Warning

Experimental. This extension is experimental and not yet ready for production use. It is built on top of WebMCP, which is itself an experimental, early-stage proposal. Both the underlying specification and this extension's API may change or break at any time without notice. Use at your own risk.

Standards and browser support

Important

Status as of 2026-10-07. WebMCP moves fast. Everything on this page can change without notice; check the linked sources before relying on it. Statements are marked as confirmed (taken from a primary source) or unconfirmed.

The specification

Confirmed: WebMCP is a Draft Community Group Report of the W3C Web Machine Learning Community Group (latest draft: 2 October 2026). It is not a W3C Standard and not on the W3C Standards Track.
Confirmed: the entry point is document.modelContext. Tools are registered one at a time with registerTool(tool, { signal }) and unregistered by aborting that AbortSignal; there is no unregisterTool(). provideContext() and clearContext() were removed from the specification (PR #132).
Confirmed: tool annotations are readOnlyHint, untrustedContentHint, consequentialHint and debugging (all boolean). See Writing tools for how this extension sets them.
Confirmed: the declarative (HTML form) API is still a TODO in the specification; only a separate explainer exists. This extension does not use it.
Confirmed: open proposals may still change the API, among them renaming execute to run (#158) and returning the registered tool from registerTool() (#234). The runtime already accepts any return value of registerTool().

Browsers

Browser	Status	Confidence
Chrome	Origin trial for desktop, milestones 149–156. Behind the flag `chrome://flags/#enable-webmcp-testing` for local testing. A default shipping milestone is not set on chromestatus.	Confirmed (shipping date unconfirmed)
Microsoft Edge	Origin trial, expiring 2027-03-30.	Confirmed
Brave	Experimental.	Unconfirmed
Firefox	Mozilla standards position: neutral.	Confirmed (position label)
Safari / WebKit	WebKit standards position: oppose.	Confirmed (position label)

There is no cross-browser consensus. Expect WebMCP to be available only in Chromium-based browsers, and only for some visitors, for the foreseeable future.

Note

Chrome is reported to have deprecated navigator.modelContext in favour of document.modelContext (Chrome 150). This could not be confirmed from a Chrome source and is therefore unconfirmed. The @mcp-b/webmcp-polyfill package still offers the navigator alias as deprecated in version 5; its upcoming major no longer provides it. This extension neither bundles nor requires a polyfill; see legacyNavigatorFallback.

Agents

Confirmed: the built-in browser of the ChatGPT desktop app uses WebMCP site tools. It only discovers tools of the top-level document (not iframes), runs a safety review per call and requires specific plans and models — see OpenAI's documentation below.
Unconfirmed: support in Gemini in Chrome has been announced but not confirmed by a primary source at the time of writing.

What this means for site operators

Treat WebMCP as progressive enhancement. Every page must work fully without it — most visitors' browsers do not offer the API.
The extension does nothing when the API is missing: no errors, no console output, no changes to the page.
Because the specification can still change, keep the extension up to date; until version 1.0.0 its API may change between minor versions.

Sources

Primary sources:

WebMCP specification draft (W3C Web Machine Learning Community Group)
webmcp issue #101 — provideContext() allows overwriting previously registered tools
webmcp PR #132 — removal of provideContext() and clearContext()
Chrome: Secure tools (Google guidance, not part of the specification)
Chrome Platform Status: WebMCP
Microsoft Edge origin trials
Mozilla standards position #1412
WebKit standards position #670
OpenAI: WebMCP in ChatGPT
OpenAI Help: Using site tools in the ChatGPT desktop app

Secondary source (not independently confirmed):

@mcp-b/webmcp-polyfill package description and changelog of the related @mcp-b packages (navigator alias deprecation)