WebMCP Documentation

WebMCP is a proposed web standard. Native support is currently a Chromium preview behind a flag. For all other browsers, use @mcp-b/webmcp-polyfill or @mcp-b/global.

Chrome’s own documentation should be your first stop for preview status and rollout details: the WebMCP early preview post, the AI on Chrome overview, and the Model Context Tool Inspector.

Browser support matrix

Browser	Native `modelContext`	Native `modelContextTesting`	Declarative API	Polyfill Support
Chrome 146+ (with flag)	Yes	Yes	Yes	N/A
Chrome/Edge (default)	No	No	No	Yes
Firefox	No	No	No	Yes
Safari	No	No	No	Yes

Chromium requirements

Version: Chrome 146.0.7672.0 or higher
Flag: chrome://flags/#enable-webmcp-testing set to Enabled
Relaunch: Required after enabling the flag

Enabling the flag

Via chrome://flags

Open Chrome and navigate to chrome://flags/#enable-webmcp-testing
Set the flag to Enabled
Click Relaunch

Via command line

macOS
Linux
Windows

/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome \
  --enable-experimental-web-platform-features \
  http://localhost:5174

google-chrome --enable-experimental-web-platform-features http://localhost:5174

& "C:\Program Files\Google\Chrome\Application\chrome.exe" `
  --enable-experimental-web-platform-features `
  http://localhost:5174

Common launch flags

Flag	Purpose	Use Case
`--enable-experimental-web-platform-features`	Enables all experimental web platform features including WebMCP	Development and testing
`--enable-features=WebModelContext`	Enables only the Web Model Context feature	Targeted testing
`--user-data-dir=/tmp/chrome-test`	Isolated browser profile	Avoiding conflicts with existing profile
`--disable-extensions`	Disables all browser extensions	Debugging polyfill-vs-native issues
`--remote-debugging-port=9222`	Enables DevTools Protocol access	Advanced debugging, `@mcp-b/chrome-devtools-mcp`
`--headless=new`	Headless mode	CI and automation

Verification

JavaScript console

console.log("modelContext:", !!navigator.modelContext);
console.log("modelContextTesting:", !!navigator.modelContextTesting);

chrome://version

Navigate to chrome://version
Find the Command Line section
Verify that your launch flags are present

For the deeper Chromium-specific notes we maintain locally, including source paths and test-focused launch setups, see CHROMIUM_FLAGS.md. For practical testing workflows, see Test Native and Polyfill.

Feature detection

Use this pattern to detect the native API and fall back to the runtime:

if (!navigator.modelContext) {
  await import("@mcp-b/global");
}

navigator.modelContext.registerTool({
  name: 'my-tool',
  description: 'Example tool',
  inputSchema: { type: 'object', properties: {} },
  execute: async () => ({ content: [{ type: 'text', text: 'ok' }] }),
});

For production use, @mcp-b/global handles detection automatically. If a native implementation exists, it leaves the core in place and layers MCP-B features on top. For the architectural reason that works, see Runtime Layering.

Security notes

The --no-sandbox flag disables critical security protections. Use it only in trusted CI environments.

Native WebMCP features are experimental and may change between Chromium versions.
Use an isolated user data directory for testing so you do not mix preview state with your regular browsing profile.
--enable-experimental-web-platform-features enables more than WebMCP.

For the current specification status, see Spec Status and Limitations. For the native-preview tutorial path, see Try the Native Chrome Preview.

Explanation

WebMCP Standard

Architecture

Design

Browser Support and Flags

Browser support matrix

Chromium requirements

Enabling the flag

Via chrome://flags

Via command line

Common launch flags

Verification

JavaScript console

chrome://version

Feature detection

Security notes

​Browser support matrix

​Chromium requirements

​Enabling the flag

​Via chrome://flags

​Via command line

​Common launch flags

​Verification

​JavaScript console

​chrome://version

​Feature detection

​Security notes

Browser support matrix

Chromium requirements

Enabling the flag

Via chrome://flags

Via command line

Common launch flags

Verification

JavaScript console

chrome://version

Feature detection

Security notes