diff --git a/src/sse/handlers/chat.js b/src/sse/handlers/chat.js
index 48ae3578..7aa530d3 100644
--- a/src/sse/handlers/chat.js
+++ b/src/sse/handlers/chat.js
@@ -7,7 +7,7 @@ import {
   extractApiKey,
   isValidApiKey,
 } from "../services/auth.js";
-import { handleAntigravityQuotaError } from "../services/antigravityQuota.js";
+import { handleAntigravityQuotaError, clearAntigravityStrikes } from "../services/antigravityQuota.js";
 import { getSettings } from "@/lib/localDb";
 import { getModelInfo, getComboModels } from "../services/model.js";
 import { handleChatCore } from "open-sse/handlers/chatCore.js";
@@ -302,6 +302,8 @@ async function handleSingleModelChat(body, modelStr, clientRawRequest = null, re
       },
       onRequestSuccess: async () => {
         await clearAccountError(credentials.connectionId, credentials, model);
+        // "Consecutive" strikes: a success clears the breaker for this pair.
+        clearAntigravityStrikes(credentials.connectionId, model);
       }
     });
 
diff --git a/src/sse/services/antigravityQuota.js b/src/sse/services/antigravityQuota.js
index e991614f..f27351f3 100644
--- a/src/sse/services/antigravityQuota.js
+++ b/src/sse/services/antigravityQuota.js
@@ -17,6 +17,56 @@ const inflightRefresh = new Map();
 
 const MIN_REFRESH_INTERVAL_MS = 30_000; // 30s between refreshes per connection
 
+// Strike-based circuit breaker (#3681): Google's quota API can report remaining
+// quota while generation endpoints keep returning 429 (sprint/weekly dual-pool
+// mismatch). After STRIKE_THRESHOLD 429s within the window for the same
+// connection+model, treat the optimistic quota reading as untrusted and
+// cache-block that pair instead of retry-storming upstream.
+const STRIKE_WINDOW_MS = 60_000; // strikes older than this reset the count
+const STRIKE_THRESHOLD = 3;
+const STRIKE_BLOCK_MS = 15 * 60_000;
+const strikeCounts = new Map(); // "connectionId|model" → { count, windowStart (anchored at first strike) }
+const strikeBlocks = new Map(); // "connectionId|model" → blockedUntil ms
+
+/**
+ * Re-apply active strike blocks onto a fresh quotas snapshot so the auth
+ * pre-filter (which reads this cache) keeps skipping the blocked pair across
+ * requests until the block expires — same channel as the exhausted-0% path.
+ */
+function applyActiveStrikeBlocks(connectionId, quotas) {
+  const now = Date.now();
+  for (const [key, until] of strikeBlocks) {
+    if (!key.startsWith(`${connectionId}|`)) continue;
+    if (until <= now) {
+      strikeBlocks.delete(key);
+      continue;
+    }
+    quotas[key.slice(connectionId.length + 1)] = {
+      remainingPercentage: 0,
+      resetAt: new Date(until).toISOString(),
+    };
+  }
+  return quotas;
+}
+
+/**
+ * Clear strike state for a connection|model after a successful request, so
+ * "consecutive" strikes means consecutive. Only removes a synthesized cache
+ * entry (resetAt == our block deadline); a real upstream 0% reading stays.
+ */
+export function clearAntigravityStrikes(connectionId, model) {
+  const key = `${connectionId}|${model}`;
+  strikeCounts.delete(key);
+  const until = strikeBlocks.get(key);
+  if (until === undefined) return;
+  strikeBlocks.delete(key);
+  const cached = quotaCache.get(connectionId);
+  if (cached?.[model]?.resetAt === new Date(until).toISOString()) {
+    delete cached[model];
+    quotaCache.set(connectionId, cached);
+  }
+}
+
 /**
  * Get the quota cache (read-only reference for auth.js pre-filter).
  */
@@ -69,7 +119,9 @@ async function _doRefresh(connectionId, accessToken, providerSpecificData, now)
     if (!usage?.quotas || usage.message) return null;
 
     // Update in-memory cache. Caller logs CACHE_BLOCK only if requested model is exhausted.
-    quotaCache.set(connectionId, usage.quotas);
+    // Strike blocks are re-asserted after every refresh so an optimistic
+    // upstream reading cannot resurrect a pair we just circuit-broke.
+    quotaCache.set(connectionId, applyActiveStrikeBlocks(connectionId, usage.quotas));
 
     return usage.quotas;
   } catch (e) {
@@ -89,7 +141,43 @@ export async function handleAntigravityQuotaError(connectionId, status, model, a
   // Throttle applies to error paths too: one quota request per account/30s.
   // The first 409/429 populates cache; concurrent or repeated errors reuse it.
   const quota = (await refreshAntigravityQuota(connectionId, accessToken, providerSpecificData))?.[model];
-  if (!quota || quota.remainingPercentage > 0 || !quota.resetAt) return null;
+
+  // Strike breaker: count every 429 whose quota reading is either optimistic
+  // (remaining > 0) or unavailable (quota API 403/error). 3 within the window
+  // => the pair is unhealthy regardless of what the API claims; block 15m.
+  // 409 counts too by design: Antigravity signals pool exhaustion with 409 as
+  // well (see #3561 — "skip exhausted account/model quota before upstream
+  // retry" was motivated by 409/429 pairs), and poisoning by transient 409s
+  // requires 3 of them inside 60 seconds on the same pair.
+  if (!quota || quota.remainingPercentage > 0) {
+    const key = `${connectionId}|${model}`;
+    const now = Date.now();
+    const strike = strikeCounts.get(key);
+    // Fixed window anchored at the FIRST qualifying strike: three 429s must
+    // all land within 60s of that first one, not within 60s of each other.
+    const windowStart = strike && now - strike.windowStart <= STRIKE_WINDOW_MS ? strike.windowStart : now;
+    const count = strike && windowStart === strike.windowStart ? strike.count + 1 : 1;
+    strikeCounts.set(key, { count, windowStart });
+    if (count >= STRIKE_THRESHOLD) {
+      strikeCounts.delete(key);
+      const blockedUntil = now + STRIKE_BLOCK_MS;
+      const reading = quota ? `${Math.round(quota.remainingPercentage)}%` : "unknown";
+      log.warn("AG_QUOTA", `${connectionId.slice(0, 8)} | STRIKE_${status} ${model} — ${count}x 429 (quota ${reading}); CACHE_BLOCK 15m`);
+      // Synthesize a 0% entry in the shared cache so the auth pre-filter skips
+      // this pair on subsequent requests too, not just the current retry loop
+      // (the chat handler does not persist modelLock_* for this path).
+      const cached = quotaCache.get(connectionId) || {};
+      cached[model] = { remainingPercentage: 0, resetAt: new Date(blockedUntil).toISOString() };
+      quotaCache.set(connectionId, cached);
+      strikeBlocks.set(key, blockedUntil);
+      return blockedUntil;
+    }
+    return null;
+  }
+
+  // Healthy-but-exhausted reading: clear strikes and use the exact resetAt.
+  strikeCounts.delete(`${connectionId}|${model}`);
+  if (!quota.resetAt) return null;
 
   const resetMs = new Date(quota.resetAt).getTime();
   if (resetMs <= Date.now()) return null;
diff --git a/tests/unit/antigravity-quota-routing.test.js b/tests/unit/antigravity-quota-routing.test.js
index c7778a3c..d8c060f6 100644
--- a/tests/unit/antigravity-quota-routing.test.js
+++ b/tests/unit/antigravity-quota-routing.test.js
@@ -27,7 +27,7 @@ vi.mock("open-sse/services/usage/google.js", () => ({
 }));
 vi.mock("@/sse/utils/logger.js", () => ({ debug: vi.fn(), info: vi.fn(), warn: vi.fn() }));
 
-const { getAntigravityQuotaCache, handleAntigravityQuotaError, refreshAntigravityQuota } = await import("@/sse/services/antigravityQuota.js");
+const { getAntigravityQuotaCache, handleAntigravityQuotaError, refreshAntigravityQuota, clearAntigravityStrikes } = await import("@/sse/services/antigravityQuota.js");
 const { getProviderCredentials } = await import("@/sse/services/auth.js");
 
 const MODEL = "claude-opus-4-6-thinking";
@@ -169,4 +169,143 @@ describe("Antigravity quota-aware routing", () => {
       vi.useRealTimers();
     }
   });
+
+  it("strike-breaks after 3 optimistic 429s within 60s and cache-blocks 15 minutes", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    // Quota API lies: reports 90% remaining while generation keeps 429ing.
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    try {
+      const first = await handleAntigravityQuotaError("ag-strike", 429, MODEL, "token", {});
+      expect(first).toBeNull();
+      const second = await handleAntigravityQuotaError("ag-strike", 429, MODEL, "token", {});
+      expect(second).toBeNull();
+
+      const third = await handleAntigravityQuotaError("ag-strike", 429, MODEL, "token", {});
+      expect(third).toBe(Date.parse("2026-08-26T00:15:00.000Z"));
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("resets the strike counter when strikes fall outside the 60s window", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    try {
+      await handleAntigravityQuotaError("ag-window", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-window", 429, MODEL, "token", {});
+      await vi.advanceTimersByTimeAsync(61_000);
+      const result = await handleAntigravityQuotaError("ag-window", 429, MODEL, "token", {});
+      expect(result).toBeNull(); // window lapsed — counter restarted at 1
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("strike-breaks when the quota API is unavailable (null reading) too", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    // Quota endpoint failing/forbidden => quota unknown. Strikes must still count.
+    mocks.getAntigravityUsage.mockResolvedValue({ message: "forbidden", quotas: {} });
+
+    try {
+      await handleAntigravityQuotaError("ag-null", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-null", 429, MODEL, "token", {});
+      const third = await handleAntigravityQuotaError("ag-null", 429, MODEL, "token", {});
+      expect(third).toBe(Date.parse("2026-08-26T00:15:00.000Z"));
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("persists the block into the shared cache so the next request skips the pair", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    try {
+      await handleAntigravityQuotaError("ag-persist", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-persist", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-persist", 429, MODEL, "token", {});
+
+      // The synthesized entry must be visible to the auth pre-filter reading
+      // the shared cache — and must survive an optimistic upstream refresh.
+      const cached = getAntigravityQuotaCache().get("ag-persist")?.[MODEL];
+      expect(cached).toMatchObject({ remainingPercentage: 0 });
+      expect(Date.parse(cached.resetAt)).toBe(Date.parse("2026-08-26T00:15:00.000Z"));
+
+      await refreshAntigravityQuota("ag-persist", "token", {});
+      expect(getAntigravityQuotaCache().get("ag-persist")?.[MODEL]).toMatchObject({
+        remainingPercentage: 0,
+      });
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("clears strike state and the synthesized block after a successful request", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    try {
+      await handleAntigravityQuotaError("ag-clear", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-clear", 429, MODEL, "token", {});
+      await handleAntigravityQuotaError("ag-clear", 429, MODEL, "token", {});
+      expect(getAntigravityQuotaCache().get("ag-clear")?.[MODEL]?.remainingPercentage).toBe(0);
+
+      clearAntigravityStrikes("ag-clear", MODEL);
+      // Synthesized entry gone — pair selectable again immediately.
+      expect(getAntigravityQuotaCache().get("ag-clear")?.[MODEL]).toBeUndefined();
+
+      // Two more 429s do NOT inherit earlier strikes: no block on the third-in-episode.
+      await handleAntigravityQuotaError("ag-clear", 429, MODEL, "token", {});
+      await expect(handleAntigravityQuotaError("ag-clear", 429, MODEL, "token", {})).resolves.toBeNull();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("anchors the window at the first strike: 3 strikes spread over 90s do not trip", async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-26T00:00:00.000Z"));
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    try {
+      await handleAntigravityQuotaError("ag-anchor", 429, MODEL, "token", {});      // t=0
+      await vi.advanceTimersByTimeAsync(45_000);
+      await handleAntigravityQuotaError("ag-anchor", 429, MODEL, "token", {});      // t=45s
+      await vi.advanceTimersByTimeAsync(45_000);
+      // t=90s: within 60s of strike #2 but outside 60s of strike #1 => new window
+      const result = await handleAntigravityQuotaError("ag-anchor", 429, MODEL, "token", {});
+      expect(result).toBeNull();
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it("keeps the optimistic path null without touching the quota cache", async () => {
+    mocks.getAntigravityUsage.mockResolvedValue({ quotas: {
+      [MODEL]: { remainingPercentage: 90, resetAt: FUTURE_RESET },
+    } });
+
+    await expect(handleAntigravityQuotaError("ag-optimistic", 429, MODEL, "token", {}))
+      .resolves.toBeNull();
+    // Optimistic reading must NOT poison the shared cache (auth pre-filter
+    // treats cached 0% as exhausted).
+    expect(getAntigravityQuotaCache().get("ag-optimistic")?.[MODEL]?.remainingPercentage).toBe(90);
+  });
 });