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WebMCP spec tracker: what changed and when

Updated · Checked against the WebMCP draft of 20 October 2026 · By the Convertlyft research team

On this page
  1. Where it stands
  2. Change log, newest first
  3. What these changes mean for your code
  4. Coming up

The WebMCP spec is a Draft Community Group Report from the W3C Web Machine Learning Community Group. The latest draft is dated 20 October 2026. It is not a W3C standard and not on the W3C Standards Track, and it changes often. This page logs each change with its date, newest first. We update it when changes land. Last checked 24 October 2026.

The spec lives at webmachinelearning.github.io/webmcp. The source and explainers live in the webmachinelearning/webmcp GitHub repository. Numbers like #352 below are pull requests in that repository.

Where it stands

Status. WebMCP is a Community Group (CG) draft. A W3C TAG review (#1238) was opened as a “CG early review”, in connection with a charter proposal to adopt WebMCP as a tentative deliverable of the Web Machine Learning Working Group. That adoption is a proposal. We have not seen it confirmed.

Editors (as of 20 October 2026):

  • Brandon Walderman (Microsoft)
  • Khushal Sagar (Google)
  • Dominic Farolino (Google)
  • Samuel Dallstream (Microsoft)

Browser positions:

  • Chrome: developer trial from Chrome 146. Origin trial from Chrome 149 to 156 inclusive. Shipping is planned for Chrome 157.
  • Edge: the repository lists an origin trial in Edge 150. Microsoft’s trial page says the trial expires on 30 March 2027.
  • Mozilla: position: neutral on standards-positions #1412, closed on 5 August 2026. Mozilla raised concerns about adversarial use and prompt injection. It called the “MCP” name “a strange and unnecessary branding decision”. As of 9 October 2026, a refreshed Mozilla position is pending.
  • WebKit: oppose on standards-positions #670 (11 June 2026). WebKit argues the gaps should be fixed in HTML and ARIA. It says an agent acting for a user “is, in effect, assistive technology”, and that the Machine Learning group is the wrong venue.

W3C TAG review. Review #1238 opened on 11 June 2026 and is still open. On 6 August 2026 the TAG wrote that a “formal position has not yet been reached”. On 9 October 2026 the editors asked the TAG to wait for Mozilla’s refreshed position and for TPAC.

Other consumers. The repository’s status page lists experimental support in Brave’s Leo AI chat, and ChatGPT Desktop as “Supported” (added 26 August 2026). Meta Ray-Ban Display is listed as “coming soon” for web apps. At Google I/O on 19 May 2026, Google said Gemini in Chrome “will soon support WebMCP APIs”. We found no confirmation that it has shipped.

Change log, newest first

October 2026

  • 20 October 2026. New draft published. Samuel Dallstream added as the fourth editor (#352).
  • 9 October 2026. The editors asked the W3C TAG to wait for Mozilla’s refreshed position and for TPAC.
  • 8 October 2026. The declarative (HTML form) API was removed from the spec “for now” (#338). It still lives in declarative-api-explainer.md and in Chrome’s implementation. Also, #339 replaced UnknownError with specific DOMExceptions.
  • 2 October 2026. continuations-explainer.md added to the repository (#327).

September 2026

  • 30 September 2026. The origin-keyed agent cluster requirement was removed (#330).
  • 28 September 2026. The spec now references the MCP specification, non-normatively (#315).
  • 17 September 2026. The debugging annotation was added (#253). ToolActivatedEvent and ToolCancelEvent were added (#245).
  • 4 September 2026. Headless use was made explicitly in scope in the explainer (#296).
  • 3 September 2026. The consequentialHint annotation was added (#217).

August 2026

  • 26 August 2026. ChatGPT Desktop listed as “Supported” on the implementation status page (#258).
  • 17 August 2026. executeTool() now takes an object, not a JSON string (#246).
  • 14 August 2026. executeTool() specced (#226). RegisteredTool.inputSchema changed from a string to an object (#241).
  • 5 August 2026. Shopify announced WebMCP tools on every Liquid storefront, and Hydrogen support in developer preview. Mozilla closed #1412 with a neutral position.

July 2026

  • 21 July 2026. getTools() specced (#223). Annotations moved into a separate annotations object (#225).
  • 8 July 2026. The README started mentioning the webmcp-types npm package (#220).

June 2026

  • 22 June 2026. Chrome’s “agent-ready toolkit” post. It covers the Lighthouse agentic browsing category from Chrome 150.
  • 17 June 2026. registerTool() now rejects if its signal is already aborted (#202).
  • 11 June 2026. ModelContextClient removed “for now” (#205). WebKit posted its oppose position. The W3C TAG review #1238 opened.
  • 9 June 2026. Chrome blog: “Join the WebMCP origin trial” (Chrome 149).
  • 8 June 2026. registerTool() now returns a Promise (#200).

May 2026

  • 28 May 2026. Mozilla and WebKit standards-position requests filed.
  • 20 May 2026. The toolchange event and the Permissions Policy (tools) added (#179).
  • 19 May 2026. ModelContext moved from navigator to Document (#177). The entry point is now document.modelContext. Google I/O announced the origin trial in Chrome 149.
  • 18 May 2026. Chrome’s WebMCP docs hub published. Chrome’s Intent to Experiment (posted 15 May) got approval for an origin trial from Chrome 149 to 156.
  • Early May. Lighthouse 13.2.0 (1 May) added the “Agentic browsing” category with WebMCP audits. Lighthouse 13.3.0 (7 May) turned it on by default. The release page does not show the year; 2026 is implied by the docs.

April 2026

  • 23 April 2026. The untrustedContentHint annotation specced (#169).
  • 20 April 2026. The WordPress AI plugin opened issue #448, “Add WebMCP experiment”. It is still open, and draft PR #1081 is not merged.

March 2026

  • 27 March 2026. unregisterTool() removed (#156). Tools are now removed by aborting the signal passed to registerTool().
  • 11 March 2026. Chrome published its “WebMCP vs MCP” comparison page.
  • 5 March 2026. provideContext() and clearContext() removed (#132). They could overwrite tools registered by other scripts (issue #101).
  • 2 March 2026. “Add basic tool infra and complete registerTool()” (#113).

February 2026

  • 10 February 2026. Chrome blog: “WebMCP is available for early preview”.

2025

  • 13 August 2025. The WebMCP explainer was first published.

What these changes mean for your code

If you wrote WebMCP code before September 2026, check it against this list:

  • Use document.modelContext, not navigator.modelContext.
  • Remove calls to provideContext(), clearContext() and unregisterTool(). Use an AbortController signal instead.
  • await the Promise from registerTool() and catch errors.
  • Put hints in an annotations object. Put title on the tool itself.
  • Pass exposedTo and signal in the second (options) argument.
  • Pass an object, not a JSON string, to executeTool().

Our how-to guide shows current code for each of these.

Coming up

  • Chrome: the origin trial ends with Chrome 156. Shipping is planned for Chrome 157. We have not verified calendar dates for these releases.
  • Edge: the trial expires on 30 March 2027.
  • Standards: a refreshed Mozilla position, the TAG review, and the proposed move to the Working Group are all still open.

We will add each change here as it lands.

Questions people ask

Is WebMCP a W3C standard?

No. The current draft is a Draft Community Group Report from the Web Machine Learning Community Group, dated 20 October 2026. The spec itself says it is not a W3C Standard nor on the W3C Standards Track.

Where is the WebMCP GitHub repository?

The spec and explainers live at github.com/webmachinelearning/webmcp. The rendered spec is at webmachinelearning.github.io/webmcp.

Is there a WebMCP polyfill?

There is a community package, @mcp-b/webmcp-polyfill, from the WebMCP-org (MCP-B) project. Its installWebMCP() keeps native support when the browser has it. It is community work, not part of the spec.

Which browsers support WebMCP?

Chrome runs an origin trial from Chrome 149 to 156 and plans to ship in Chrome 157. Edge has an origin trial. Mozilla's position is neutral and WebKit's is oppose.

Sources

  1. WebMCP, Draft Community Group Report, 20 October 2026 (read 2026-10-23)
  2. WebMCP repository (README) (read 2026-10-23)
  3. WebMCP implementation status (read 2026-10-23)
  4. WebMCP commit history (page 1) (read 2026-10-23)
  5. WebMCP commit history (page 2) (read 2026-10-23)
  6. WebMCP pull requests: navigator to Document (#177, #179) (read 2026-10-23)
  7. WebMCP pull requests: unregisterTool removal (#156) (read 2026-10-23)
  8. WebMCP pull request #132: remove provideContext and clearContext (read 2026-10-23)
  9. WebMCP-org community npm packages (read 2026-10-23)
  10. Blink-dev: Intent to Experiment, WebMCP (read 2026-10-23)
  11. Chrome blog: WebMCP is available for early preview (read 2026-10-23)
  12. Chrome blog: Join the WebMCP origin trial (read 2026-10-23)
  13. Chrome blog: Chrome at I/O 2026 (read 2026-10-23)
  14. Chrome blog: agent-ready toolkit (read 2026-10-23)
  15. Chrome for Developers: WebMCP vs MCP (read 2026-10-23)
  16. Chrome for Developers: WebMCP overview (read 2026-10-23)
  17. Lighthouse releases (read 2026-10-23)
  18. Microsoft Edge origin trial: WebMCP (read 2026-10-23)
  19. Mozilla standards-positions #1412 (read 2026-10-23)
  20. WebKit standards-positions #670 (read 2026-10-23)
  21. W3C TAG design review #1238 (read 2026-10-23)
  22. Shopify changelog: WebMCP support for Liquid and Hydrogen storefronts (read 2026-10-23)
  23. WordPress AI plugin: Add WebMCP experiment (issue #448) (read 2026-10-23)