Remote MCP servers are still in a strange space. Anthropic updated the MCP spec about a month ago with a new Streamable HTTP transport, but it doesn't appear that Claude supports that transport yet.

When I hooked up our remote MCP server, Claude sends a GET request to the endpoint. According to the spec, clients that want to support both transports should first attempt to POST an InitializeRequest to the server URL. If that returns a 4xx, it should then assume the SSE integration.