HTTP Streamable vs SSE endpoints

[pksorensen]

When having worked with the spec in node and other platforms, there seem to be a clear developer path if one are creating legacy SSE + Message endpoint vs http streamble transport.

Coming to c# sdk its hard to understand when its using http streamble vs sse endpoints. Right now it is mixed up with disabling sateless which is confusing.

Example from source code: If its not stateless it automatically enable /sse endpoints. It means you cant do a statefull MCP server in c# atm without enabling the /sse endpoint, which then lead to additional endpoints that some clients might use due to fallback principals if an error happens on the http streamble endpoint "/".

This issue asks that we consider adding another option to controll if legacy endpoints should be enabled. What do people prefer? I am personally thinking that a option/feature flag where we basically sets the transport more clearly when configuring the app similar to when setting up stdio. I think the current approach makes it hard to couble code/setup to the spec.

        if (!streamableHttpHandler.HttpServerTransportOptions.Stateless)
        {
            // The GET and DELETE endpoints are not mapped in Stateless mode since there's no way to send unsolicited messages
            // for the GET to handle, and there is no server-side state for the DELETE to clean up.
            streamableHttpGroup.MapGet("", streamableHttpHandler.HandleGetRequestAsync)
                .WithMetadata(new ProducesResponseTypeMetadata(StatusCodes.Status200OK, contentTypes: ["text/event-stream"]));
            streamableHttpGroup.MapDelete("", streamableHttpHandler.HandleDeleteRequestAsync);

            // Map legacy HTTP with SSE endpoints only if not in Stateless mode, because we cannot guarantee the /message requests
            // will be handled by the same process as the /sse request.
            var sseHandler = endpoints.ServiceProvider.GetRequiredService<SseHandler>();
            var sseGroup = mcpGroup.MapGroup("")
                .WithDisplayName(b => $"MCP HTTP with SSE | {b.DisplayName}");

            sseGroup.MapGet("/sse", sseHandler.HandleSseRequestAsync)
                .WithMetadata(new ProducesResponseTypeMetadata(StatusCodes.Status200OK, contentTypes: ["text/event-stream"]));
            sseGroup.MapPost("/message", sseHandler.HandleMessageRequestAsync)
                .WithMetadata(new AcceptsMetadata(["application/json"]))
                .WithMetadata(new ProducesResponseTypeMetadata(StatusCodes.Status202Accepted));
        }