diff --git a/packages/playwright-core/src/tools/mcp/cdpRelay.ts b/packages/playwright-core/src/tools/mcp/cdpRelay.ts
index b1a654131afe1..a75db1791d53e 100644
--- a/packages/playwright-core/src/tools/mcp/cdpRelay.ts
+++ b/packages/playwright-core/src/tools/mcp/cdpRelay.ts
@@ -34,7 +34,7 @@ import ws, { WebSocketServer as wsServer } from 'ws';
 import { ManualPromise } from '@isomorphic/manualPromise';
 import { registry } from '../../server/registry/index';
 
-import { findPlaywrightExtensionProfile, playwrightExtensionId } from '../utils/extension';
+import { defaultUserDataDir, findPlaywrightExtensionProfile, playwrightExtensionId } from '../utils/extension';
 import { addressToString } from '../utils/mcp/http';
 import { logUnhandledError } from './log';
 import { ExtensionProtocolV2 } from './cdpRelayV2';
@@ -142,7 +142,7 @@ export class CDPRelayServer {
     const testUserDataDir = process.env.PWTEST_EXTENSION_USER_DATA_DIR;
     if (testUserDataDir)
       args.push(`--user-data-dir=${testUserDataDir}`);
-    const userDataDir = testUserDataDir ?? this._userDataDir;
+    const userDataDir = testUserDataDir ?? this._userDataDir ?? defaultUserDataDir(channel);
     const profileDirectory = userDataDir ? await findPlaywrightExtensionProfile(userDataDir) : undefined;
     if (profileDirectory)
       args.push(`--profile-directory=${profileDirectory}`);
diff --git a/packages/playwright-core/src/tools/utils/extension.ts b/packages/playwright-core/src/tools/utils/extension.ts
index 94a459766cffe..15fb179fdc3c0 100644
--- a/packages/playwright-core/src/tools/utils/extension.ts
+++ b/packages/playwright-core/src/tools/utils/extension.ts
@@ -15,6 +15,7 @@
  */
 
 import fs from 'fs';
+import os from 'os';
 import path from 'path';
 
 // Also pinned via the "key" field in packages/extension/manifest.json.
@@ -22,6 +23,34 @@ export const playwrightExtensionId = 'mmlmfjhmonkocbjadbfplnigmagldckm';
 
 export const playwrightExtensionInstallUrl = `https://chromewebstore.google.com/detail/playwright-extension/${playwrightExtensionId}`;
 
+export function defaultUserDataDir(channel: string): string | undefined {
+  const home = os.homedir();
+  switch (os.platform()) {
+    case 'win32': {
+      const localAppData = process.env.LOCALAPPDATA;
+      if (!localAppData)
+        return undefined;
+      if (channel === 'chrome')
+        return path.join(localAppData, 'Google', 'Chrome', 'User Data');
+      if (channel === 'msedge')
+        return path.join(localAppData, 'Microsoft', 'Edge', 'User Data');
+      return undefined;
+    }
+    case 'darwin':
+      if (channel === 'chrome')
+        return path.join(home, 'Library', 'Application Support', 'Google', 'Chrome');
+      if (channel === 'msedge')
+        return path.join(home, 'Library', 'Application Support', 'Microsoft Edge');
+      return undefined;
+    default:
+      if (channel === 'chrome')
+        return path.join(home, '.config', 'google-chrome');
+      if (channel === 'msedge')
+        return path.join(home, '.config', 'microsoft-edge');
+      return undefined;
+  }
+}
+
 export async function findPlaywrightExtensionProfile(userDataDir: string): Promise<string | undefined> {
   const profiles = await listProfileDirectories(userDataDir);
   const lastUsed = await readLastUsedProfile(userDataDir);