diff --git a/src/queue/processors.ts b/src/queue/processors.ts
index 01eeccc05e..195b2dba70 100644
--- a/src/queue/processors.ts
+++ b/src/queue/processors.ts
@@ -261,6 +261,7 @@ import {
 } from "../settings/agent-execution";
 import {
   ISSUE_WAKE_MAX_PRS,
+  MERGE_WAKE_MAX_PRS,
   SWEEP_FANOUT_DEDUP_MS,
   SWEEP_MAX_PRS,
   isRegateSweepDraining,
@@ -1730,6 +1731,55 @@ async function maybeEnqueueRagReindexForMergedPr(
   });
 }
 
+/**
+ * Event-driven re-gate trigger on sibling PR merge (#4005): companion to the merge-train gate. When a PR
+ * MERGES, every OTHER open PR's gate verdict can be invalidated by it (a newly-conflicting base, a duplicate
+ * cluster now missing its winner, a linked-issue cap that just freed up) with nothing proactively re-checking
+ * it -- the scheduled sweep is bounded to SWEEP_MAX_PRS per repo per ~2-minute tick and can take several
+ * cycles to reach a given sibling. Enqueue a bounded, staggered `agent-regate-pr` job per sibling right away
+ * instead of waiting for the next sweep pass to notice the drift.
+ *
+ * Fires ONLY on a genuine merge -- `action === "closed"` AND a `merged_at` timestamp (an ordinary close changed
+ * nothing on the base branch, so siblings have nothing new to react to; mirrors maybeEnqueueRagReindexForMergedPr's
+ * own merge check just above). Scoped to the SAME repos the re-gate sweep already covers (self-host convergence-
+ * allowlisted OR hosted agent-configured) -- this closes the "stale sibling" latency gap for repos already
+ * getting proactive re-gates, not a scope expansion to repos that never were. `otherOpenPullRequests` is the
+ * caller's already-fetched, already-bounded (100-row, ascending-by-number) sibling list — reused as-is rather than
+ * re-querying, so the lowest-numbered open siblings are re-gated first, same tie-break the duplicate-winner
+ * election uses elsewhere. Best-effort: enqueue failures are logged by the caller, never surfaced to the gate.
+ */
+async function maybeEnqueueSiblingRegateForMergedPr(
+  env: Env,
+  deliveryId: string,
+  repoFullName: string,
+  action: string | undefined,
+  mergedAt: string | null | undefined,
+  installationId: number,
+  settings: RepositorySettings,
+  otherOpenPullRequests: readonly PullRequestRecord[],
+): Promise<void> {
+  // action is only ever undefined before shouldProcessPullRequestPublicSurface's own action-set check has
+  // already passed at the call site, so a direct comparison (no nullish fallback needed) keeps this line's
+  // branches exhaustively reachable -- unlike maybeEnqueueRagReindexForMergedPr's `?? ""`, which predates this.
+  if (action !== "closed" || !mergedAt) return;
+  if (!(isConvergenceRepoAllowed(env, repoFullName) || isAgentConfigured(settings.autonomy))) return;
+  const siblings = otherOpenPullRequests.slice(0, MERGE_WAKE_MAX_PRS);
+  for (const [index, sibling] of siblings.entries()) {
+    const job: JobMessage = {
+      type: "agent-regate-pr",
+      deliveryId,
+      repoFullName,
+      prNumber: sibling.number,
+      installationId,
+      ...(sibling.createdAt ? { prCreatedAt: sibling.createdAt } : {}),
+    };
+    const delaySeconds = Math.min(index * 10, 600);
+    await (delaySeconds > 0
+      ? env.JOBS.send(job, { delaySeconds })
+      : env.JOBS.send(job));
+  }
+}
+
 // Recompute the DETERMINISTIC gate verdict for a repo's stalest open PRs and record it as an audit event —
 // ADVISORY ONLY: nothing is published to GitHub (no check, comment, or label) and no PR is mutated. This is
 // the Phase-0 scheduling rail; the action layer (#778) is what will later turn a flagged verdict into a real
@@ -6165,6 +6215,31 @@ async function processGitHubWebhook(
             }),
           );
         });
+        // Event-driven sibling re-gate (#4005): a merge can invalidate every OTHER open PR's gate verdict, and
+        // otherwise nothing re-checks them until the next bounded sweep tick reaches each one. Enqueued (not run
+        // inline), same shape as the RAG re-index just above. Best-effort.
+        await maybeEnqueueSiblingRegateForMergedPr(
+          env,
+          deliveryId,
+          repoFullName,
+          payload.action,
+          payload.pull_request.merged_at,
+          installationId,
+          settings,
+          otherOpenPullRequests,
+        ).catch((error) => {
+          /* v8 ignore next -- best-effort: a sibling re-gate enqueue failure is logged, never surfaced to the gate. */
+          console.error(
+            JSON.stringify({
+              level: "warn",
+              event: "sibling_regate_enqueue_failed",
+              deliveryId,
+              repository: repoFullName,
+              pullNumber: pr.number,
+              error: errorMessage(error),
+            }),
+          );
+        });
       }
     }
 
diff --git a/src/settings/agent-sweep.ts b/src/settings/agent-sweep.ts
index 4d2e701330..8979198217 100644
--- a/src/settings/agent-sweep.ts
+++ b/src/settings/agent-sweep.ts
@@ -26,6 +26,16 @@ export const SWEEP_MAX_PRS = 3;
 // worst case ~25 x 9 = 225 REST calls, staggered by the same delaySeconds window the caller already uses.
 export const ISSUE_WAKE_MAX_PRS = 25;
 
+// Sibling-merge wake budget (#4005): companion to the merge-train gate -- when a PR MERGES, every OTHER open PR's
+// verdict can be invalidated (a newly-conflicting base, a duplicate cluster missing its winner, a linked-issue cap
+// that just freed up) with nothing proactively re-checking it until the next sweep tick. This handler fires ONCE
+// per merge, same one-shot shape as ISSUE_WAKE_MAX_PRS, but a merge is a far MORE common trigger than an issue
+// label/assignment change -- a busy repo can merge many PRs an hour, each firing this fan-out, so reusing
+// ISSUE_WAKE_MAX_PRS's 25 would let repeated merges inside one rate-limit window compound in a way the rarer
+// issue-wake trigger never does. 15 keeps each merge's worst case at 15 x 9 ≈ 135 REST calls (same ~9-REST-GET
+// per-PR re-review cost as the other agent-regate-pr fan-outs), staggered by the same delaySeconds window.
+export const MERGE_WAKE_MAX_PRS = 15;
+
 // Skip-if-fresh window: a PR touched within this span was almost certainly just gated by its webhook, so the
 // sweep leaves it alone for that brief moment to avoid racing the in-flight webhook review. Kept SHORT (2 min)
 // because the sweep is now LIGHT (re-gate + act, no AI) and runs every ~2 min — a just-approved PR must be
diff --git a/test/unit/queue.test.ts b/test/unit/queue.test.ts
index 0ea063d51a..e78d446cf9 100644
--- a/test/unit/queue.test.ts
+++ b/test/unit/queue.test.ts
@@ -67,7 +67,7 @@ import {
   fetchPullRequestFreshness,
 } from "../../src/github/pr-freshness";
 import { createTestEnv } from "../helpers/d1";
-import { ISSUE_WAKE_MAX_PRS, SWEEP_MAX_PRS } from "../../src/settings/agent-sweep";
+import { ISSUE_WAKE_MAX_PRS, MERGE_WAKE_MAX_PRS, SWEEP_MAX_PRS } from "../../src/settings/agent-sweep";
 import { AGENT_LABEL_PENDING_CLOSURE, DEFAULT_LINKED_ISSUE_HARD_RULES } from "../../src/review/linked-issue-hard-rules";
 
 vi.mock("../../src/github/pr-freshness", async (importOriginal) => {
@@ -3287,6 +3287,145 @@ describe("queue processors", () => {
     );
   });
 
+  it("sibling re-gate fan-out (#4005): a merged PR enqueues a bounded agent-regate-pr job for each open sibling PR", async () => {
+    const sent: Array<{ message: import("../../src/types").JobMessage; options?: QueueSendOptions }> = [];
+    const env = createTestEnv({
+      GITHUB_APP_PRIVATE_KEY: await generatePrivateKeyPem(),
+      GITTENSORY_REVIEW_REPOS: "owner/agent-repo",
+      JOBS: {
+        async send(message: import("../../src/types").JobMessage, options?: QueueSendOptions) {
+          sent.push(options ? { message, options } : { message });
+        },
+      } as unknown as Queue,
+    });
+    await upsertInstallation(env, { action: "created", installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" }, target_type: "Organization", repository_selection: "selected", permissions: {}, events: [] } });
+    await upsertRepositoryFromGitHub(env, { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } }, 9001);
+    await upsertRepositorySettings(env, { repoFullName: "owner/agent-repo", autonomy: { merge: "auto" }, aiReviewMode: "off", gatePack: "oss-anti-slop", gateCheckMode: "enabled", checkRunMode: "off", commentMode: "off", publicSurface: "off" });
+    for (const number of [10, 11, 12]) {
+      await upsertPullRequestFromGitHub(env, "owner/agent-repo", { number, title: `Sibling PR ${number}`, state: "open", user: { login: "contributor" }, head: { sha: `sib${number}` }, labels: [], body: "No linked issue.", created_at: `2026-07-0${number - 9}T00:00:00.000Z` });
+    }
+    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("/check-runs")) return Response.json({ id: 900 }, { status: 201 });
+      if (url.includes("/files")) return Response.json([]);
+      return new Response("not found", { status: 404 });
+    });
+
+    await processJob(env, {
+      type: "github-webhook",
+      deliveryId: "sibling-merge-fanout",
+      eventName: "pull_request",
+      payload: {
+        action: "closed",
+        installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" } },
+        repository: { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } },
+        pull_request: { number: 999, title: "Merged PR", state: "closed", merged_at: "2026-07-08T00:00:00.000Z", user: { login: "contributor" }, head: { sha: "mergedsha" }, labels: [], body: "No linked issue." },
+      } as never,
+    });
+
+    // Bounded, staggered fan-out for each OTHER open PR — never for the merged PR's own number.
+    const regateJobs = sent.filter(({ message }) => message.type === "agent-regate-pr");
+    expect(regateJobs.map(({ message }) => message)).toEqual([
+      expect.objectContaining({ type: "agent-regate-pr", repoFullName: "owner/agent-repo", prNumber: 10, installationId: 9001, prCreatedAt: "2026-07-01T00:00:00.000Z" }),
+      expect.objectContaining({ type: "agent-regate-pr", repoFullName: "owner/agent-repo", prNumber: 11, installationId: 9001, prCreatedAt: "2026-07-02T00:00:00.000Z" }),
+      expect.objectContaining({ type: "agent-regate-pr", repoFullName: "owner/agent-repo", prNumber: 12, installationId: 9001, prCreatedAt: "2026-07-03T00:00:00.000Z" }),
+    ]);
+    expect(regateJobs.map(({ options }) => options)).toEqual([undefined, { delaySeconds: 10 }, { delaySeconds: 20 }]);
+  });
+
+  it("sibling re-gate fan-out (#4005): closing a PR WITHOUT a merge does not enqueue any sibling re-gate", async () => {
+    const sent: Array<{ message: import("../../src/types").JobMessage; options?: QueueSendOptions }> = [];
+    const env = createTestEnv({
+      GITHUB_APP_PRIVATE_KEY: await generatePrivateKeyPem(),
+      GITTENSORY_REVIEW_REPOS: "owner/agent-repo",
+      JOBS: {
+        async send(message: import("../../src/types").JobMessage, options?: QueueSendOptions) {
+          sent.push(options ? { message, options } : { message });
+        },
+      } as unknown as Queue,
+    });
+    await upsertInstallation(env, { action: "created", installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" }, target_type: "Organization", repository_selection: "selected", permissions: {}, events: [] } });
+    await upsertRepositoryFromGitHub(env, { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } }, 9001);
+    await upsertRepositorySettings(env, { repoFullName: "owner/agent-repo", autonomy: { merge: "auto" }, aiReviewMode: "off", gatePack: "oss-anti-slop", gateCheckMode: "enabled", checkRunMode: "off", commentMode: "off", publicSurface: "off" });
+    for (const number of [10, 11, 12]) {
+      await upsertPullRequestFromGitHub(env, "owner/agent-repo", { number, title: `Sibling PR ${number}`, state: "open", user: { login: "contributor" }, head: { sha: `sib${number}` }, labels: [], body: "No linked issue." });
+    }
+    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("/check-runs")) return Response.json({ id: 900 }, { status: 201 });
+      if (url.includes("/files")) return Response.json([]);
+      return new Response("not found", { status: 404 });
+    });
+
+    await processJob(env, {
+      type: "github-webhook",
+      deliveryId: "sibling-close-no-merge",
+      eventName: "pull_request",
+      payload: {
+        action: "closed",
+        installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" } },
+        repository: { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } },
+        pull_request: { number: 999, title: "Closed without merge", state: "closed", merged_at: null, user: { login: "contributor" }, head: { sha: "closedsha" }, labels: [], body: "No linked issue." },
+      } as never,
+    });
+
+    // An ordinary close changed nothing on the base branch — no sibling has anything new to react to.
+    const regateJobs = sent.filter(({ message }) => message.type === "agent-regate-pr");
+    expect(regateJobs).toEqual([]);
+  });
+
+  it("sibling re-gate fan-out (#4005): the fan-out is capped at MERGE_WAKE_MAX_PRS even with more open siblings", async () => {
+    const sent: Array<{ message: import("../../src/types").JobMessage; options?: QueueSendOptions }> = [];
+    const env = createTestEnv({
+      GITHUB_APP_PRIVATE_KEY: await generatePrivateKeyPem(),
+      GITTENSORY_REVIEW_REPOS: "owner/agent-repo",
+      JOBS: {
+        async send(message: import("../../src/types").JobMessage, options?: QueueSendOptions) {
+          sent.push(options ? { message, options } : { message });
+        },
+      } as unknown as Queue,
+    });
+    await upsertInstallation(env, { action: "created", installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" }, target_type: "Organization", repository_selection: "selected", permissions: {}, events: [] } });
+    await upsertRepositoryFromGitHub(env, { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } }, 9001);
+    await upsertRepositorySettings(env, { repoFullName: "owner/agent-repo", autonomy: { merge: "auto" }, aiReviewMode: "off", gatePack: "oss-anti-slop", gateCheckMode: "enabled", checkRunMode: "off", commentMode: "off", publicSurface: "off" });
+    for (let number = 1; number <= MERGE_WAKE_MAX_PRS + 2; number += 1) {
+      await upsertPullRequestFromGitHub(env, "owner/agent-repo", { number, title: `Sibling PR ${number}`, state: "open", user: { login: "contributor" }, head: { sha: `sib${number}` }, labels: [], body: "No linked issue." });
+    }
+    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("/check-runs")) return Response.json({ id: 900 }, { status: 201 });
+      if (url.includes("/files")) return Response.json([]);
+      return new Response("not found", { status: 404 });
+    });
+
+    await processJob(env, {
+      type: "github-webhook",
+      deliveryId: "sibling-merge-fanout-bounded",
+      eventName: "pull_request",
+      payload: {
+        action: "closed",
+        installation: { id: 9001, account: { login: "owner", id: 1, type: "Organization" } },
+        repository: { name: "agent-repo", full_name: "owner/agent-repo", private: false, owner: { login: "owner" } },
+        pull_request: { number: 999, title: "Merged PR", state: "closed", merged_at: "2026-07-08T00:00:00.000Z", user: { login: "contributor" }, head: { sha: "mergedsha" }, labels: [], body: "No linked issue." },
+      } as never,
+    });
+
+    // MERGE_WAKE_MAX_PRS + 2 siblings are open, but only the first MERGE_WAKE_MAX_PRS (lowest-numbered, same
+    // ordering listOtherOpenPullRequests already returns) are enqueued -- a repo with many open PRs must not be
+    // able to turn one merge into an unbounded burst of ~9-REST-GET re-gates.
+    const regateJobs = sent.filter(({ message }) => message.type === "agent-regate-pr");
+    expect(regateJobs).toHaveLength(MERGE_WAKE_MAX_PRS);
+    expect(regateJobs.map(({ message }) => (message as { prNumber: number }).prNumber)).toEqual(
+      Array.from({ length: MERGE_WAKE_MAX_PRS }, (_, index) => index + 1),
+    );
+  });
+
   it("REGRESSION (#2371): a coalesced issue-side signal schedules a trailing re-review so an add-then-remove sequence is never lost", async () => {
     // Unlike CI-completion events, same-PR issue-side events are NOT interchangeable within the window: a
     // label ADD immediately followed by a REMOVE carries genuinely different states. The first event's