StreamableHttpHandler rejects concurrent GET SSE streams despite spec allowing them

[deyanp]

Description

StreamableHttpHandler enforces a single active GET SSE stream per session via TryStartGetRequest(). When a second GET arrives (without Last-Event-ID), the server returns:

{"error":{"code":-32000,"message":"Bad Request: This server does not support multiple GET requests. Start a new session or use Last-Event-ID header to resume."},"id":"","jsonrpc":"2.0"}

However, the MCP spec (2025-06-18) explicitly allows multiple concurrent SSE streams:

Multiple Connections

1. The client MAY remain connected to multiple SSE streams simultaneously.
2. The server MUST send each of its JSON-RPC messages on only one of the connected streams; that is, it MUST NOT broadcast the same message across multiple streams.

Observed behavior

In production, the Anthropic/ClaudeAI MCP client opens a GET SSE stream and then attempts to open a second one a few seconds later. The server rejects it with 400.

19:46:28 GET /api/v1/mcp → 200 text/event-stream (SSE stream opened)
19:46:36 GET /api/v1/mcp → 400 "does not support multiple GET requests"

Expected behavior

Either:

1. Support multiple concurrent GET streams per the spec's "Multiple Connections" section, or
2. Provide a configuration option (e.g., HttpTransportOptions.AllowMultipleGetStreams) so server implementers can opt in

Environment

• ModelContextProtocol.AspNetCore v1.2.0
• .NET 10.0
• MCP client: Anthropic/ClaudeAI 1.0.0

[halter73]

Multiple Connections

The client MAY remain connected to multiple SSE streams simultaneously.
The server MUST send each of its JSON-RPC messages on only one of the connected streams; that is, it MUST NOT broadcast the same message across multiple streams.

Neither of these requirements require the server to support multiple concurrent GET streams. It just allows the client to attempt it (maybe, but I think that language is more about concurrent POST requests than GET requests), and sets requirements for the server if it does allow multiple streams (which we do because of the aforementioned POST requests).

https://csharp.sdk.modelcontextprotocol.io/concepts/stateless/stateless.html#transports-and-sessions explains why the Streamable HTTP transport works the way it does.

And if you disagree, there's an even lower level StreamableHttpServerTransport that lets you implement the spec in your own specific way more similar to the low-level APIs offered by some of the other SDKs as their only Streamable HTTP API.

[deyanp]

well, I am just observing the following (client is Claude Desktop) in the logs:

2026-04-01T19:46:36.118775632Z info: Microsoft.AspNetCore.Hosting.Diagnostics[1] Request starting HTTP/1.1 GET http://aaa.bbb.com/api/v1/mcp - - -
2026-04-01T19:46:36.127728698Z info: Microsoft.AspNetCore.Http.Result.JsonResult[1] Setting HTTP status code 400.
2026-04-01T19:46:36.130584471Z info: Microsoft.AspNetCore.Http.Result.JsonResult[3] Writing value of type 'JsonRpcError' as Json.
2026-04-01T19:46:36.135064666Z info: Framework.WebApi.GET__mcp_[132] GET /api/v1/mcp responded 400: {"error":{"code":-32000,"message":"Bad Request: This server does not support multiple GET requests. Start a new session or use Last-Event-ID header to resume."},"id":"","jsonrpc":"2.0"} (CorId: 8b373e68-0939-49c2-bdd5-532a9abbf371)
2026-04-01T19:46:36.138169611Z info: Microsoft.AspNetCore.Hosting.Diagnostics[2] Request finished HTTP/1.1 GET http://aaa.bbb.com/api/v1/mcp - 400 - application/json;+charset=utf-8 18.0522ms

not sure what I can do about it ...