Skip to main content
The declarative API lets a page expose HTML workflows, especially forms, as WebMCP tools. This surface is evolving quickly, so the Chrome and W3C documents are the source of truth.
Use this page as a quick summary. For the current explainer, attribute names, schema synthesis rules, constraint mapping, and open design questions, read the official declarative explainer and the declarative draft.

​
Current prototype shape

The current Chromium prototype centers on three form-level ideas:
ConceptPurpose
toolnameDeclare that a form is a tool
tooldescriptionProvide agent-facing description text
toolautosubmitLet the browser submit automatically after fields are filled
Parameter names and validation come from ordinary HTML. The browser derives a schema from form controls, labels, and validation attributes instead of asking authors to maintain a second JSON schema by hand. For the current browser support and flag requirements, see Browser Support and Flags. For the broader safety model behind toolautosubmit, see Security and Human-in-the-Loop. For why this page stays intentionally small, see What Is WebMCP.

​
Example

This example matches the current upstream explainer direction: 
<form
  action="/flights/search"
  method="get"
  toolname="search_flights"
  tooldescription="Search flights by route/date"
  toolautosubmit
>
  <label>
    Origin
    <input name="origin" required toolparamdescription="IATA code or city" />
  </label>

  <label>
    Destination
    <input name="destination" required />
  </label>

  <label>
    Date
    <input type="date" name="date" required />
  </label>

  <button type="submit">Search</button>
</form>

​
What happens

At a high level, the browser:
  1. Finds forms that declare a tool.
  2. Synthesizes an input schema from the form controls.
  3. Fills controls when an agent invokes the tool.
  4. Either auto-submits or pauses for user review.
  5. Returns the result through browser-managed completion paths.
The exact synthesis rules, event model, CSS pseudo-classes, and cross-document behavior are still being refined upstream. That is why this page does not duplicate the full mapping tables.

​
Same-document completion

The current prototype direction includes a SubmitEvent.respondWith(...) path for same-document completion: 
document.querySelector("form[toolname='search_flights']")?.addEventListener("submit", (event) => {
  if (!event.agentInvoked) {
    return;
  }

  event.preventDefault();
  event.respondWith(
    Promise.resolve({
      content: [{ type: "text", text: "Search submitted and processed." }],
    })
  );
});
For the current WebIDL additions and cross-document completion discussion, use the official explainer. For how MCP-B runtime choices relate to native declarative experiments, see Native vs Polyfill vs Global.