feat: Implement `ui/update-model-context` for MCP Apps

[aharvard]

Summary

Implement the ui/update-model-context method from the MCP Apps specification (ext-apps PR #125, now merged).

This allows MCP Apps to update the host's model context without triggering a follow-up action - unlike ui/message which triggers agent processing, or notifications/message which is for logging.

Use Case

An MCP App can inform the agent about app state changes (e.g., "User selected 3 items totaling $150") that should influence future responses, without immediately prompting the agent to act.

What's Missing

1. UI Layer (ui/desktop/src/components/McpApps/)

   • Handle ui/update-model-context requests from Guest UI
   • Advertise capability in hostCapabilities during ui/initialize

2. Server Layer (crates/goose-server/)

   • New endpoint to receive and store context updates

3. Agent Layer (crates/goose/src/agents/)

   • Storage for MCP App contexts (keyed by extension + resource URI)
   • Include stored context in model prompts for future turns

Spec Reference

// Request (Guest UI → Host)
{
  method: "ui/update-model-context",
  params: {
    content?: ContentBlock[],
    structuredContent?: Record<string, unknown>
  }
}

// Response
{ result: {} }

Host behavior per spec:

• SHOULD provide context to model in future turns
• MAY overwrite previous context with new update
• If multiple updates before next user message, SHOULD only send the last

Related

• Spec: https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx
• PR: feat: add ui/update-model-context modelcontextprotocol/ext-apps#125

[aharvard]

The UI layer is fairly straightforward. Any thoughts on how to approach the server and agent layers?

[taniacryptid]

Thank you for starting this discussion! Let me tag folks to start chiming in :) @block/goose-maintainers @block/goose-core-maintainers

[habuma]

I'm quite keen on seeing this work, too. I'm using 1.27.2 and it doesn't seem that ui/update-model-context has been implemented yet.

In case this helps...

I turned on developer mode and at the point where the app is sending a ui/update-model-context message, I see the following error:

app-with-deps.js:1 
Uncaught (in promise) $ZodError: [
  {
    "code": "unrecognized_keys",
    "keys": [
      "status",
      "message"
    ],
    "path": [],
    "message": "Unrecognized keys: \"status\", \"message\""
  }
]
    at new $ZodError (app-with-deps.js:1:5513)
    at app-with-deps.js:2:703
    at Xv (app-with-deps.js:40:13125)
    at app-with-deps.js:40:71280
    at oc._onresponse (app-with-deps.js:40:68814)
    at _transport.onmessage (app-with-deps.js:40:64846)
    at O$.messageListener (app-with-deps.js:40:76735)

I set a breakpoint on line 40 of app-with-deps and see that the n parameter passed to onmessage has the following:

id: 2
jsonrpc: "2.0"
result: 
{
  status: 'error', 
  message: 'Unhandled JSON-RPC method: ui/update-model-context'
}

I'd love to use Goose for my MCP App, but because of this I cannot. MCP Jam and even Claude Desktop handle it just fine, so until Goose supports this, I will have to use those.

[aharvard]

Thanks, @habuma! You're correct, we don't have this in goose yet. We'll share updates here once we do.

[habuma]

Thanks. One more unfortunate data point, though. The Goose Blog even talks about using updateModelContext() in https://block.github.io/goose/blog/2026/01/30/5-tips-building-mcp-apps/#updatemodelcontext. Will be good to have that implemented in Goose so that the article doesn't send anyone down a path that is not yet supported.

[aharvard]

Just picked this up to work on. Stay tuned!