Extension declaration for MCP Apps

The MCP Apps extension denotes that a client supporting MCP Apps should declare its capability as follows.

ref. https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx#clientserver-capability-negotiation

{
  "method": "initialize",
  "params": {
    "protocolVersion": "2024-11-05",
    "capabilities": {
      "extensions": {
        "io.modelcontextprotocol/ui": {
          "mimeTypes": ["text/html;profile=mcp-app"]
        }
      }
    },
    "clientInfo": {
      "name": "claude-desktop",
      "version": "1.0.0"
    }
  }
}

However, ClientCapabilities does not have a field for extensions, so we cannot implement a client that supports MCP Apps extension.

Similarly, MCP servers built with this SDK cannot verify client capabilities in extensions.

I can send a PR if it's okay to add a field for extensions.

If I understand correctly this is basically about implementing SEP-2133. It's been accepted so I think it should be fine. If you would be interested to do it, you can propose a change and we can discuss the design.

Thank you. I'll work on it. SEP-2133

Created PR #794. Please add your feedback there.

mcp: add Extensions field to capabilities per SEP-2133 (modelcontextp…

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Extension declaration for MCP Apps #777

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Extension declaration for MCP Apps #777

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ymmt2005 commented on Feb 4, 2026

maciej-kisiel commented on Feb 13, 2026

ymmt2005 commented on Feb 13, 2026

ymmt2005 commented on Feb 16, 2026

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