GET endpoint for unsolicited messages is called in stateless mode

[AndreaVielCGN]

I'm developing a MCP client and a server. I noticed that if I'm in stateful mode on the server (Stateless = false) the client will make only a POST / call to initialize the server. However if I enable the stateless mode (Stateless = true) the client will make an additional GET / call (that will get a 405 error) to the server.

In debugging I noticed the GET call is coming from here

csharp-sdk/src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

Line 154 in 07435dc

_getReceiveTask ??= ReceiveUnsolicitedMessagesAsync();

What I don't get is why the GET call is made only when stateless and how it is deciding that (since state is a feature of the server and not the client) and if it makes sense to do that at all since as far as I understand in stateless mode unsolicited messages aren't supported. I should be the opposite in principle

csharp-sdk/docs/concepts/stateless/stateless.md

Line 321 in 07435dc

The Streamable HTTP client automatically reconnects its SSE event stream when the connection drops. This only applies to **stateful sessions** — the GET event stream is how the server sends unsolicited messages to the client, and it requires an active session. Stream reconnection is separate from session expiry: reconnection recovers the event stream within an existing session, while the example above handles creating a new session after the server has terminated the old one.

.

If that matters the client is set to TransportMode = HttpTransportMode.StreamableHttp avoiding SSE.

[mikekistler]

Can you provide a simple repro for this issue, preferably in a GitHub repo that we can just clone, build, and run?