diff --git a/src/queue/processors.ts b/src/queue/processors.ts
index 385fd80027..df8df255b4 100644
--- a/src/queue/processors.ts
+++ b/src/queue/processors.ts
@@ -1151,6 +1151,10 @@ export function hasVerifiedRequiredContexts(
   return requiredContexts != null && requiredContexts.size > 0;
 }
 
+export function agentMaintenanceHeadMatchesGate(reviewedHeadSha: string | null | undefined, currentHeadSha: string | null | undefined): boolean {
+  return reviewedHeadSha == null || currentHeadSha == null || currentHeadSha === reviewedHeadSha;
+}
+
 /**
  * #778 maintainer auto-maintain trigger. After the gate runs on a PR webhook, if the repo opted the agent in
  * (an acting autonomy level), reuse the CANONICAL verdict produced by the full gate evaluation, plan the
@@ -1186,6 +1190,10 @@ async function maybeRunAgentMaintenance(
   /* v8 ignore next -- defensive: the PR was upserted earlier in this same webhook, so it is always present. */
   if (!pr) return;
   if (pr.state !== "open") return;
+  // The gate verdict belongs to the webhook/re-review head that produced it. Under concurrent self-host queues, a
+  // newer synchronize can advance the stored row before this job acts; fail closed rather than pairing a stale
+  // passing gate with a newer, unreviewed head for CI, planning, or merge execution.
+  if (!agentMaintenanceHeadMatchesGate(args.pr.headSha, pr.headSha)) return;
   // Drafts are work-in-progress: never auto-approve / merge / close / label a draft. Symmetric with the re-gate
   // sweep, which drops drafts (agent-sweep.ts). A draft signals "not ready"; the agent acts once it is marked
   // ready_for_review (which re-triggers this path on the now-undrafted PR). The converted_to_draft draft-dodge
diff --git a/test/unit/queue.test.ts b/test/unit/queue.test.ts
index 0447fea8e3..5406b76b09 100644
--- a/test/unit/queue.test.ts
+++ b/test/unit/queue.test.ts
@@ -39,7 +39,7 @@ import {
   upsertRepositoryFromGitHub,
   putCachedAiReview,
 } from "../../src/db/repositories";
-import { changedPathsForGuardrail, processJob } from "../../src/queue/processors";
+import { agentMaintenanceHeadMatchesGate, changedPathsForGuardrail, processJob } from "../../src/queue/processors";
 import { upsertRepoFocusManifest } from "../../src/signals/focus-manifest-loader";
 import { normalizeRegistryPayload } from "../../src/registry/normalize";
 import { persistRegistrySnapshot } from "../../src/registry/sync";
@@ -6944,6 +6944,82 @@ describe("changedPathsForGuardrail", () => {
   });
 });
 
+describe("agentMaintenanceHeadMatchesGate", () => {
+  it("allows maintenance only when the stored PR head still matches the reviewed gate head", () => {
+    expect(agentMaintenanceHeadMatchesGate("reviewed", "reviewed")).toBe(true);
+    expect(agentMaintenanceHeadMatchesGate("reviewed", "new-unreviewed")).toBe(false);
+  });
+
+  it("keeps legacy no-SHA paths fail-open because no exact reviewed head can be pinned", () => {
+    expect(agentMaintenanceHeadMatchesGate(undefined, "current")).toBe(true);
+    expect(agentMaintenanceHeadMatchesGate("reviewed", null)).toBe(true);
+  });
+
+  it("REGRESSION (#stale-head): a newer synchronize that advances the stored head before maintenance acts blocks the stale-gate auto-merge", async () => {
+    const env = createTestEnv({ GITHUB_APP_PRIVATE_KEY: await generatePrivateKeyPem() });
+    await upsertInstallation(env, {
+      action: "created",
+      installation: {
+        id: 123,
+        account: { login: "JSONbored", id: 1, type: "User" },
+        target_type: "User",
+        repository_selection: "all",
+        permissions: { metadata: "read", pull_requests: "write", issues: "write" },
+        events: ["pull_request"],
+      },
+      repositories: [{ name: "gittensory", full_name: "JSONbored/gittensory", private: false, owner: { login: "JSONbored" } }],
+    });
+    // Clean, mergeable, approved, green CI + merge:auto + approve:auto + close:auto — this PR WOULD be auto-acted.
+    // The ONLY thing that must stop it is the stale-head guard in maybeRunAgentMaintenance.
+    await upsertRepositorySettings(env, {
+      repoFullName: "JSONbored/gittensory",
+      commentMode: "off",
+      publicSurface: "off",
+      autoLabelEnabled: false,
+      checkRunMode: "off",
+      gateCheckMode: "enabled",
+      autonomy: { merge: "auto", approve: "auto", close: "auto" },
+    });
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL) => {
+      const url = input.toString();
+      if (url === "https://api.gittensor.io/miners") return Response.json([]);
+      if (url.includes("/access_tokens")) return Response.json({ token: "installation-token" });
+      if (url.includes("/commits/stale1/check-runs")) return Response.json({ total_count: 0, check_runs: [] });
+      if (url.includes("/commits/stale1/status")) return Response.json({ statuses: [] });
+      if (url.includes("/check-runs")) return Response.json({ id: 902 }, { status: 201 });
+      return new Response("not found", { status: 404 });
+    });
+
+    // Simulate the concurrent-queue race the guard defends against: the gate evaluated head "stale1", but by the
+    // time maintenance re-reads the persisted row a newer `synchronize` has advanced the stored head to "newer2".
+    // maybeRunAgentMaintenance re-reads via getPullRequest, so divert that read to the advanced head.
+    const realGetPullRequest = repositoriesModule.getPullRequest;
+    const spy = vi.spyOn(repositoriesModule, "getPullRequest").mockImplementation(async (...callArgs) => {
+      const row = await realGetPullRequest(...callArgs);
+      return row ? { ...row, headSha: "newer2" } : row;
+    });
+    try {
+      await processJob(env, {
+        type: "github-webhook",
+        deliveryId: "stale-head-no-maintenance",
+        eventName: "pull_request",
+        payload: {
+          action: "opened",
+          installation: { id: 123, account: { login: "JSONbored", id: 1, type: "User" } },
+          repository: { name: "gittensory", full_name: "JSONbored/gittensory", private: false, owner: { login: "JSONbored" } },
+          pull_request: { number: 71, title: "Clean PR", state: "open", user: { login: "contributor" }, head: { sha: "stale1" }, labels: [], body: "Closes #1", mergeable_state: "clean", reviewDecision: "APPROVED" },
+        },
+      });
+    } finally {
+      spy.mockRestore();
+    }
+
+    // No terminal maintenance action of ANY class fires: the gate verdict belonged to the now-stale head.
+    const acted = await env.DB.prepare("select count(*) as n from audit_events where event_type in ('agent.action.merge','agent.action.approve','agent.action.close')").first<{ n: number }>();
+    expect(acted?.n).toBe(0);
+  });
+});
+
 describe("one-shot reopen prevention", () => {
   beforeEach(() => {
     clearInstallationTokenCacheForTest();