fix(mcp): report invalid tool arguments

packages/playwright-core/src/tools/backend/browserBackend.ts

@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+import * as z from 'zod';
 import debug from 'debug';
 import { Context } from './context';
 import { Response } from './response';
@@ -63,8 +64,14 @@ export class BrowserBackend implements ServerBackend {
     const tool = this._tools.find(tool => tool.schema.name === name)!;
     if (!tool)
       return formatError(`Tool "${name}" not found`);
-    // eslint-disable-next-line no-restricted-syntax
-    const parsedArguments = tool.schema.inputSchema.parse(rawArguments) as any;
+    let parsedArguments: any;
+    try {
+      parsedArguments = tool.schema.inputSchema.parse(rawArguments);
+    } catch (error) {
+      if (error instanceof z.ZodError)
+        return formatError(`Invalid arguments for tool "${name}":\n${z.prettifyError(error)}`);
+      throw error;
+    }
     const cwd = rawArguments._meta?.cwd;
     const raw = !!rawArguments._meta?.raw;
     const context = this._context!;

tests/mcp/validation.spec.ts

@@ -0,0 +1,45 @@
+/**
+ * Copyright (c) Microsoft Corporation.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { test, expect, parseResponse } from './fixtures';
+
+test('reports missing required tool arguments', async ({ client }) => {
+  const response = await client.callTool({
+    name: 'browser_navigate',
+    arguments: {},
+  });
+
+  expect(response).toHaveResponse({
+    isError: true,
+    error: expect.stringContaining('Invalid arguments for tool "browser_navigate":'),
+  });
+  const parsed = parseResponse(response);
+  expect(parsed.error).toContain('Invalid input: expected string');
+  expect(parsed.error).toContain('at url');
+});
+
+test('reports invalid tool argument types', async ({ client }) => {
+  const response = await client.callTool({
+    name: 'browser_navigate',
+    arguments: { url: 123 },
+  });
+
+  const parsed = parseResponse(response);
+  expect(parsed.isError).toBe(true);
+  expect(parsed.error).toContain('Invalid arguments for tool "browser_navigate":');
+  expect(parsed.error).toContain('Invalid input: expected string, received number');
+  expect(parsed.error).toContain('at url');
+});