Prototype app-rendered elicitation in MCP Apps

[krubenok]

Summary

This draft provides a reference implementation for app-rendered elicitations as an experimental addition to the existing MCP Apps extension.

It:

• adds an experimental elicitation capability under io.modelcontextprotocol/ui;
• attaches _meta.ui.resourceUri to an otherwise standard form elicitation only when the client negotiated both core form elicitation and MCP Apps elicitation support;
• preserves an ordinary form elicitation for clients that support elicitation but not MCP Apps;
• works with the 2026-07-28 stateless MRTR InputRequiredResult / retry flow;
• adds strongly typed helpers, tests, documentation, and a minimal end-to-end server, host, and app sample.

The public API is marked with the existing MCP Apps experimental diagnostic (MCPEXP003) while the proposal is discussed.

Proposed wire convention

{
  "elicitation": { "form": {} },
  "extensions": {
    "io.modelcontextprotocol/ui": {
      "mimeTypes": ["text/html;profile=mcp-app"],
      "elicitation": {}
    }
  }
}

When negotiated, the server may add:

{
  "_meta": {
    "ui": {
      "resourceUri": "ui://portfolio/assign-manager"
    }
  }
}

The underlying request remains a standard elicitation/create, including a complete requestedSchema. A client that does not negotiate the nested Apps capability therefore receives the native form request unchanged.

Validation

• dotnet build ModelContextProtocol.slnx --no-restore
• dotnet build src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj --no-restore (net10.0, net9.0, net8.0, netstandard2.0)
• 9 focused McpAppElicitation tests
• live Streamable HTTP validation under protocol 2026-07-28:
  • form-only client: InputRequiredResult contains a normal form elicitation without app metadata;
  • Apps-capable client: the same request additionally contains _meta.ui.resourceUri;
  • MRTR retry completes with the same typed result.

Context

• [Proposal] Rich UI elicitation — attach an MCP App to an elicitation/create request ext-apps#511
• modelcontextprotocol/ext-apps discussion 如何使用sse协议进行更多的控制，对服务器的配置以及如何处理客户端请求进行细粒度控制？ #514
• Feat/create elicitation UI ext-apps#531

SEP-3118 cites this draft as its reference implementation. Until that proposal settles, the earlier two-extension prototype remains available on krubenok:feature/apps-elicitation for experiments that need an independently negotiated extension.

AI assistance disclosure

This proposal and implementation were developed with substantial assistance from OpenAI Codex. The author directed the design, reviewed the implementation, and validated the end-to-end behavior.