diff --git a/lib/plugins/__tests__/github-approve-on-green.test.ts b/lib/plugins/__tests__/github-approve-on-green.test.ts
index 65887ac7..c390e10c 100644
--- a/lib/plugins/__tests__/github-approve-on-green.test.ts
+++ b/lib/plugins/__tests__/github-approve-on-green.test.ts
@@ -94,6 +94,7 @@ interface Captured {
   approvePosts: Array<Record<string, unknown>>;
   commentPosts: Array<Record<string, unknown>>;
   mergePuts: Array<Record<string, unknown>>;
+  updateBranchPuts: Array<Record<string, unknown>>;
 }
 
 function ciCompletionPayload(): Record<string, unknown> {
@@ -105,7 +106,7 @@ function ciCompletionPayload(): Record<string, unknown> {
 }
 
 function stubFetch(opts: StubOpts): Captured {
-  const captured: Captured = { approvePosts: [], commentPosts: [], mergePuts: [] };
+  const captured: Captured = { approvePosts: [], commentPosts: [], mergePuts: [], updateBranchPuts: [] };
   const json = (body: unknown, status = 200) =>
     new Response(JSON.stringify(body), { status, headers: { "Content-Type": "application/json" } });
 
@@ -360,11 +361,13 @@ interface SweepPr {
   mergeStatus?: number;
   /** ISO created_at for the pulls listing (default: undefined — reads as "too young" for rescue). */
   createdAt?: string;
+  /** mergeable_state for a single-PR GET (default "blocked"). */
+  mergeableState?: string;
 }
 
 /** Stub GitHub REST for a reconciliation sweep over one repo's open PRs. */
 function stubSweepFetch(prs: SweepPr[]): Captured {
-  const captured: Captured = { approvePosts: [], commentPosts: [], mergePuts: [] };
+  const captured: Captured = { approvePosts: [], commentPosts: [], mergePuts: [], updateBranchPuts: [] };
   const json = (body: unknown, status = 200) =>
     new Response(JSON.stringify(body), { status, headers: { "Content-Type": "application/json" } });
   const byNum = new Map(prs.map((p) => [p.number, p]));
@@ -405,6 +408,16 @@ function stubSweepFetch(prs: SweepPr[]): Captured {
       captured.approvePosts.push(JSON.parse(String(init?.body ?? "{}")));
       return json({ id: 999 });
     }
+    // update-behind (fifth stall mode): PUT /pulls/{n}/update-branch
+    if (url.match(/\/pulls\/\d+\/update-branch$/) && method === "PUT") {
+      captured.updateBranchPuts.push({ url, ...JSON.parse(String(init?.body ?? "{}")) });
+      return json({ message: "Updating pull request branch." }, 202);
+    }
+    // single-PR GET (mergeable_state probe on a 405 refusal)
+    if (url.match(/\/pulls\/\d+$/) && method === "GET") {
+      const p = byNumFromUrl(url);
+      return json({ number: p?.number, mergeable_state: p?.mergeableState ?? "blocked" });
+    }
     // merge-completion backstop (ws-5sc): PUT /pulls/{n}/merge
     if (url.match(/\/pulls\/\d+\/merge$/) && method === "PUT") {
       const p = byNumFromUrl(url);
@@ -584,6 +597,16 @@ describe("_reconcileApproveOnGreen — level-triggered backstop (#879)", () => {
     expect(dispatched.length).toBe(0);
   });
 
+  test("405 on a BEHIND branch → sweep updates the branch (expected head SHA) and drops the watch", async () => {
+    const behind = { ...approvedGreenPr(913, "bhd0913", "main"), mergeStatus: 405, mergeableState: "behind" };
+    const { captured, plugin } = await runSweepWithPlugin([behind], { seedWatch: { key: `${OWNER}/${REPO}#913@bhd0913`, ageMs: 10 * 60_000 } });
+    expect(captured.mergePuts.length).toBe(1); // merge attempted first
+    expect(captured.updateBranchPuts.length).toBe(1);
+    expect(captured.updateBranchPuts[0]!.expected_head_sha).toBe("bhd0913");
+    const watch = (plugin as unknown as { mergeCompletionFirstSeen: Map<string, number> }).mergeCompletionFirstSeen;
+    expect(watch.size).toBe(0); // head will change — stale watch dropped
+  });
+
   test("covers a reviewed-but-UNREGISTERED repo (the approve-on-green gap fix)", async () => {
     // Registry is empty (repo not tagged / not in EXPLICIT), but Quinn reviewed
     // it this process → the sweep must still cover it via reviewedRepoCoords.
diff --git a/lib/plugins/github.ts b/lib/plugins/github.ts
index 73aa750f..e01854a2 100644
--- a/lib/plugins/github.ts
+++ b/lib/plugins/github.ts
@@ -1613,9 +1613,47 @@ export class GitHubPlugin implements Plugin {
       if (!res.ok) {
         const body = await res.text();
         // 409 = head moved since we started watching — the watch is stale,
-        // drop it (a fresh approve cycle re-evaluates the new SHA). Any other
-        // refusal (405 not-mergeable, 403) keeps the watch for the next sweep.
-        if (res.status === 409) this.mergeCompletionFirstSeen.delete(key);
+        // drop it (a fresh approve cycle re-evaluates the new SHA).
+        if (res.status === 409) {
+          this.mergeCompletionFirstSeen.delete(key);
+          log.warn(`merge-completion (${source}): PUT merge → 409 for ${key} (head moved): ${body.slice(0, 200)}`);
+          return false;
+        }
+        // 405 on a BEHIND branch = strict protection refusing an out-of-date
+        // head — the fifth stall mode: two PRs in flight, the first merge
+        // moves main under the second, and neither native auto-merge nor
+        // anything else updates the branch. Update it ourselves: the
+        // synchronize event runs CI + re-review on the fresh head and the
+        // normal promote→merge cycle converges. expected_head_sha guards the
+        // same push race as the merge call. Any other refusal keeps the
+        // watch for the next sweep.
+        if (res.status === 405) {
+          const detail = (await this._ghGet(getToken, owner, repo, `/repos/${owner}/${repo}/pulls/${number}`)) as
+            | { mergeable_state?: string }
+            | null;
+          if (detail?.mergeable_state === "behind") {
+            const upd = await withCircuitBreaker("github-api", () =>
+              fetch(`https://api.github.com/repos/${owner}/${repo}/pulls/${number}/update-branch`, {
+                method: "PUT",
+                headers: {
+                  Authorization: `Bearer ${token}`,
+                  "Content-Type": "application/json",
+                  Accept: "application/vnd.github+json",
+                  "User-Agent": "protoWorkstacean/1.0",
+                  "X-GitHub-Api-Version": "2022-11-28",
+                },
+                body: JSON.stringify({ expected_head_sha: headSha }),
+              }),
+            );
+            this.mergeCompletionFirstSeen.delete(key); // head will change either way
+            if (upd.ok) {
+              log.info(`merge-completion (${source}): ${key} is behind main — branch updated, cycle re-runs on the fresh head`);
+            } else {
+              log.warn(`merge-completion (${source}): update-branch → ${upd.status} for ${key}: ${(await upd.text()).slice(0, 200)}`);
+            }
+            return false;
+          }
+        }
         log.warn(`merge-completion (${source}): PUT merge → ${res.status} for ${key}: ${body.slice(0, 200)}`);
         return false;
       }