fix(mcp): report invalid tool arguments

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

@@ -63,8 +63,12 @@ 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) {
+      return formatError(formatValidationError(name, error));
+    }
     const cwd = rawArguments._meta?.cwd;
     const raw = !!rawArguments._meta?.raw;
     const context = this._context!;
@@ -94,3 +98,46 @@ function formatRejectionReason(reason: unknown): string {
     return reason.stack ?? reason.message;
   return String(reason);
 }
+
+type ValidationIssue = {
+  path?: unknown[];
+  message?: string;
+};
+
+function isValidationError(error: unknown): error is { issues: ValidationIssue[] } {
+  return !!error && typeof error === 'object' && Array.isArray((error as { issues?: unknown }).issues);
+}
+
+function formatValidationError(toolName: string, error: unknown): string {
+  if (!isValidationError(error))
+    return String(error);
+
+  const lines = error.issues.map(issue => `- ${formatIssuePath(issue.path)}: ${issue.message || 'Invalid input'}`);
+  if (!lines.length)
+    return `Invalid arguments for tool "${toolName}".`;
+  return [
+    `Invalid arguments for tool "${toolName}":`,
+    ...lines,
+  ].join('\n');
+}
+
+function formatIssuePath(path: unknown[] | undefined): string {
+  if (!path?.length)
+    return '<root>';
+
+  let result = '';
+  for (const segment of path) {
+    if (typeof segment === 'number') {
+      result += `[${segment}]`;
+      continue;
+    }
+    const text = String(segment);
+    if (!result)
+      result = text;
+    else if (/^[a-zA-Z_$][\w$]*$/.test(text))
+      result += `.${text}`;
+    else
+      result += `[${JSON.stringify(text)}]`;
+  }
+  return result;
+}

tests/mcp/validation.spec.ts

@@ -0,0 +1,123 @@
+/**
+ * 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 * as z from 'zod';
+import { tools } from '../../packages/playwright-core/lib/coreBundle';
+import { test, expect, parseResponse } from './fixtures';
+
+function createBackend() {
+  return new tools.BrowserBackend({}, {
+    once() {},
+    browser: () => undefined,
+  } as any, [{
+    capability: 'core',
+    schema: {
+      name: 'browser_validate',
+      title: 'Validate',
+      description: 'Validate arguments',
+      inputSchema: z.object({
+        url: z.string(),
+      }),
+      type: 'action',
+    },
+    handle: async () => {
+      throw new Error('Tool handler should not run for invalid arguments');
+    },
+  }]);
+}
+
+test('reports missing required tool arguments', async () => {
+  const response = await createBackend().callTool('browser_validate', {});
+
+  expect(response).toHaveResponse({
+    isError: true,
+    error: expect.stringContaining('Invalid arguments for tool "browser_validate":'),
+  });
+  const parsed = parseResponse(response);
+  expect(parsed.error).toContain('- url:');
+  expect(parsed.error).toContain('string');
+});
+
+test('reports invalid tool argument types', async () => {
+  const response = await createBackend().callTool('browser_validate', {
+    url: 123,
+  });
+
+  const parsed = parseResponse(response);
+  expect(parsed.isError).toBe(true);
+  expect(parsed.error).toContain('Invalid arguments for tool "browser_validate":');
+  expect(parsed.error).toContain('- url:');
+  expect(parsed.error).toContain('string');
+});
+
+test('reports validation errors in json mode', async () => {
+  const response = await createBackend().callTool('browser_validate', {
+    _meta: { json: true },
+  });
+
+  expect(response.isError).toBe(true);
+  expect(response.content[0].type).toBe('text');
+  if (response.content[0].type !== 'text')
+    throw new Error('Expected a text response');
+  const payload = JSON.parse(response.content[0].text);
+  expect(payload).toEqual(expect.objectContaining({
+    isError: true,
+    error: expect.stringContaining('Invalid arguments for tool "browser_validate":'),
+  }));
+  expect(payload.error).toContain('- url:');
+  expect(payload.error).toContain('string');
+});
+
+test('reports nested tool argument paths', async () => {
+  const backend = new tools.BrowserBackend({}, {
+    once() {},
+    browser: () => undefined,
+  } as any, [{
+    capability: 'core',
+    schema: {
+      name: 'browser_validate_nested',
+      title: 'Validate nested',
+      description: 'Validate nested arguments',
+      inputSchema: z.object({
+        items: z.array(z.object({
+          label: z.string(),
+        })),
+      }),
+      type: 'action',
+    },
+    handle: async () => {
+      throw new Error('Tool handler should not run for invalid arguments');
+    },
+  }]);
+
+  const response = await backend.callTool('browser_validate_nested', {
+    items: [{ label: 123 }],
+  });
+
+  const parsed = parseResponse(response);
+  expect(parsed.isError).toBe(true);
+  expect(parsed.error).toContain('- items[0].label:');
+  expect(parsed.error).toContain('string');
+});
+
+test('reports unknown tools before validation', async () => {
+  const response = await createBackend().callTool('browser_unknown', {});
+
+  expect(response).toHaveResponse({
+    isError: true,
+    error: 'Tool "browser_unknown" not found',
+  });
+});