diff --git a/src/adapters/client-fingerprint.ts b/src/adapters/client-fingerprint.ts
index 91a51ea39a..e01880a938 100644
--- a/src/adapters/client-fingerprint.ts
+++ b/src/adapters/client-fingerprint.ts
@@ -49,15 +49,19 @@ const ANTIGRAVITY_IDE_PLATFORM = "windows/amd64";
 export const ANTIGRAVITY_GOOG_API_CLIENT_UA = "google-api-nodejs-client/10.3.0";
 
 /**
- * The real Antigravity IDE User-Agent, e.g.
- * `antigravity/ide/2.5.5 (aidev_client; os_type=windows; arch=amd64)`.
+ * Real Antigravity IDE User-Agent format, decompiled from 2.5.5 Go LS (`setHeaders` @ `0x1018fbe00`):
+ * `antigravity/ide/${version} (os_type=${osType}; arch=${arch}; aidev_client; auth_method=oauth)`
  *
- * Must be the IDE client family, NOT `antigravity/cli/...`: the Cloud Code Assist backend gates
+ * Token ordering from decompiled binary: `os_type` -> `arch` -> `aidev_client` -> `auth_method=oauth`.
+ *
+ * Must be the IDE client family (`antigravity/ide/...`): Cloud Code Assist backend gates
  * newer agent models (e.g. `gemini-3.7-flash`) by User-Agent and answers 404 NOT_FOUND to
  * CLI-shaped UAs even with a valid OAuth token. Only `antigravity/ide/<ver>` unlocks them.
  * A `GOOGLE_ANTIGRAVITY_USER_AGENT` override (set by the caller) takes precedence upstream.
  */
-export function antigravityUserAgent(version = ANTIGRAVITY_IDE_VERSION): string {
+export function antigravityUserAgent(version = ANTIGRAVITY_IDE_VERSION, authMethod = "oauth"): string {
+  const ov = process.env.GOOGLE_ANTIGRAVITY_USER_AGENT?.trim();
+  if (ov) return ov;
   const [osType, arch] = ANTIGRAVITY_IDE_PLATFORM.split("/");
-  return `antigravity/ide/${version} (${ANTIGRAVITY_IDE_CLIENT_NAME}; os_type=${osType}; arch=${arch})`;
+  return `antigravity/ide/${version} (os_type=${osType}; arch=${arch}; ${ANTIGRAVITY_IDE_CLIENT_NAME}; auth_method=${authMethod})`;
 }
diff --git a/src/adapters/google-antigravity-wire.ts b/src/adapters/google-antigravity-wire.ts
index e516b64c63..b4b0ca3176 100644
--- a/src/adapters/google-antigravity-wire.ts
+++ b/src/adapters/google-antigravity-wire.ts
@@ -9,7 +9,7 @@ import { antigravityUserAgent } from "./client-fingerprint";
  * 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();
+export const ANTIGRAVITY_REQUEST_UA = antigravityUserAgent();
 
 /**
  * Whether a stored `OcxToolCall.thoughtSignature` is a REAL upstream Gemini signature versus a
diff --git a/tests/client-fingerprint.test.ts b/tests/client-fingerprint.test.ts
index 9e30d494ef..9f681fdb51 100644
--- a/tests/client-fingerprint.test.ts
+++ b/tests/client-fingerprint.test.ts
@@ -21,24 +21,38 @@ function parsed(): OcxParsedRequest {
 describe("client fingerprint — helpers", () => {
   test("antigravity UA has the real IDE shape, never the literal giveaway", async () => {
     const ua = antigravityUserAgent();
-    expect(ua).toBe(`antigravity/ide/${ANTIGRAVITY_IDE_VERSION} (aidev_client; os_type=windows; arch=amd64)`);
+    expect(ua).toBe(`antigravity/ide/${ANTIGRAVITY_IDE_VERSION} (os_type=windows; arch=amd64; aidev_client; auth_method=oauth)`);
     expect(ua).not.toBe("antigravity");
   });
 
-  test("antigravity UA honors an explicit version override", async () => {
-    expect(antigravityUserAgent("9.9.9")).toBe("antigravity/ide/9.9.9 (aidev_client; os_type=windows; arch=amd64)");
+  test("antigravity UA honors explicit version and authMethod overrides", async () => {
+    expect(antigravityUserAgent("9.9.9")).toBe("antigravity/ide/9.9.9 (os_type=windows; arch=amd64; aidev_client; auth_method=oauth)");
+    expect(antigravityUserAgent(ANTIGRAVITY_IDE_VERSION, "api_key")).toBe(
+      `antigravity/ide/${ANTIGRAVITY_IDE_VERSION} (os_type=windows; arch=amd64; aidev_client; auth_method=api_key)`,
+    );
   });
 
-  test("GOOGLE_ANTIGRAVITY_USER_AGENT env override wins over the default UA", async () => {
-    const prev = process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
-    process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = "custom-ua/1.2.3";
+  test("GOOGLE_ANTIGRAVITY_USER_AGENT env override trims surrounding whitespace", async () => {
+    const prevGoogle = process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
     try {
-      // Fresh module instance so the env-driven constant is re-evaluated at import time.
-      const mod = await import(`../src/adapters/google-antigravity-wire?override=${Date.now()}`);
-      expect(mod.ANTIGRAVITY_REQUEST_UA).toBe("custom-ua/1.2.3");
+      process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = "  custom-ua/1.2.3  ";
+      expect(antigravityUserAgent()).toBe("custom-ua/1.2.3");
     } finally {
-      if (prev === undefined) delete process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
-      else process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = prev;
+      if (prevGoogle === undefined) delete process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
+      else process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = prevGoogle;
+    }
+  });
+
+  test("whitespace-only GOOGLE_ANTIGRAVITY_USER_AGENT falls back to default UA", async () => {
+    const prevGoogle = process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
+    try {
+      process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = "   ";
+      expect(antigravityUserAgent()).toBe(
+        `antigravity/ide/${ANTIGRAVITY_IDE_VERSION} (os_type=windows; arch=amd64; aidev_client; auth_method=oauth)`,
+      );
+    } finally {
+      if (prevGoogle === undefined) delete process.env.GOOGLE_ANTIGRAVITY_USER_AGENT;
+      else process.env.GOOGLE_ANTIGRAVITY_USER_AGENT = prevGoogle;
     }
   });
 
diff --git a/tests/google-antigravity-wire.test.ts b/tests/google-antigravity-wire.test.ts
index cc3b0d458f..b7274aed31 100644
--- a/tests/google-antigravity-wire.test.ts
+++ b/tests/google-antigravity-wire.test.ts
@@ -59,7 +59,7 @@ describe("antigravity CCA envelope", () => {
     // 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)",
+      "antigravity/ide/2.5.5 (os_type=windows; arch=amd64; aidev_client; auth_method=oauth)",
     );
     // The literal "antigravity" giveaway UA must no longer be sent.
     expect(req.headers["User-Agent"]).not.toBe("antigravity");