diff --git a/src/browser.ts b/src/browser.ts
index c5a671e0..2956e74d 100644
--- a/src/browser.ts
+++ b/src/browser.ts
@@ -54,21 +54,22 @@ async function ensureBrowserConnected(browserURL: string) {
   return browser;
 }
 
-async function ensureBrowserLaunched(
-  headless: boolean,
-  isolated: boolean,
-  customDevToolsPath?: string,
-  executablePath?: string,
-  channel?: Channel,
-): Promise<Browser> {
-  if (browser?.connected) {
-    return browser;
-  }
+type McpLaunchOptions = {
+  executablePath?: string;
+  customDevTools?: string;
+  channel?: Channel;
+  userDataDir?: string;
+  headless: boolean;
+  isolated: boolean;
+};
+
+export async function launch(options: McpLaunchOptions): Promise<Browser> {
+  const {channel, executablePath, customDevTools, headless, isolated} = options;
   const profileDirName =
     channel && channel !== 'stable' ? `mcp-profile-${channel}` : 'mcp-profile';
 
-  let userDataDir: string | undefined;
-  if (!isolated) {
+  let userDataDir = options.userDataDir;
+  if (!isolated && !userDataDir) {
     userDataDir = path.join(
       os.homedir(),
       '.cache',
@@ -85,8 +86,8 @@ async function ensureBrowserLaunched(
     '--no-first-run',
     '--hide-crash-restore-bubble',
   ];
-  if (customDevToolsPath) {
-    args.push(`--custom-devtools-frontend=file://${customDevToolsPath}`);
+  if (customDevTools) {
+    args.push(`--custom-devtools-frontend=file://${customDevTools}`);
   }
   let puppeterChannel: ChromeReleaseChannel | undefined;
   if (!executablePath) {
@@ -95,16 +96,45 @@ async function ensureBrowserLaunched(
         ? (`chrome-${channel}` as ChromeReleaseChannel)
         : 'chrome';
   }
-  browser = await puppeteer.launch({
-    ...connectOptions,
-    channel: puppeterChannel,
-    executablePath,
-    defaultViewport: null,
-    userDataDir,
-    pipe: true,
-    headless,
-    args,
-  });
+
+  try {
+    return await puppeteer.launch({
+      ...connectOptions,
+      channel: puppeterChannel,
+      executablePath,
+      defaultViewport: null,
+      userDataDir,
+      pipe: true,
+      headless,
+      args,
+    });
+  } catch (error) {
+    // TODO: check browser logs for `Failed to create a ProcessSingleton for
+    // your profile directory` instead.
+    if (
+      userDataDir &&
+      (error as Error).message.includes(
+        '(Target.setDiscoverTargets): Target closed',
+      )
+    ) {
+      throw new Error(
+        `The browser is already running for ${userDataDir}. Use --isolated to run multiple browser instances.`,
+        {
+          cause: error,
+        },
+      );
+    }
+    throw error;
+  }
+}
+
+async function ensureBrowserLaunched(
+  options: McpLaunchOptions,
+): Promise<Browser> {
+  if (browser?.connected) {
+    return browser;
+  }
+  browser = await launch(options);
   return browser;
 }
 
@@ -118,13 +148,7 @@ export async function resolveBrowser(options: {
 }) {
   const browser = options.browserUrl
     ? await ensureBrowserConnected(options.browserUrl)
-    : await ensureBrowserLaunched(
-        options.headless,
-        options.isolated,
-        options.customDevTools,
-        options.executablePath,
-        options.channel,
-      );
+    : await ensureBrowserLaunched(options);
 
   return browser;
 }
diff --git a/tests/browser.test.ts b/tests/browser.test.ts
new file mode 100644
index 00000000..fd2e392a
--- /dev/null
+++ b/tests/browser.test.ts
@@ -0,0 +1,40 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import {describe, it} from 'node:test';
+import assert from 'node:assert';
+import os from 'node:os';
+import path from 'node:path';
+import {launch} from '../src/browser.js';
+
+describe('browser', () => {
+  it('cannot launch multiple times with the same profile', async () => {
+    const tmpDir = os.tmpdir();
+    const folderPath = path.join(tmpDir, `temp-folder-${crypto.randomUUID()}`);
+    const browser1 = await launch({
+      headless: true,
+      isolated: false,
+      userDataDir: folderPath,
+    });
+    try {
+      try {
+        const browser2 = await launch({
+          headless: true,
+          isolated: false,
+          userDataDir: folderPath,
+        });
+        await browser2.close();
+        assert.fail('not reached');
+      } catch (err) {
+        assert.strictEqual(
+          err.message,
+          `The browser is already running for ${folderPath}. Use --isolated to run multiple browser instances.`,
+        );
+      }
+    } finally {
+      await browser1.close();
+    }
+  });
+});