), bool(m_ref), bool(m_pr), pick_msg,
+                    )
 
                 leo_token = get_agent_token("leo")
                 comment_body = (