SSE not working in latest MCP version

[HesamZamanpour]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

Hi team,
I’ve noticed that in the latest release of the MCP protocol, older MCP clients using the SSE transport no longer appear to be supported.

I have two main questions:

1. Is SSE planned to be deprecated entirely going forward?
2. Would it be possible for the MCP system to support both the new streamableHttp protocol and the previous SSE-based streaming in parallel, to ensure backward compatibility?

This would help with smooth transitions for clients still relying on the SSE-based setup.

Example Code

Python, Pydantic AI & LLM client version

python==3.12.9
pydantic-ai==0.2.12

[DouweM]

@HesamZamanpour Sorry you're running into this -- we'd been under the impression that #1716 was backward-compatible with SSE servers, but it appears not. We do intend to keep supporting them, but we may need separate classes for SSE and Streamable HTTP servers.

@Kludex was looking into this earlier today, I'll have to sync with him to see what the status is.

[DouweM]

There's some more discussion in #1880, I'll close this as a dupe of that.