SEP: Replace `clientCapabilities?.extensions` with `clientCapabilities?.experimental`

[matteo8p]

Issue

The current MCP Apps SEP proposes to use clientCapabilities?.extensions. However, extensions is not a valid object in clientCapabilities in accordance with the MCP spec. Instead, we should be using clientCapabilities?.experimental. This is consistent with the official MCPClient spec and SDK.

See ClientCapabilities in the MCP spec here.

Existing

The existing initialize message uses extensions. Example:

{
  "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"
    }
  }
}

Solution

Instead, we should be using capabilities?.experimental:

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

[idosal]

Thanks @matteo8p ! SEP-1865 (MCP Apps) depends on SEP-1724 (Extensions), which should add the extensions attribute to capabilities.

[matteo8p]

@idosal Gotcha, that makes sense. Seems like SEP-1724 hasn't been pushed or updated for several months now. I'll ask.

[matteo8p]

@idosal It seems like the Extensions proposal is stuck and not approved by core maintainers. Might be worth it to move off of using extension for the client negotiation, or rethink how we do the initialize handshake.

I do think experiment works fine for what it is if the extensions sep gets stuck. Wouldn't want approval of Extensions to be a blocker on MCP apps as Apps is moving faster than Model Context Protocol

[liady]

@matteo8p as Ido mentioned currently this SEP depends on the Extensions SEP (also as a use-case display of a proper spec extension), so the right path forward would be to approve the extension SEP first. MCP Apps is an extension of MCP, so for now we should still keep the proper process.