fix: allow screenshot capture when filename is an empty string (#37678)

Author: ogadra

packages/playwright/src/mcp/browser/tools/screenshot.ts

@@ -51,7 +51,7 @@ const screenshot = defineTabTool({
       throw new Error('fullPage cannot be used with element screenshots.');
 
     const fileType = params.type || 'png';
-    const fileName = await tab.context.outputFile(params.filename ?? dateAsFileName(fileType), { origin: 'llm', reason: 'Saving screenshot' });
+    const fileName = await tab.context.outputFile(params.filename || dateAsFileName(fileType), { origin: 'llm', reason: 'Saving screenshot' });
     const options: playwright.PageScreenshotOptions = {
       type: fileType,
       quality: fileType === 'png' ? undefined : 90,

tests/mcp/screenshot.spec.ts

@@ -176,6 +176,49 @@ test('browser_take_screenshot (default type should be png)', async ({ startClien
   expect(files[0]).toMatch(/^page-\d{4}-\d{2}-\d{2}T\d{2}-\d{2}-\d{2}-\d{3}Z\.png$/);
 });
 
+test('browser_take_screenshot (filename is empty string)', async ({ startClient, server }, testInfo) => {
+  const outputDir = testInfo.outputPath('output');
+  const { client } = await startClient({
+    config: { outputDir },
+  });
+  expect(await client.callTool({
+    name: 'browser_navigate',
+    arguments: { url: server.HELLO_WORLD },
+  })).toHaveResponse({
+    code: expect.stringContaining(`page.goto('http://localhost`),
+  });
+
+  expect(await client.callTool({
+    name: 'browser_take_screenshot',
+    arguments: {
+      filename: '',
+    },
+  })).toEqual({
+    content: [
+      {
+        text: expect.stringMatching(
+            new RegExp(`page-\\d{4}-\\d{2}-\\d{2}T\\d{2}-\\d{2}-\\d{2}\\-\\d{3}Z\\.png`)
+        ),
+        type: 'text',
+      },
+      {
+        data: expect.any(String),
+        mimeType: 'image/png',
+        type: 'image',
+      },
+    ],
+  });
+
+  const files = [...fs.readdirSync(outputDir)].filter(f => f.endsWith('.png'));
+
+  expect(fs.existsSync(outputDir)).toBeTruthy();
+  expect(files).toHaveLength(1);
+  expect(files[0]).toMatch(
+      new RegExp(`^page-\\d{4}-\\d{2}-\\d{2}T\\d{2}-\\d{2}-\\d{2}-\\d{3}Z\\.png$`)
+  );
+});
+
+
 test('browser_take_screenshot (filename: "output.png")', async ({ startClient, server }, testInfo) => {
   const outputDir = testInfo.outputPath('output');
   const { client } = await startClient({