diff --git a/packages/isomorphic/rtti.ts b/packages/isomorphic/rtti.ts
index ffef6c1cad4ca..b139d3290fd6e 100644
--- a/packages/isomorphic/rtti.ts
+++ b/packages/isomorphic/rtti.ts
@@ -20,6 +20,15 @@ export function isRegExp(obj: any): obj is RegExp {
   return obj instanceof RegExp || Object.prototype.toString.call(obj) === '[object RegExp]';
 }
 
+export function isRegexString(value: string): boolean {
+  try {
+    new RegExp(value);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 export function isObject(obj: any): obj is NonNullable<object> {
   return typeof obj === 'object' && obj !== null;
 }
diff --git a/packages/playwright-core/src/tools/backend/network.ts b/packages/playwright-core/src/tools/backend/network.ts
index ce7cd8e959f02..9f33b1540c3c5 100644
--- a/packages/playwright-core/src/tools/backend/network.ts
+++ b/packages/playwright-core/src/tools/backend/network.ts
@@ -19,6 +19,7 @@ import fs from 'fs';
 import * as z from 'zod';
 
 import { getExtensionForMimeType, isTextualMimeType } from '@isomorphic/mimeType';
+import { isRegexString } from '@isomorphic/rtti';
 
 import { defineTool, defineTabTool } from './tool';
 
@@ -34,7 +35,7 @@ const requests = defineTabTool({
     description: 'Returns a numbered list of network requests since loading the page. Use browser_network_request with the number to get full details.',
     inputSchema: z.object({
       static: z.boolean().default(false).describe('Whether to include successful static resources like images, fonts, scripts, etc. Defaults to false.'),
-      filter: z.string().optional().describe('Only return requests whose URL matches this regexp (e.g. "/api/.*user").'),
+      filter: z.string().optional().refine(v => !v || isRegexString(v), { message: 'Invalid regular expression' }).describe('Only return requests whose URL matches this regexp (e.g. "/api/.*user").'),
       filename: z.string().optional().describe('Filename to save the network requests to. If not provided, requests are returned as text.'),
     }),
     type: 'readOnly',
diff --git a/tests/mcp/network.spec.ts b/tests/mcp/network.spec.ts
index 7efc31f6f255e..f72045db6ac05 100644
--- a/tests/mcp/network.spec.ts
+++ b/tests/mcp/network.spec.ts
@@ -94,6 +94,21 @@ test('browser_network_requests filter', async ({ client, server }) => {
   }
 });
 
+test('browser_network_requests rejects invalid regex filter', async ({ client, server }) => {
+  server.setContent('/', '', 'text/html');
+
+  await client.callTool({
+    name: 'browser_navigate',
+    arguments: { url: server.PREFIX },
+  });
+
+  const response = await client.callTool({
+    name: 'browser_network_requests',
+    arguments: { filter: '[invalid(' },
+  });
+  expect(response.isError).toBe(true);
+});
+
 test('browser_network_requests numbers requests with stable indexes', async ({ client, server }) => {
   server.setContent('/', '', 'text/html');