diff --git a/extensions/google-gemini-cli-auth/README.md b/extensions/google-gemini-cli-auth/README.md
index 6e4bdbd2b..99eab057f 100644
--- a/extensions/google-gemini-cli-auth/README.md
+++ b/extensions/google-gemini-cli-auth/README.md
@@ -18,7 +18,18 @@ Restart the Gateway after enabling.
 clawdbot models auth login --provider google-gemini-cli --set-default
 ```
 
-## Env vars
+## Requirements
+
+Requires the Gemini CLI to be installed (credentials are extracted automatically):
+
+```bash
+brew install gemini-cli
+# or: npm install -g @google/gemini-cli
+```
+
+## Env vars (optional)
+
+Override auto-detected credentials with:
 
 - `CLAWDBOT_GEMINI_OAUTH_CLIENT_ID` / `GEMINI_CLI_OAUTH_CLIENT_ID`
 - `CLAWDBOT_GEMINI_OAUTH_CLIENT_SECRET` / `GEMINI_CLI_OAUTH_CLIENT_SECRET`
diff --git a/extensions/google-gemini-cli-auth/oauth.test.ts b/extensions/google-gemini-cli-auth/oauth.test.ts
new file mode 100644
index 000000000..9d186643a
--- /dev/null
+++ b/extensions/google-gemini-cli-auth/oauth.test.ts
@@ -0,0 +1,145 @@
+import { describe, expect, it, vi, beforeEach, afterEach } from "vitest";
+import { join } from "node:path";
+
+// Mock fs module before importing the module under test
+const mockExistsSync = vi.fn();
+const mockReadFileSync = vi.fn();
+const mockRealpathSync = vi.fn();
+const mockReaddirSync = vi.fn();
+
+vi.mock("node:fs", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("node:fs")>();
+  return {
+    ...actual,
+    existsSync: (...args: Parameters<typeof actual.existsSync>) => mockExistsSync(...args),
+    readFileSync: (...args: Parameters<typeof actual.readFileSync>) => mockReadFileSync(...args),
+    realpathSync: (...args: Parameters<typeof actual.realpathSync>) => mockRealpathSync(...args),
+    readdirSync: (...args: Parameters<typeof actual.readdirSync>) => mockReaddirSync(...args),
+  };
+});
+
+describe("extractGeminiCliCredentials", () => {
+  const FAKE_CLIENT_ID = "123456789-abcdef.apps.googleusercontent.com";
+  const FAKE_CLIENT_SECRET = "GOCSPX-FakeSecretValue123";
+  const FAKE_OAUTH2_CONTENT = `
+    const clientId = "${FAKE_CLIENT_ID}";
+    const clientSecret = "${FAKE_CLIENT_SECRET}";
+  `;
+
+  let originalPath: string | undefined;
+
+  beforeEach(async () => {
+    vi.resetModules();
+    vi.clearAllMocks();
+    originalPath = process.env.PATH;
+  });
+
+  afterEach(() => {
+    process.env.PATH = originalPath;
+  });
+
+  it("returns null when gemini binary is not in PATH", async () => {
+    process.env.PATH = "/nonexistent";
+    mockExistsSync.mockReturnValue(false);
+
+    const { extractGeminiCliCredentials, clearCredentialsCache } = await import("./oauth.js");
+    clearCredentialsCache();
+    expect(extractGeminiCliCredentials()).toBeNull();
+  });
+
+  it("extracts credentials from oauth2.js in known path", async () => {
+    const fakeBinDir = "/fake/bin";
+    const fakeGeminiPath = join(fakeBinDir, "gemini");
+    const fakeResolvedPath = "/fake/lib/node_modules/@google/gemini-cli/dist/index.js";
+    const fakeOauth2Path =
+      "/fake/lib/node_modules/@google/gemini-cli/node_modules/@google/gemini-cli-core/dist/src/code_assist/oauth2.js";
+
+    process.env.PATH = fakeBinDir;
+
+    mockExistsSync.mockImplementation((p: string) => {
+      if (p === fakeGeminiPath) return true;
+      if (p === fakeOauth2Path) return true;
+      return false;
+    });
+    mockRealpathSync.mockReturnValue(fakeResolvedPath);
+    mockReadFileSync.mockReturnValue(FAKE_OAUTH2_CONTENT);
+
+    const { extractGeminiCliCredentials, clearCredentialsCache } = await import("./oauth.js");
+    clearCredentialsCache();
+    const result = extractGeminiCliCredentials();
+
+    expect(result).toEqual({
+      clientId: FAKE_CLIENT_ID,
+      clientSecret: FAKE_CLIENT_SECRET,
+    });
+  });
+
+  it("returns null when oauth2.js cannot be found", async () => {
+    const fakeBinDir = "/fake/bin";
+    const fakeGeminiPath = join(fakeBinDir, "gemini");
+    const fakeResolvedPath = "/fake/lib/node_modules/@google/gemini-cli/dist/index.js";
+
+    process.env.PATH = fakeBinDir;
+
+    mockExistsSync.mockImplementation((p: string) => p === fakeGeminiPath);
+    mockRealpathSync.mockReturnValue(fakeResolvedPath);
+    mockReaddirSync.mockReturnValue([]); // Empty directory for recursive search
+
+    const { extractGeminiCliCredentials, clearCredentialsCache } = await import("./oauth.js");
+    clearCredentialsCache();
+    expect(extractGeminiCliCredentials()).toBeNull();
+  });
+
+  it("returns null when oauth2.js lacks credentials", async () => {
+    const fakeBinDir = "/fake/bin";
+    const fakeGeminiPath = join(fakeBinDir, "gemini");
+    const fakeResolvedPath = "/fake/lib/node_modules/@google/gemini-cli/dist/index.js";
+    const fakeOauth2Path =
+      "/fake/lib/node_modules/@google/gemini-cli/node_modules/@google/gemini-cli-core/dist/src/code_assist/oauth2.js";
+
+    process.env.PATH = fakeBinDir;
+
+    mockExistsSync.mockImplementation((p: string) => {
+      if (p === fakeGeminiPath) return true;
+      if (p === fakeOauth2Path) return true;
+      return false;
+    });
+    mockRealpathSync.mockReturnValue(fakeResolvedPath);
+    mockReadFileSync.mockReturnValue("// no credentials here");
+
+    const { extractGeminiCliCredentials, clearCredentialsCache } = await import("./oauth.js");
+    clearCredentialsCache();
+    expect(extractGeminiCliCredentials()).toBeNull();
+  });
+
+  it("caches credentials after first extraction", async () => {
+    const fakeBinDir = "/fake/bin";
+    const fakeGeminiPath = join(fakeBinDir, "gemini");
+    const fakeResolvedPath = "/fake/lib/node_modules/@google/gemini-cli/dist/index.js";
+    const fakeOauth2Path =
+      "/fake/lib/node_modules/@google/gemini-cli/node_modules/@google/gemini-cli-core/dist/src/code_assist/oauth2.js";
+
+    process.env.PATH = fakeBinDir;
+
+    mockExistsSync.mockImplementation((p: string) => {
+      if (p === fakeGeminiPath) return true;
+      if (p === fakeOauth2Path) return true;
+      return false;
+    });
+    mockRealpathSync.mockReturnValue(fakeResolvedPath);
+    mockReadFileSync.mockReturnValue(FAKE_OAUTH2_CONTENT);
+
+    const { extractGeminiCliCredentials, clearCredentialsCache } = await import("./oauth.js");
+    clearCredentialsCache();
+
+    // First call
+    const result1 = extractGeminiCliCredentials();
+    expect(result1).not.toBeNull();
+
+    // Second call should use cache (readFileSync not called again)
+    const readCount = mockReadFileSync.mock.calls.length;
+    const result2 = extractGeminiCliCredentials();
+    expect(result2).toEqual(result1);
+    expect(mockReadFileSync.mock.calls.length).toBe(readCount);
+  });
+});
diff --git a/extensions/google-gemini-cli-auth/oauth.ts b/extensions/google-gemini-cli-auth/oauth.ts
index 0fc68aa5a..405b94641 100644
--- a/extensions/google-gemini-cli-auth/oauth.ts
+++ b/extensions/google-gemini-cli-auth/oauth.ts
@@ -1,6 +1,7 @@
 import { createHash, randomBytes } from "node:crypto";
-import { readFileSync } from "node:fs";
+import { existsSync, readFileSync, readdirSync, realpathSync } from "node:fs";
 import { createServer } from "node:http";
+import { delimiter, dirname, join } from "node:path";
 
 const CLIENT_ID_KEYS = ["CLAWDBOT_GEMINI_OAUTH_CLIENT_ID", "GEMINI_CLI_OAUTH_CLIENT_ID"];
 const CLIENT_SECRET_KEYS = [
@@ -47,15 +48,98 @@ function resolveEnv(keys: string[]): string | undefined {
   return undefined;
 }
 
-function resolveOAuthClientConfig(): { clientId: string; clientSecret?: string } {
-  const clientId = resolveEnv(CLIENT_ID_KEYS);
-  if (!clientId) {
-    throw new Error(
-      "Missing Gemini OAuth client ID. Set CLAWDBOT_GEMINI_OAUTH_CLIENT_ID (or GEMINI_CLI_OAUTH_CLIENT_ID).",
-    );
+let cachedGeminiCliCredentials: { clientId: string; clientSecret: string } | null = null;
+
+/** @internal */
+export function clearCredentialsCache(): void {
+  cachedGeminiCliCredentials = null;
+}
+
+/** Extracts OAuth credentials from the installed Gemini CLI's bundled oauth2.js. */
+export function extractGeminiCliCredentials(): { clientId: string; clientSecret: string } | null {
+  if (cachedGeminiCliCredentials) return cachedGeminiCliCredentials;
+
+  try {
+    const geminiPath = findInPath("gemini");
+    if (!geminiPath) return null;
+
+    const resolvedPath = realpathSync(geminiPath);
+    const geminiCliDir = dirname(dirname(resolvedPath));
+
+    const searchPaths = [
+      join(geminiCliDir, "node_modules", "@google", "gemini-cli-core", "dist", "src", "code_assist", "oauth2.js"),
+      join(geminiCliDir, "node_modules", "@google", "gemini-cli-core", "dist", "code_assist", "oauth2.js"),
+    ];
+
+    let content: string | null = null;
+    for (const p of searchPaths) {
+      if (existsSync(p)) {
+        content = readFileSync(p, "utf8");
+        break;
+      }
+    }
+    if (!content) {
+      const found = findFile(geminiCliDir, "oauth2.js", 10);
+      if (found) content = readFileSync(found, "utf8");
+    }
+    if (!content) return null;
+
+    const idMatch = content.match(/(\d+-[a-z0-9]+\.apps\.googleusercontent\.com)/);
+    const secretMatch = content.match(/(GOCSPX-[A-Za-z0-9_-]+)/);
+    if (idMatch && secretMatch) {
+      cachedGeminiCliCredentials = { clientId: idMatch[1], clientSecret: secretMatch[1] };
+      return cachedGeminiCliCredentials;
+    }
+  } catch {
+    // Gemini CLI not installed or extraction failed
   }
-  const clientSecret = resolveEnv(CLIENT_SECRET_KEYS);
-  return { clientId, clientSecret };
+  return null;
+}
+
+function findInPath(name: string): string | null {
+  const exts = process.platform === "win32" ? [".cmd", ".bat", ".exe", ""] : [""];
+  for (const dir of (process.env.PATH ?? "").split(delimiter)) {
+    for (const ext of exts) {
+      const p = join(dir, name + ext);
+      if (existsSync(p)) return p;
+    }
+  }
+  return null;
+}
+
+function findFile(dir: string, name: string, depth: number): string | null {
+  if (depth <= 0) return null;
+  try {
+    for (const e of readdirSync(dir, { withFileTypes: true })) {
+      const p = join(dir, e.name);
+      if (e.isFile() && e.name === name) return p;
+      if (e.isDirectory() && !e.name.startsWith(".")) {
+        const found = findFile(p, name, depth - 1);
+        if (found) return found;
+      }
+    }
+  } catch {}
+  return null;
+}
+
+function resolveOAuthClientConfig(): { clientId: string; clientSecret?: string } {
+  // 1. Check env vars first (user override)
+  const envClientId = resolveEnv(CLIENT_ID_KEYS);
+  const envClientSecret = resolveEnv(CLIENT_SECRET_KEYS);
+  if (envClientId) {
+    return { clientId: envClientId, clientSecret: envClientSecret };
+  }
+
+  // 2. Try to extract from installed Gemini CLI
+  const extracted = extractGeminiCliCredentials();
+  if (extracted) {
+    return extracted;
+  }
+
+  // 3. No credentials available
+  throw new Error(
+    "Gemini CLI not found. Install it first: brew install gemini-cli (or npm install -g @google/gemini-cli), or set GEMINI_CLI_OAUTH_CLIENT_ID.",
+  );
 }
 
 function isWSL(): boolean {