diff --git a/src/providers/quota/antigravity.ts b/src/providers/quota/antigravity.ts
index 928b10e2570..3fb6489b6ab 100644
--- a/src/providers/quota/antigravity.ts
+++ b/src/providers/quota/antigravity.ts
@@ -167,6 +167,17 @@ const ANTIGRAVITY_ACCOUNT_QUOTA_BASE = "https://daily-cloudcode-pa.googleapis.co
 const ANTIGRAVITY_QUOTA_SUMMARY_URL = `${ANTIGRAVITY_ACCOUNT_QUOTA_BASE}/v1internal:retrieveUserQuotaSummary`;
 const ANTIGRAVITY_QUOTA_MODELS_URL = `${ANTIGRAVITY_ACCOUNT_QUOTA_BASE}/v1internal:fetchAvailableModels`;
 
+/**
+ * Compatibility fingerprint for quota accounting only.
+ *
+ * Some otherwise healthy Antigravity OAuth identities return HTTP 403 from
+ * retrieveUserQuotaSummary when the request uses the current IDE fingerprint while accepting the
+ * same bearer/project with the older Antigravity client family. Inference and model discovery must
+ * keep the IDE fingerprint because model availability is UA-gated; this one-shot fallback is
+ * deliberately scoped to a 403 from the quota-summary endpoint.
+ */
+export const ANTIGRAVITY_QUOTA_COMPAT_USER_AGENT = "antigravity/1.0";
+
 /** Only these fixed accounting destinations may use transparent Fake-IP DNS. */
 export function isCanonicalAntigravityQuotaUrl(name: string, url: string): boolean {
   return name === "google-antigravity"
@@ -253,17 +264,26 @@ function antigravityUnavailableFailure(
 }
 
 export async function probeAntigravityUsageQuota(accessToken: string, projectId: string): Promise<AntigravityQuotaProbeResult> {
-  const fetchQuota = (url: string) => providerOutboundPost("google-antigravity", { baseUrl: ANTIGRAVITY_ACCOUNT_QUOTA_BASE }, url, {
+  const fetchQuota = (url: string, userAgent = antigravityUserAgent()) => providerOutboundPost("google-antigravity", { baseUrl: ANTIGRAVITY_ACCOUNT_QUOTA_BASE }, url, {
     headers: {
       Accept: "application/json", "Content-Type": "application/json",
-      "User-Agent": antigravityUserAgent(), Authorization: `Bearer ${accessToken}`,
+      "User-Agent": userAgent, Authorization: `Bearer ${accessToken}`,
     },
     body: JSON.stringify({ project: projectId }), signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
   }, antigravityOutboundDependencies);
   let summaryFailure: QuotaFailureCode | undefined;
   try {
-    const response = await fetchQuota(ANTIGRAVITY_QUOTA_SUMMARY_URL);
+    let response = await fetchQuota(ANTIGRAVITY_QUOTA_SUMMARY_URL);
     if (await providerRedirectError(response, ANTIGRAVITY_QUOTA_SUMMARY_URL)) return unavailableAntigravityQuota("redirect_blocked");
+    if (response.status === 403) {
+      // Google currently admits some healthy Antigravity accounts to inference and quota
+      // accounting while rejecting the newer IDE fingerprint on the summary endpoint itself.
+      // Retry exactly once with the older Antigravity client family; 401 is deliberately not
+      // retried because it is an authentication failure, not a fingerprint compatibility case.
+      try { await response.body?.cancel(); } catch { /* best effort */ }
+      response = await fetchQuota(ANTIGRAVITY_QUOTA_SUMMARY_URL, ANTIGRAVITY_QUOTA_COMPAT_USER_AGENT);
+      if (await providerRedirectError(response, ANTIGRAVITY_QUOTA_SUMMARY_URL)) return unavailableAntigravityQuota("redirect_blocked");
+    }
     if (response.status === 401 || response.status === 403) return unavailableAntigravityQuota("access_denied");
     if (response.ok) {
       const quota = parseAntigravityQuotaSummary(asRecord(await readQuotaJson(response)));
diff --git a/tests/providers/provider-account-quota.test.ts b/tests/providers/provider-account-quota.test.ts
index 2ddae55d6dd..d09b33059b9 100644
--- a/tests/providers/provider-account-quota.test.ts
+++ b/tests/providers/provider-account-quota.test.ts
@@ -18,6 +18,8 @@ import {
   providerOAuthAccountQuotaMode,
 } from "../../src/providers/quota";
 import { PROXY_ENV_KEYS } from "../../src/lib/proxy-env";
+import { antigravityUserAgent } from "../../src/adapters/client-fingerprint";
+import { ANTIGRAVITY_QUOTA_COMPAT_USER_AGENT } from "../../src/providers/quota/antigravity";
 import { removeTreeWithRetry } from "../helpers/remove-tree";
 
 const originalFetch = globalThis.fetch;
@@ -738,6 +740,66 @@ describe("google-antigravity per-account quota (#1082)", () => {
     }
   });
 
+  test("retries a 403 quota summary once with the Antigravity compatibility UA", async () => {
+    const expires = Date.now() + 60 * 60_000;
+    await saveCredential("google-antigravity", {
+      access: "agy-compat", refresh: "r-compat", expires,
+      projectId: "proj-compat", accountId: "agy-compat-account", email: "compat@example.com",
+    });
+    globalThis.fetch = (async () => { throw new Error("plain fetch must not be used for account bearers"); }) as typeof fetch;
+
+    const seen: Array<{ url: string; userAgent: string | null }> = [];
+    setAntigravityAccountQuotaTransportForTests({
+      resolveAddresses: async () => ({
+        hostname: "daily-cloudcode-pa.googleapis.com",
+        addresses: [{ address: "142.250.0.1", family: 4 }],
+        privateNetwork: false,
+      }),
+      pinnedPost: async (url, _pinned, _body, _signal, requestOptions) => {
+        const userAgent = new Headers(requestOptions?.headers).get("user-agent");
+        seen.push({ url, userAgent });
+        if (seen.length === 1) return new Response(null, { status: 403 });
+        return new Response(antigravitySummaryBody(0.86, 0.38), {
+          status: 200,
+          headers: { "content-type": "application/json" },
+        });
+      },
+    });
+
+    const [row] = await fetchProviderAccountQuotas("google-antigravity", true);
+    expect(row?.unavailable).not.toBe(true);
+    expect(row?.quotaFailure).toBeUndefined();
+    expect(row?.quota?.customWindows).toHaveLength(4);
+    expect(seen).toEqual([
+      { url: summaryUrl, userAgent: antigravityUserAgent() },
+      { url: summaryUrl, userAgent: ANTIGRAVITY_QUOTA_COMPAT_USER_AGENT },
+    ]);
+  });
+
+  test("does not hide a 401 behind the quota compatibility retry", async () => {
+    const expires = Date.now() + 60 * 60_000;
+    await saveCredential("google-antigravity", {
+      access: "agy-auth-fail", refresh: "r-auth-fail", expires,
+      projectId: "proj-auth-fail", accountId: "agy-auth-fail-account", email: "auth-fail@example.com",
+    });
+    const seen: string[] = [];
+    setAntigravityAccountQuotaTransportForTests({
+      resolveAddresses: async () => ({
+        hostname: "daily-cloudcode-pa.googleapis.com",
+        addresses: [{ address: "142.250.0.1", family: 4 }],
+        privateNetwork: false,
+      }),
+      pinnedPost: async (url, _pinned, _body, _signal, requestOptions) => {
+        seen.push(new Headers(requestOptions?.headers).get("user-agent") ?? "");
+        return new Response(null, { status: 401 });
+      },
+    });
+
+    const [row] = await fetchProviderAccountQuotas("google-antigravity", true);
+    expect(row).toMatchObject({ unavailable: true, quotaFailure: "access_denied" });
+    expect(seen).toEqual([antigravityUserAgent()]);
+  });
+
   test("falls back to fetchAvailableModels when retrieveUserQuotaSummary returns 404", async () => {
     const expires = Date.now() + 60 * 60_000;
     await saveCredential("google-antigravity", { access: "agy-first", refresh: "r1", expires, projectId: "proj-first", accountId: "agy-a", email: "a@example.com" });
@@ -926,7 +988,7 @@ describe("google-antigravity per-account quota (#1082)", () => {
           },
         });
         expect(await fetchProviderAccountQuotas("google-antigravity")).toEqual([{ accountId: idFor("a@example.com"), quota: null, unavailable: true, quotaFailure: status < 400 ? "redirect_blocked" : "access_denied" }]);
-        expect(posted).toEqual(fallback ? [summaryUrl, modelsUrl] : [summaryUrl]);
+        expect(posted).toEqual(fallback ? [summaryUrl, modelsUrl] : status === 403 ? [summaryUrl, summaryUrl] : [summaryUrl]);
         expect(plainFetchCalls).toBe(0);
       });
     }
diff --git a/tests/providers/provider-quota.test.ts b/tests/providers/provider-quota.test.ts
index 139daa50d04..2235e33f7b4 100644
--- a/tests/providers/provider-quota.test.ts
+++ b/tests/providers/provider-quota.test.ts
@@ -3557,7 +3557,7 @@ describe("fetchProviderQuotaReports", () => {
           pinnedPost: async url => { posted.push(url); return new Response(null, { status, headers: { location: modelsUrl } }); },
         });
         expect((await fetchProviderQuotaReports(config(), true)).reports).toEqual([]);
-        expect(posted).toEqual([summaryUrl]);
+        expect(posted).toEqual(status === 403 ? [summaryUrl, summaryUrl] : [summaryUrl]);
         expect(plainFetchCalls).toEqual([]);
       });
     }