fix: include errors in slim responses (#3085)

Slim mode now returns rejected `evaluate` calls as one `Error: …`
response with `isError: true`, matching full responses. Successful
response text is unchanged.

Fixes #2938.

## Validation

- Passed `npm run build`, `npm run typecheck`, and `npm run
check-format` on the updated PR head.
- Passed the full `npm run test` suite on the updated PR head, including
Chrome-backed browser tests.

## AI disclosure

OpenAI Codex assisted with issue analysis, implementation, testing, and
code review.

src/SlimMcpResponse.ts

@@ -13,9 +13,13 @@ export class SlimMcpResponse extends McpResponse {
     content: Array<TextContent | ImageContent>;
     structuredContent: object;
   }> {
+    const error = this.error;
+    const responseText = error
+      ? [...this.responseLines, `Error: ${error.message}`].join('\n')
+      : this.responseLines.join('\n');
     const text: TextContent = {
       type: 'text',
-      text: this.responseLines.join('\n'),
+      text: responseText,
     };
     return {
       content: [text],

src/tools/slim/tools.ts

@@ -100,11 +100,7 @@ export const evaluate = definePageTool(() => ({
   verifyFilesSchema: {},
   handler: async (request, response) => {
     const page = request.page;
-    try {
-      const result = await page.pptrPage.evaluate(request.params.script);
-      response.appendResponseLine(JSON.stringify(result));
-    } catch (err) {
-      response.appendResponseLine(String(err.message));
-    }
+    const result = await page.pptrPage.evaluate(request.params.script);
+    response.appendResponseLine(JSON.stringify(result));
   },
 }));

tests/SlimMcpResponse.test.ts

@@ -0,0 +1,43 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import assert from 'node:assert';
+import {afterEach, describe, it} from 'node:test';
+
+import sinon from 'sinon';
+
+import {SlimMcpResponse} from '../src/SlimMcpResponse.js';
+import {createMockMcpContext, createMockParsedArguments} from './mocks.js';
+
+describe('SlimMcpResponse', () => {
+  afterEach(() => {
+    sinon.restore();
+  });
+
+  it('preserves response lines when there is no error', async () => {
+    const response = new SlimMcpResponse(createMockParsedArguments());
+    response.appendResponseLine('Navigation complete');
+
+    const {content} = await response.handle(createMockMcpContext());
+
+    assert.strictEqual(content[0].type, 'text');
+    assert.strictEqual(content[0].text, 'Navigation complete');
+  });
+
+  it('includes stored errors in the response text', async () => {
+    const response = new SlimMcpResponse(createMockParsedArguments());
+    response.appendResponseLine('Navigation started');
+    response.setError(new Error('Navigation failed'));
+
+    const {content} = await response.handle(createMockMcpContext());
+
+    assert.strictEqual(content[0].type, 'text');
+    assert.strictEqual(
+      content[0].text,
+      'Navigation started\nError: Navigation failed',
+    );
+  });
+});

tests/tools/slim/tools.test.ts

@@ -33,18 +33,18 @@ describe('slim', () => {
     sinon.assert.calledOnceWithExactly(response.appendResponseLine, '10');
   });
 
-  it('handles errors', async () => {
+  it('propagates evaluation errors for response handling', async () => {
     const {page, context, response, args} = createHandlerMocks();
     const script = "throw new Error('test error')";
     page.pptrPage.evaluate.rejects(new Error('test error'));
 
-    await evaluate(args).handler({params: {script}, page}, response, context);
+    await assert.rejects(
+      evaluate(args).handler({params: {script}, page}, response, context),
+      {message: 'test error'},
+    );
 
     sinon.assert.calledOnceWithExactly(page.pptrPage.evaluate, script);
-    sinon.assert.calledOnceWithExactly(
-      response.appendResponseLine,
-      'test error',
-    );
+    sinon.assert.notCalled(response.appendResponseLine);
   });
 
   it('navigates to correct page', async t => {