feat: support testing light and dark mode

docs/tool-reference.md

@@ -206,6 +206,7 @@
 
 **Parameters:**
 
+- **colorScheme** (enum: "dark", "light", "auto") _(optional)_: [`Emulate`](#emulate) the dark or the light mode. Set to "auto" to reset to the default.
 - **cpuThrottlingRate** (number) _(optional)_: Represents the CPU slowdown factor. Set the rate to 1 to disable throttling. If omitted, throttling remains unchanged.
 - **geolocation** (unknown) _(optional)_: Geolocation to [`emulate`](#emulate). Set to null to clear the geolocation override.
 - **networkConditions** (enum: "No emulation", "Offline", "Slow 3G", "Fast 3G", "Slow 4G", "Fast 4G") _(optional)_: Throttle network. Set to "No emulation" to disable. If omitted, conditions remain unchanged.

src/McpContext.ts

@@ -124,6 +124,7 @@ export class McpContext implements Context {
   #geolocationMap = new WeakMap<Page, GeolocationOptions>();
   #viewportMap = new WeakMap<Page, Viewport>();
   #userAgentMap = new WeakMap<Page, string>();
+  #colorSchemeMap = new WeakMap<Page, 'dark' | 'light'>();
   #dialog?: Dialog;
 
   #pageIdMap = new WeakMap<Page, number>();
@@ -353,6 +354,20 @@ export class McpContext implements Context {
     return this.#userAgentMap.get(page) ?? null;
   }
 
+  setColorScheme(scheme: 'dark' | 'light' | null): void {
+    const page = this.getSelectedPage();
+    if (scheme === null) {
+      this.#colorSchemeMap.delete(page);
+    } else {
+      this.#colorSchemeMap.set(page, scheme);
+    }
+  }
+
+  getColorScheme(): 'dark' | 'light' | null {
+    const page = this.getSelectedPage();
+    return this.#colorSchemeMap.get(page) ?? null;
+  }
+
   setIsRunningPerformanceTrace(x: boolean): void {
     this.#isRunningTrace = x;
   }

src/McpResponse.ts

@@ -435,6 +435,7 @@ export class McpResponse implements Response {
       viewport?: object;
       userAgent?: string;
       cpuThrottlingRate?: number;
+      colorScheme?: string;
       dialog?: {
         type: string;
         message: string;
@@ -482,6 +483,13 @@ export class McpResponse implements Response {
       structuredContent.cpuThrottlingRate = cpuThrottlingRate;
     }
 
+    const colorScheme = context.getColorScheme();
+    if (colorScheme) {
+      response.push(`## Color Scheme emulation`);
+      response.push(`Emulating: ${colorScheme}`);
+      structuredContent.colorScheme = colorScheme;
+    }
+
     const dialog = context.getDialog();
     if (dialog) {
       const defaultValueIfNeeded =

src/tools/ToolDefinition.ts

@@ -122,6 +122,7 @@ export type Context = Readonly<{
   getViewport(): Viewport | null;
   setUserAgent(userAgent: string | null): void;
   getUserAgent(): string | null;
+  setColorScheme(scheme: 'dark' | 'light' | null): void;
   saveTemporaryFile(
     data: Uint8Array<ArrayBufferLike>,
     mimeType: 'image/png' | 'image/jpeg' | 'image/webp',

src/tools/emulation.ts

@@ -63,6 +63,12 @@ export const emulate = defineTool({
       .describe(
         'User agent to emulate. Set to null to clear the user agent override.',
       ),
+    colorScheme: zod
+      .enum(['dark', 'light', 'auto'])
+      .optional()
+      .describe(
+        'Emulate the dark or the light mode. Set to "auto" to reset to the default.',
+      ),
     viewport: zod
       .object({
         width: zod.number().int().min(0).describe('Page width in pixels.'),
@@ -158,6 +164,23 @@ export const emulate = defineTool({
       }
     }
 
+    if (request.params.colorScheme) {
+      if (request.params.colorScheme === 'auto') {
+        await page.emulateMediaFeatures([
+          {name: 'prefers-color-scheme', value: ''},
+        ]);
+        context.setColorScheme(null);
+      } else {
+        await page.emulateMediaFeatures([
+          {
+            name: 'prefers-color-scheme',
+            value: request.params.colorScheme,
+          },
+        ]);
+        context.setColorScheme(request.params.colorScheme);
+      }
+    }
+
     if (viewport !== undefined) {
       if (viewport === null) {
         await page.setViewport(null);

tests/McpResponse.test.js.snapshot

@@ -177,6 +177,18 @@ exports[`McpResponse > adds an alert dialog 2`] = `
 }
 `;
 
+exports[`McpResponse > adds color scheme emulation setting when it is set 1`] = `
+# test response
+## Color Scheme emulation
+Emulating: dark
+`;
+
+exports[`McpResponse > adds color scheme emulation setting when it is set 2`] = `
+{
+  "colorScheme": "dark"
+}
+`;
+
 exports[`McpResponse > adds console messages when the setting is true 1`] = `
 # test response
 ## Console messages

tests/McpResponse.test.ts

@@ -383,6 +383,20 @@ describe('McpResponse', () => {
     });
   });
 
+  it('adds color scheme emulation setting when it is set', async t => {
+    await withMcpContext(async (response, context) => {
+      context.setColorScheme('dark');
+      const {content, structuredContent} = await response.handle(
+        'test',
+        context,
+      );
+      t.assert.snapshot?.(getTextContent(content[0]));
+      t.assert.snapshot?.(
+        JSON.stringify(stabilizeStructuredContent(structuredContent), null, 2),
+      );
+    });
+  });
+
   it('adds a prompt dialog', async t => {
     await withMcpContext(async (response, context) => {
       const page = context.getSelectedPage();

tests/tools/emulation.test.ts

@@ -467,4 +467,109 @@ describe('emulation', () => {
       });
     });
   });
+
+  describe('colorScheme', () => {
+    it('emulates color scheme', async () => {
+      await withMcpContext(async (response, context) => {
+        await emulate.handler(
+          {
+            params: {
+              colorScheme: 'dark',
+            },
+          },
+          response,
+          context,
+        );
+
+        assert.strictEqual(context.getColorScheme(), 'dark');
+        const page = context.getSelectedPage();
+        const scheme = await page.evaluate(() =>
+          window.matchMedia('(prefers-color-scheme: dark)').matches
+            ? 'dark'
+            : 'light',
+        );
+        assert.strictEqual(scheme, 'dark');
+      });
+    });
+
+    it('updates color scheme', async () => {
+      await withMcpContext(async (response, context) => {
+        await emulate.handler(
+          {
+            params: {
+              colorScheme: 'dark',
+            },
+          },
+          response,
+          context,
+        );
+        assert.strictEqual(context.getColorScheme(), 'dark');
+
+        await emulate.handler(
+          {
+            params: {
+              colorScheme: 'light',
+            },
+          },
+          response,
+          context,
+        );
+        assert.strictEqual(context.getColorScheme(), 'light');
+        const page = context.getSelectedPage();
+        const scheme = await page.evaluate(() =>
+          window.matchMedia('(prefers-color-scheme: light)').matches
+            ? 'light'
+            : 'dark',
+        );
+        assert.strictEqual(scheme, 'light');
+      });
+    });
+
+    it('resets color scheme when set to auto', async () => {
+      await withMcpContext(async (response, context) => {
+        const page = context.getSelectedPage();
+
+        const initial = await page.evaluate(
+          () => window.matchMedia('(prefers-color-scheme: dark)').matches,
+        );
+
+        await emulate.handler(
+          {
+            params: {
+              colorScheme: 'dark',
+            },
+          },
+          response,
+          context,
+        );
+        assert.strictEqual(context.getColorScheme(), 'dark');
+        // Check manually that it is dark
+
+        assert.strictEqual(
+          await page.evaluate(
+            () => window.matchMedia('(prefers-color-scheme: dark)').matches,
+          ),
+          true,
+        );
+
+        await emulate.handler(
+          {
+            params: {
+              colorScheme: 'auto',
+            },
+          },
+          response,
+          context,
+        );
+
+        assert.strictEqual(context.getColorScheme(), null);
+        assert.strictEqual(
+          await page.evaluate(
+            () => window.matchMedia('(prefers-color-scheme: dark)').matches,
+          ),
+          initial,
+        );
+      });
+    });
+  });
 });