WebMCP convergence

[ochafik]

Option 2 below seems best, as discussed at today's meeting (I'll send a PR along its lines)

Option 1: Support new WebMCP mime type for resources

Strawman proposal we've just brainstormed on w/ @liady (as alternative to #796)

• New mime type: text/html;profile=webmcp
• Tool._meta.ui.resourceUri points to an WebMcp page
• Tool result CAN have structuredContent, which gets encoded as url fragment params of the iframe URL, e.g. in this case #query=...

import { registerAppTool, registerAppResource } from "@modelcontextprotocol/ext-apps/server";
import { McpServer } from "@modelcontextprotocol/sdk/...";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/...";

const server = new McpServer({name: 'Example Server', version: '1.0.0', webMcp: 'required'});

const uiHtml = `
  document.modelContext.registerTool({ name: 'start', description: ..., inputSchema: ..., execute: ... }) 
`;

const resourceUri = 'ui://example';

registerAppResource(server, {
  uri: resourceUri,
  mimeType: 'text/html;profile=webmcp',
  content: uiHtml,
  _meta: {
    ui: {
      csp: {
        connectDomains: ['api.myhost.com']
      }
    }
  }
})
registerAppTool(server, 'show-embed', {
  inputSchema: z.object({q: z.string()}).shape,
  _meta: {
    ui: { resourceUri }
  }
}, ({message}) => {
   let query = '...';
   return {
     content: [],
     structuredContent: {query}
   }
})
  
server.server.connect(new StdioServerTransport())
.then(() => console.error('Server is running');

Option 2: Remove MCP App Tools from draft spec / just adopt WebMCP

Might be a winner haha

Pros

• No host implemented app tools yet
• Sandbox wrapper can poke at view's document.modelContext directly (same origin) so host can inject polyfill in html payload of view

Cons

• What about elicitation, etc? -> Liad: responsibility of WebMCP :-D

Option 3: Thin interop layer

#796

Potential blockers:

• Polyfill (left to web page) and their (lack of standard) protocol w/ host
• Two techs coexisting in same page w/ slight

[domfarolino]

Tool._meta.ui.resourceUri points to an WebMcp page

What is a "WebMcp page" in particular? Just an app blob that happens to use the WebMCP API?

[idosal]

To follow up on the meeting, I don't think this side is necessary. It'd be amazing to position WebMCP as MCP App's recommended app tool implementation and provide an official bridge, deprecating the existing implementation.