Throw an error if trying to set network condition if allowlist or blocklist are set

Author: Natallia Harshunova

src/McpContext.ts

@@ -347,27 +347,31 @@ export class McpContext implements Context {
     const mcpPage = this.#getMcpPage(page);
     const newSettings: EmulationSettings = {...mcpPage.emulationSettings};
 
-    // Skip network emulation if blocklist/allowlist is configured, as it is rejected by Puppeteer.
-    if (!this.#options.hasNetworkBlockOrAllowlist) {
-      if (!options.networkConditions) {
-        await page.emulateNetworkConditions(null);
-        delete newSettings.networkConditions;
-      } else if (options.networkConditions === 'Offline') {
-        await page.emulateNetworkConditions({
-          offline: true,
-          download: 0,
-          upload: 0,
-          latency: 0,
-        });
-        newSettings.networkConditions = 'Offline';
-      } else if (options.networkConditions in PredefinedNetworkConditions) {
-        const networkCondition =
-          PredefinedNetworkConditions[
-            options.networkConditions as keyof typeof PredefinedNetworkConditions
-          ];
-        await page.emulateNetworkConditions(networkCondition);
-        newSettings.networkConditions = options.networkConditions;
+    // Skip network emulation if blocklist/allowlist is configured, as it conflicts with blocking rules in Puppeteer.
+    if (this.#options.hasNetworkBlockOrAllowlist) {
+      if (options.networkConditions !== undefined) {
+        throw new Error(
+          'Network throttling is not supported when network blocking (allowlist/blocklist) is configured.',
+        );
       }
+    } else if (!options.networkConditions) {
+      await page.emulateNetworkConditions(null);
+      delete newSettings.networkConditions;
+    } else if (options.networkConditions === 'Offline') {
+      await page.emulateNetworkConditions({
+        offline: true,
+        download: 0,
+        upload: 0,
+        latency: 0,
+      });
+      newSettings.networkConditions = 'Offline';
+    } else if (options.networkConditions in PredefinedNetworkConditions) {
+      const networkCondition =
+        PredefinedNetworkConditions[
+          options.networkConditions as keyof typeof PredefinedNetworkConditions
+        ];
+      await page.emulateNetworkConditions(networkCondition);
+      newSettings.networkConditions = options.networkConditions;
     }
 
     const secondarySession = this.getDevToolsUniverse(mcpPage)?.session;

tests/network_blocking.test.ts

@@ -7,6 +7,7 @@
 import assert from 'node:assert/strict';
 import {describe, it} from 'node:test';
 
+import {emulate} from '../src/tools/emulation.js';
 import {lighthouseAudit} from '../src/tools/lighthouse.js';
 import {navigatePage} from '../src/tools/pages.js';
 import {evaluateScript} from '../src/tools/script.js';
@@ -239,4 +240,44 @@ describe('Network Blocking Integration', () => {
       },
     );
   });
+
+  it('throws error when trying to emulate network conditions while blocklist is configured', async () => {
+    const blockedUrlPattern = ['*://*/*'];
+    await withMcpContext(
+      async (response, context) => {
+        // Attempting to emulate network conditions should throw an error.
+        await assert.rejects(async () => {
+          await emulate.handler(
+            {
+              params: {
+                networkConditions: 'Offline',
+              },
+              page: context.getSelectedMcpPage(),
+            },
+            response,
+            context,
+          );
+        }, /Network throttling is not supported when network blocking \(allowlist\/blocklist\) is configured\./);
+
+        // Attempting to emulate CPU rate or other things should succeed without errors.
+        await emulate.handler(
+          {
+            params: {
+              cpuThrottlingRate: 2,
+            },
+            page: context.getSelectedMcpPage(),
+          },
+          response,
+          context,
+        );
+        assert.strictEqual(
+          response.responseLines[0],
+          'Emulation configured successfully',
+        );
+      },
+      {
+        blockedUrlPattern,
+      },
+    );
+  });
 });