The Chrome DevTools Protocol allows for tools to instrument, inspect, debug and profile Chromium, Chrome and other Blink-based browsers. Many existing projects currently use the protocol. The Chrome DevTools uses this protocol and the team maintains its API.
Instrumentation is divided into a number of domains (DOM, Debugger, Network etc.). Each domain defines a number of commands it supports and events it generates. Both commands and events are serialized JSON objects of a fixed structure. You can either debug over the wire using the raw messages as they are described in the corresponding domain documentation, or use extension JavaScript API.
Protocol API Docs
The latest (tip-of-tree) protocol (tot)
It changes frequently and can break at any time. However it captures the full capabilities of the Protocol, whereas the stable release is a subset. There is no backwards compatibility support guaranteed for the capabilities it introduces.
v8-inspector protocol (v8)
It is available in node 6.3+ and enables debugging & profiling of Node.js apps.
stable 1.2 protocol (1-2)
The stable release of the protocol, tagged at Chrome 54. It includes a smaller subset of the complete protocol compatibilities.
stable 1.3 protocol (1-3)
The stable release of the protocol, tagged at Chrome 64. It includes a smaller subset of the complete protocol compatibilities.
Resources
Consider subscribing to the chrome-debugging-protocol mailing list.
The devtools-protocol repo issue tracker can also be used for concerns with the protocol. It also hosts the canonical copy of the json files.
Useful: Getting Started with Headless Chrome and the Headless Chromium readme.
The chrome-remote-interface node module is recommended, and its wiki and issue tracker are full of useful recipes.
The awesome-chrome-devtools page links to many of the tools in the protocol ecosystem, including protocol API libraries in JavaScript, TypeScript, Python, Java, and Go. Many applications and libraries already use the protocol.
Basics: Using DevTools as protocol client
The Developer Tools front-end can attach to a remotely running Chrome instance for debugging. For this scenario to work, you should start your host Chrome instance with the remote-debugging-port command line switch:
chrome.exe --remote-debugging-port=9222
Then you can start a separate client Chrome instance, using a distinct user profile:
chrome.exe --user-data-dir=<some directory>
Now you can navigate to the given port from your client and attach to any of the discovered tabs for debugging: http://localhost:9222
You will find the Developer Tools interface identical to the embedded one and here is why:
- When you navigate your client browser to the remote's Chrome port, Developer Tools front-end is being served from the host Chrome as a Web Application from the Web Server.
- It fetches HTML, JavaScript and CSS files over HTTP
- Once loaded, Developer Tools establishes a Web Socket connection to its host and starts exchanging JSON messages with it.
In this scenario, you can substitute Developer Tools front-end with your own implementation. Instead of navigating to the HTML page at http://localhost:9222, your application can discover available pages by requesting: http://localhost:9222/json and getting a JSON object with information about inspectable pages along with the WebSocket addresses that you could use in order to start instrumenting them.
Remote debugging is especially useful when debugging remote instances of the browser or attaching to the embedded devices. Blink port owners are responsible for exposing debugging connections to the external users.
If you need the protocol version for a specific Chrome client, glance at fetchFromChromeRepo from chrome-remote-interface's devtools.js.
Sniffing the protocol
This is especially handy to understand how the DevTools frontend makes use of the protocol. First, run Chrome with the debugging port open:
alias canary="/Applications/Google\ Chrome\ Canary.app/Contents/MacOS/Google\ Chrome\ Canary" canary --remote-debugging-port=9222 http://localhost:9222 http://chromium.orgThen, select the Chromium Projects item in the Inspectable Pages list. Now that DevTools is up and fullscreen, open DevTools to inspect it. Cmd-R in the new inspector to make the first restart. Now head to Network Panel, filter by Websocket, select the connection and click the Frames tab. Now you can easily see the frames of WebSocket activity as you use the first instance of the DevTools.
DevTools protocol via Chrome extension
To allow chrome extensions to interact with the protocol, we introduced chrome.debugger extension API that exposes this JSON message transport interface. As a result, you can not only attach to the remotely running Chrome instance, but also instrument it from its own extension.
Chrome Debugger Extension API provides a higher level API where command domain, name and body are provided explicitly in the sendCommand call. This API hides request ids and handles binding of the request with its response, hence allowing sendCommand to report result in the callback function call. One can also use this API in combination with the other Extension APIs.
If you are developing a Web-based IDE, you should implement an extension that exposes debugging capabilities to your page and your IDE will be able to open pages with the target application, set breakpoints there, evaluate expressions in console, live edit JavaScript and CSS, display live DOM, network interaction and any other aspect that Developer Tools is instrumenting today.
Opening embedded Developer Tools will terminate the remote connection and thus detach the extension.
Frequently Asked Questions
How is the protocol defined?
The canonical protocol definitions live in the Chromium source tree: (browser_protocol.json and js_protocol.json). They are maintained manually by the DevTools engineering team. These files are mirrored (hourly) on GitHub in the devtools-protocol repo.
The declarative protocol definitions are used across tools. Within Chromium, a binding layer is created for the Chrome DevTools to interact with, and separately the protocol is used for Chrome Headless’s C++ interface.
What’s the protocol_externs file?
It’s created via generate_protocol_externs.py and useful for tools using closure compiler. The TypeScript story is here.
Are the HTTP endpoints documented?
Not yet. See bugger-daemon’s third-party docs. See also the endpoints implementation in Chromium. /json/protocol was added in Chrome 60.
How do I access the browser target?
The endpoint is exposed as webSocketDebuggerUrl in /json/version. Note the browser in the URL, rather than page. If Chrome was launched with --remote-debugging-port=0 and chose an open port, the browser endpoint is written to both stderr and the DevToolsActivePort file in browser profile folder.
Does the protocol support multiple simultaneous clients?
We currently do not support multiple clients connected to the protocol simultaneously. This includes opening DevTools while another client is connected. On the bug tracker, crbug.com/129539 follows the issue; you can star it for email updates.
Upon disconnnection, the outgoing client will receive a detached event. For example: {"method":"Inspector.detached","params":{"reason":"replaced_with_devtools"}}. View the enum of possible reasons. (For reference: the original patch). After disconnection, some apps have chosen to pause their state and offer a reconnect button.