Transport error: SSE stream disconnected while MCP usage works

[weipienlee]

What happened?

Almost every second I see an error appear in the log (see ex log below) while the MCP "tool" just works. It regards the remote Tavily search MCP server. On the surface it all seems to work just fine ie query and results can be viewed in the webapp without any (error) messaging.

Version Information

docker images | grep librechat
ghcr.io/danny-avila/librechat-dev latest cc7ac083836c 29 hours ago 1.56GB
ghcr.io/danny-avila/librechat-rag-api-dev-lite latest 174ed0a539bc 2 weeks ago 1.6GB

git rev-parse HEAD
a309c20d85eb0f7ce1e7ff141f8a5edf5650c732

Steps to Reproduce

1. Add Tavily to mcp config (see below).
2. Start docker compose.
3. Look add LibreChat container logs for errors.

---

tavily-search:
  type: streamable-http
  url: "https://mcp.tavily.com/mcp/?tavilyApiKey=${TAVILY_API_KEY}"
  initTimeout: 20000 # thought this would help
  timeout: 10000 # thought this would help

What browsers are you seeing the problem on?

Firefox

Relevant log output

2025-09-13 22:54:42 info: [MCP][tavily-search] Creating streamable-http transport: https://mcp.tavily.com/mcp/?tavilyApiKey=xxxxxxxxxx
2025-09-13 22:54:42 error: [MCP][tavily-search] Transport error: SSE stream disconnected: AbortError: This operation was aborted
2025-09-13 22:54:47 error: [MCP][tavily-search] Transport error: SSE stream disconnected: TypeError: terminated

Screenshots

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[weipienlee]

type sse is also not working. Work around is (but should not be necessary):

tavily-search:
  command: npx
  args:
    - -y
    - mcp-remote
    - "https://mcp.tavily.com/mcp/?tavilyApiKey=${TAVILY_API_KEY}"

Extra info on the remote MCP server:

• https://docs.tavily.com/documentation/mcp
• https://www.npmjs.com/package/tavily-mcp

[danny-avila]

This is an MCP server that uses streamable transport, without a fallback to SSE transport, so it won't work.

SSE transports are working for MCP servers that strictly use it.

[weipienlee]

So, I understand it's by design that LibreChat "forces" mcp servers to fallback to sse. If correct then keep in mind sse will (soon) be deprecated:

https://mcp-framework.com/docs/Transports/sse/

[danny-avila]

No, streamable HTTP also makes use of "SSE":
https://modelcontextprotocol.io/specification/draft/basic/transports#streamable-http

[weipienlee]

Ah, maintaining backwards compatibility makes more sense. Just for my understanding. Does this statement:

This is an MCP server that uses streamable transport, without a fallback to SSE transport, so it won't work.

mean LibreChat currently "requires" sse fallback for the Streamable transport type and, therefore, only uses sse in case of Streamable transport type servers configs?

[changkun]

@danny-avila do you aware if this SDK follows the SSE transport for streamable transport? https://github.com/mark3labs/mcp-go

We see the MCP servers build based on this SDK generally fail and throw similar errors with LibreChat.

What should be a correct way to fix this error when using this SDK to build MCP server?

[danny-avila]

Almost every second I see an error appear in the log (see ex log below) while the MCP "tool" just works.

This can happen as the MCP server initializes, it can disconnect as part of the connection flow. If everything is working, there is no real issue.