WebMCP
WebMCP, maintained by W3C Web Machine Learning Community Group (editors from Microsoft and Google). A browser API that lets a page register JavaScript functions as tools, with descriptions and schemas, that an in-browser agent can call. Status: Draft specification; Chrome and Edge test it in an origin trial, Brave behind a flag.
Fact sheet
| Name | WebMCP |
|---|---|
| Maintainer | W3C Web Machine Learning Community Group (editors from Microsoft and Google) |
| What it is | A browser API that lets a page register JavaScript functions as tools, with descriptions and schemas, that an in-browser agent can call. |
| Status and version | Draft specification; Chrome and Edge test it in an origin trial, Brave behind a flag, no Firefox or Safari implementation (nekuda FAQ, October 2026). |
| What it does not do | Does not work without a browser agent that reads the page; does not replace a server-side MCP endpoint; the declarative form-based API is only being explored. |
Dated facts
- The draft documents document.modelContext with registerTool(), getTools(), executeTool() and a toolchange event. (github.com, read 4 November 2026)
- The imperative registerTool() API is in the spec; a declarative API built from HTML forms is being explored and is not in it. (github.com, read 4 November 2026)
- WebMCP is on by default in top-level windows and same-origin iframes; cross-origin iframes need the Permissions Policy allow="tools". (github.com, read 4 November 2026)
- The explainer was first published on 13 Aug 2025; TypeScript definitions ship as the webmcp-types npm package. (github.com, dated 13 August 2025, read 4 November 2026)
- webmcp.com, maintained by nekuda, reported 8,556 verified sites and 52,491 indexed tools. (webmcp.com, read 4 November 2026)
- nekuda reports that ChatGPT's desktop app began using WebMCP tools in its built-in browser on 25 Aug 2026. (webmcp.com, dated 25 August 2026, read 4 November 2026)
- Cloudflare Browser Run moved its WebMCP support to document.modelContext on 28 Sep 2026. (developers.cloudflare.com, dated 28 September 2026, read 4 November 2026)
- Mintlify ships WebMCP tools such as search_docs and read_page on Pro and Enterprise plans. (www.mintlify.com, read 4 November 2026)
Seen in the shared crawl
The shared crawl looks for WebMCP in the initial HTML of each home page. It found a WebMCP hint on 25 of 7,545 home pages in the Tranco top 10,000 and on 59 of 3,393 on Ora's leaderboard.