diff --git a/src/adapters/client-fingerprint.ts b/src/adapters/client-fingerprint.ts
index ca090c5669..91a51ea39a 100644
--- a/src/adapters/client-fingerprint.ts
+++ b/src/adapters/client-fingerprint.ts
@@ -40,20 +40,24 @@ export function claudeCodeSessionId(token: string | undefined): string {
   return `${h.slice(0, 8)}-${h.slice(8, 12)}-4${h.slice(13, 16)}-${variant}${h.slice(17, 20)}-${h.slice(20, 32)}`;
 }
 
-// ── Antigravity CLI ──
-/** Pinned fallback Antigravity CLI version (real client fetches a manifest; we pin to avoid the network dependency). */
-export const ANTIGRAVITY_CLI_VERSION = "1.1.12";
-const ANTIGRAVITY_CLI_CLIENT_NAME = "aidev_client";
-const ANTIGRAVITY_CLI_PLATFORM = "darwin/arm64";
+// ── Antigravity IDE ──
+/** Pinned fallback Antigravity IDE language-server version (matches the bundled LS 2.5.5). */
+export const ANTIGRAVITY_IDE_VERSION = "2.5.5";
+const ANTIGRAVITY_IDE_CLIENT_NAME = "aidev_client";
+const ANTIGRAVITY_IDE_PLATFORM = "windows/amd64";
 /** Secondary Google API client UA the Antigravity client library reports. */
 export const ANTIGRAVITY_GOOG_API_CLIENT_UA = "google-api-nodejs-client/10.3.0";
 
 /**
- * The real Antigravity CLI User-Agent, e.g.
- * `antigravity/cli/1.1.12 (aidev_client; os_type=darwin; arch=arm64)`.
+ * The real Antigravity IDE User-Agent, e.g.
+ * `antigravity/ide/2.5.5 (aidev_client; os_type=windows; arch=amd64)`.
+ *
+ * Must be the IDE client family, NOT `antigravity/cli/...`: the Cloud Code Assist backend gates
+ * newer agent models (e.g. `gemini-3.7-flash`) by User-Agent and answers 404 NOT_FOUND to
+ * CLI-shaped UAs even with a valid OAuth token. Only `antigravity/ide/<ver>` unlocks them.
  * A `GOOGLE_ANTIGRAVITY_USER_AGENT` override (set by the caller) takes precedence upstream.
  */
-export function antigravityUserAgent(version = ANTIGRAVITY_CLI_VERSION): string {
-  const [osType, arch] = ANTIGRAVITY_CLI_PLATFORM.split("/");
-  return `antigravity/cli/${version} (${ANTIGRAVITY_CLI_CLIENT_NAME}; os_type=${osType}; arch=${arch})`;
+export function antigravityUserAgent(version = ANTIGRAVITY_IDE_VERSION): string {
+  const [osType, arch] = ANTIGRAVITY_IDE_PLATFORM.split("/");
+  return `antigravity/ide/${version} (${ANTIGRAVITY_IDE_CLIENT_NAME}; os_type=${osType}; arch=${arch})`;
 }
diff --git a/src/adapters/google-antigravity-wire.ts b/src/adapters/google-antigravity-wire.ts
index 7c7a7714f6..e516b64c63 100644
--- a/src/adapters/google-antigravity-wire.ts
+++ b/src/adapters/google-antigravity-wire.ts
@@ -3,10 +3,11 @@ import type { OcxContentPart, OcxParsedRequest } from "../types";
 import { antigravityUserAgent } from "./client-fingerprint";
 
 /**
- * Antigravity request User-Agent. Mirrors the real Antigravity CLI UA
- * (`antigravity/cli/{ver} (aidev_client; os_type=darwin; arch=arm64)`) so the request fingerprint
+ * Antigravity request User-Agent. Mirrors the real Antigravity IDE UA
+ * (`antigravity/ide/{ver} (aidev_client; os_type=windows; arch=amd64)`) so the request fingerprint
  * matches the OAuth credential — the prior literal `"antigravity"` was a giveaway no real client
- * sends. A `GOOGLE_ANTIGRAVITY_USER_AGENT` override still wins.
+ * sends. The IDE client family is also required to unlock newer agent models (the backend 404s
+ * CLI-shaped UAs for `gemini-3.7-*`). A `GOOGLE_ANTIGRAVITY_USER_AGENT` override still wins.
  */
 export const ANTIGRAVITY_REQUEST_UA = process.env.GOOGLE_ANTIGRAVITY_USER_AGENT || antigravityUserAgent();
 
diff --git a/src/adapters/google.ts b/src/adapters/google.ts
index 9ccba7f33d..9a074c0506 100644
--- a/src/adapters/google.ts
+++ b/src/adapters/google.ts
@@ -469,7 +469,7 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
         const envelope = {
           model: wireModelId,
           // The envelope's `userAgent` field is a protocol constant ("antigravity"), distinct from
-          // the HTTP `User-Agent` header (the real CLI UA). CLIProxyAPI `geminiToAntigravity` hardcodes
+          // the HTTP `User-Agent` header (the real IDE UA). CLIProxyAPI `geminiToAntigravity` hardcodes
           // the body field; only the header carries the versioned client string.
           userAgent: "antigravity",
           requestType: "agent",
diff --git a/tests/client-fingerprint.test.ts b/tests/client-fingerprint.test.ts
index cf155df948..9e30d494ef 100644
--- a/tests/client-fingerprint.test.ts
+++ b/tests/client-fingerprint.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import {
-  ANTIGRAVITY_CLI_VERSION,
+  ANTIGRAVITY_IDE_VERSION,
   ANTIGRAVITY_GOOG_API_CLIENT_UA,
   CLAUDE_CODE_HEADERS,
   antigravityUserAgent,
@@ -19,14 +19,14 @@ function parsed(): OcxParsedRequest {
 }
 
 describe("client fingerprint — helpers", () => {
-  test("antigravity UA has the real CLI shape, never the literal giveaway", async () => {
+  test("antigravity UA has the real IDE shape, never the literal giveaway", async () => {
     const ua = antigravityUserAgent();
-    expect(ua).toBe("antigravity/cli/1.1.12 (aidev_client; os_type=darwin; arch=arm64)");
+    expect(ua).toBe(`antigravity/ide/${ANTIGRAVITY_IDE_VERSION} (aidev_client; os_type=windows; arch=amd64)`);
     expect(ua).not.toBe("antigravity");
   });
 
   test("antigravity UA honors an explicit version override", async () => {
-    expect(antigravityUserAgent("9.9.9")).toBe("antigravity/cli/9.9.9 (aidev_client; os_type=darwin; arch=arm64)");
+    expect(antigravityUserAgent("9.9.9")).toBe("antigravity/ide/9.9.9 (aidev_client; os_type=windows; arch=amd64)");
   });
 
   test("GOOGLE_ANTIGRAVITY_USER_AGENT env override wins over the default UA", async () => {
diff --git a/tests/google-antigravity-wire.test.ts b/tests/google-antigravity-wire.test.ts
index a5427c5584..5ea15eff2a 100644
--- a/tests/google-antigravity-wire.test.ts
+++ b/tests/google-antigravity-wire.test.ts
@@ -56,7 +56,11 @@ describe("antigravity CCA envelope", () => {
     expect(env.request.model).toBeUndefined();
     expect(env.request.safetySettings).toBeUndefined();
     expect(req.headers["Authorization"]).toBe("Bearer ya29.token");
-    expect(req.headers["User-Agent"]).toMatch(/^antigravity\/cli\/[\d.]+ \(aidev_client; os_type=\w+; arch=\w+\)$/);
+    // The exact default must not drift: Google gates models by family AND version,
+    // so any change to version/platform could silently re-lock gemini-3.7-flash.
+    expect(req.headers["User-Agent"]).toBe(
+      "antigravity/ide/2.5.5 (aidev_client; os_type=windows; arch=amd64)",
+    );
     // The literal "antigravity" giveaway UA must no longer be sent.
     expect(req.headers["User-Agent"]).not.toBe("antigravity");
     // x-goog-api-client is NOT sent on runtime requests (CLIProxyAPI only uses it during onboarding).