Chrome DevTools Protocol (CDP)

Get tab WebSocket URL first: Never connect to ws://localhost:9222/devtools/browser directly. Fetch http://localhost:9222/json/list and use webSocketDebuggerUrl from the active tab.

Enable domains before use: Runtime.enable and Page.enable must be called before any Runtime.evaluate or Page.navigate commands.

CDP is async: Wait for response before sending next command. Use Promise-based wrapper with response ID tracking.

Screenshot on high-DPI: Include fromSurface: true and scale: 2 in Page.captureScreenshot params for Retina displays.

Get response body separately: Network.responseReceived doesn't include body. Call Network.getResponseBody with requestId after response completes.

Chrome Extension Manifest V3

Permissions split: Use permissions for APIs, host_permissions for URLs. Never use http://*/* in permissions.

Service workers terminate: No persistent state. Use chrome.storage.local instead of global variables. Use chrome.alarms instead of setInterval.

Content script isolation: Can't access page globals. Use chrome.scripting.executeScript with func for page context. Use window.postMessage for content↔page communication.