fix: set emulatedUserAgent and use finalDisplayedUrl in lighthouse_audit (#2795)

`lighthouse_audit` never set `emulatedUserAgent`, so desktop audits ran
with Lighthouse's default mobile UA. It now mirrors the UA constants
Lighthouse's own presets use. It also reported `URL: undefined` in
snapshot mode, because `mainDocumentUrl` is only populated for
navigations; it now uses `finalDisplayedUrl`.

src/tools/lighthouse.ts

@@ -21,6 +21,16 @@ import {ToolCategory} from './categories.js';
 import {startTrace} from './performance.js';
 import {definePageTool} from './ToolDefinition.js';
 
+// Kept in sync with `constants.userAgents` in Lighthouse's
+// core/config/constants.js, which `lighthouse:default` and the `desktop`
+// preset use. The bundle in src/third_party only re-exports the runner
+// entrypoints, so these cannot be imported and are mirrored here alongside the
+// screen emulation metrics below. Refresh via scripts/update-lighthouse.ts.
+const MOBILE_USER_AGENT =
+  'Mozilla/5.0 (Linux; Android 11; moto g power (2022)) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Mobile Safari/537.36';
+const DESKTOP_USER_AGENT =
+  'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36';
+
 export const lighthouseAudit = definePageTool((args: ParsedArguments) => ({
   name: 'lighthouse_audit',
   description: `Get Lighthouse score and reports for accessibility, SEO, best practices, and agentic browsing. This excludes performance. For performance audits, run ${startTrace(args).name}`,
@@ -79,6 +89,7 @@ export const lighthouseAudit = definePageTool((args: ParsedArguments) => ({
         deviceScaleFactor: 1,
         disabled: false,
       };
+      flags.emulatedUserAgent = DESKTOP_USER_AGENT;
     } else {
       flags.formFactor = 'mobile';
       flags.screenEmulation = {
@@ -88,6 +99,7 @@ export const lighthouseAudit = definePageTool((args: ParsedArguments) => ({
         deviceScaleFactor: 1.75,
         disabled: false,
       };
+      flags.emulatedUserAgent = MOBILE_USER_AGENT;
     }
 
     let result: RunnerResult | undefined;
@@ -162,7 +174,9 @@ export const lighthouseAudit = definePageTool((args: ParsedArguments) => ({
       summary: {
         mode,
         device,
-        url: lhr.mainDocumentUrl,
+        // `mainDocumentUrl` is only set for navigations, whereas
+        // `finalDisplayedUrl` is populated for every gather mode.
+        url: lhr.finalDisplayedUrl,
         scores: categoryScores,
         audits: {
           failed: failedAudits,

tests/mocks.ts

@@ -201,12 +201,16 @@ export function createHandlerMocks(options: Partial<ParsedArguments> = {}): {
   return {page, context, response, args};
 }
 
-export function createMockRunnerResult(): RunnerResult {
+export function createMockRunnerResult(
+  lhrOverrides: Partial<Result> = {},
+): RunnerResult {
   const lhr = {
+    finalDisplayedUrl: 'http://localhost',
     mainDocumentUrl: 'http://localhost',
     categories: {},
     audits: {},
     timing: {total: 0},
+    ...lhrOverrides,
   };
   return {
     lhr: lhr as unknown as Result,

tests/tools/lighthouse.test.ts

@@ -108,6 +108,84 @@ describe('lighthouse', () => {
       sinon.assert.calledOnceWithExactly(page.restoreEmulation);
     });
 
+    it('emulates a desktop user agent for desktop audits', async () => {
+      const {page, context, response, args} = createHandlerMocks();
+      context.saveTemporaryFile.resolves({filepath: 'report.json'});
+      const navigation = sinon
+        .stub(lighthouseRunner, 'navigation')
+        .resolves(createMockRunnerResult());
+
+      await lighthouseAudit(args).handler(
+        {
+          params: {
+            mode: 'navigation',
+            device: 'desktop',
+          },
+          page,
+        },
+        response,
+        context,
+      );
+
+      const {flags} = navigation.firstCall.args[2];
+      assert.equal(flags?.formFactor, 'desktop');
+      assert.match(String(flags?.emulatedUserAgent), /Macintosh/);
+      assert.doesNotMatch(String(flags?.emulatedUserAgent), /Mobile/);
+    });
+
+    it('emulates a mobile user agent for mobile audits', async () => {
+      const {page, context, response, args} = createHandlerMocks();
+      context.saveTemporaryFile.resolves({filepath: 'report.json'});
+      const snapshot = sinon
+        .stub(lighthouseRunner, 'snapshot')
+        .resolves(createMockRunnerResult());
+
+      await lighthouseAudit(args).handler(
+        {
+          params: {
+            mode: 'snapshot',
+            device: 'mobile',
+          },
+          page,
+        },
+        response,
+        context,
+      );
+
+      const {flags} = snapshot.firstCall.args[1];
+      assert.equal(flags?.formFactor, 'mobile');
+      assert.match(String(flags?.emulatedUserAgent), /Mobile Safari/);
+    });
+
+    it('reports the URL in snapshot mode, where mainDocumentUrl is unset', async () => {
+      const {page, context, response, args} = createHandlerMocks();
+      context.saveTemporaryFile.resolves({filepath: 'report.json'});
+      sinon.stub(lighthouseRunner, 'snapshot').resolves(
+        createMockRunnerResult({
+          mainDocumentUrl: undefined,
+          finalDisplayedUrl: 'https://example.com/page',
+        }),
+      );
+
+      await lighthouseAudit(args).handler(
+        {
+          params: {
+            mode: 'snapshot',
+            device: 'mobile',
+          },
+          page,
+        },
+        response,
+        context,
+      );
+
+      sinon.assert.calledOnce(response.attachLighthouseResult);
+      assert.equal(
+        response.attachLighthouseResult.firstCall.args[0].summary.url,
+        'https://example.com/page',
+      );
+    });
+
     it('runs Lighthouse in snapshot mode with mobile device', async () => {
       server.addHtmlRoute('/test-mobile', html`<div>Test Mobile</div>`);