Questions about MCP's Streamable HTTP Behavior

[qnnn]

Hello Team,

I'm currently testing the streamable HTTP client in mcp-javasdk, using modelcontextprotocol/server-everything@latest as my MCP server. I have a few questions that I'd appreciate your help with:

1. Regarding SSE behavior: If GET:/mcp is a standalone SSE endpoint, when multiple clients (from different deployments) connect to it, will only one client receive notifications (like change events) while others miss them? This question comes from the following TypeScript implementation I observed:

   https://github.com/modelcontextprotocol/typescript-sdk/blob/9bf5c629796220c39ada3d6b5a004cbba30cfb4d/src/server/streamableHttp.ts#L201-L211

2. About stateless mode: Since GET:/mcp is required to return 405 in stateless mode, does this mean the MCP server cannot proactively notify MCP clients?

   https://github.com/modelcontextprotocol/typescript-sdk/blob/ddc1a0c292602b3133f92f04710379ed1e450325/src/examples/server/simpleStatelessStreamableHttp.ts#L127-L137

I apologize for not being very familiar with TypeScript and Python implementations.

Thank you!

[evalstate]

1. You need to update connected Clients on a per-session basis.
2. You can open an SSE Stream on the /POST to send e.g. tool update notifications.

[qnnn]

Regarding point 2, a /POST request does not exist at all times. It seems that the stateless mode of streamable HTTP is conditionally full-duplex? For example, in the notifications/initialized request, the Mcp server callback might not be able to request the Mcp Client.

[evalstate]

Adding a link to a write up I did recently on Transports https://huggingface.co/blog/building-hf-mcp . Let me know if your questions are answered and I'll close this thread.

[qnnn]

@evalstate Thanks a lot—this post is really clear and helpful