Turning WebMCP on in Chrome

There are two separate ways WebMCP becomes available, and confusing them is the most common reason a developer concludes the API is broken.

Local development: the flag

For your own machine, enable the developer trial. Chrome 146 was the first stable release to carry it.

1. Open chrome://flags/#enable-webmcp-testing.
2. Set it to Enabled and restart the browser.
3. Confirm in DevTools: typeof document.modelContext should report object.

The Experimental Web Platform Features flag also carries it in some builds. If the dedicated flag is missing from your version, that is the fallback worth trying before concluding your channel does not have it.

Real visitors: the origin trial

A flag only affects the browser it is set in. To have WebMCP work for people who visit your site, register the origin trial and serve the token. The trial covers Chrome 149 through 156.

<meta http-equiv="origin-trial" content="YOUR_TOKEN_HERE">

Origin-Trial: YOUR_TOKEN_HERE

Permissions policy

Access is gated behind a policy-controlled feature named tools, with a default allowlist of ['self']. Top-level documents and same-origin iframes get it automatically. A cross-origin iframe does not, and calling into it throws NotAllowedError.

<iframe src="https://widget.example.com/booking" allow="tools"></iframe>

That matters more than it first appears: booking widgets, chat, configurators and checkout steps are routinely third-party iframes, and those are exactly the surfaces you would most want an agent to operate.

Confirming registration actually happened

Code running is not the same as a tool being registered — a rejected name, a missing secure context or a thrown handler all fail without a visible symptom. Ask the browser directly.

const tools = await document.modelContext.getTools();
console.table(tools.map(t => ({
  name: t.name,
  description: t.description.slice(0, 60),
  readOnly: t.annotations?.readOnlyHint ?? false,
})));

getTools() is also how our own scanner will read a live tool list rather than inferring one from source, and it is the only way to see tools that a bundle registers at runtime.

Common reasons nothing appears