MCP Streamable HTTP is one URL. The client POSTs JSON-RPC to it and may open a GET stream on the same URL; nothing else is part of the address. Most connection failures we see in our own server log are a client talking to a different address than the one it was given, or sending the wrong headers.
Source: [Dev.to](https://dev.to/tanod/mcp-connection-errors-explained-404-on-sse-406-not-acceptable-session-400s-402-dik)