WebMCP in Chrome: setup, origin trial and tool testing

Enable WebMCP for local testing or use an available origin trial for a deployed origin. Record your browser version and configuration, then verify registration and a controlled call. The browser tool test guide provides a fictional read-only demo.

Local development: the flag

Chrome's documentation describes a local WebMCP testing flag. Availability and exposed APIs depend on the browser build; check the installed version before following an older example.

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 local flag affects that browser. Chrome documents an origin trial starting with Chrome 149 for deployed testing. Check the trial's current eligibility, supported versions and expiry before registering a token for your exact origin.

<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 in the current draft, but your browser build may not expose it. Chrome's Model Context Tool Inspector offers another way to inspect registration and manually call a tool. Our public checker inspects source signals and does not execute this browser test.

Common reasons nothing appears