StreamableHttpClient Transport hangs on connection to other mcp servers

[wagnerjt]

Describe the bug

When trying to use this go-sdk http client transport to connect to other MCP servers outside of the ones created in this sdk. The transport never connects and eventually hits the default timeout provided by the internal httpclient.

To Reproduce

Simple client example with:

transport = mcp.NewStreamableClientTransport("127.0.0.1:8080/mcp", &mcp.StreamableClientTransportOptions{})

Using something like mcp-go's everything server with go run main.go -t http which serves on :8080.

I also tested this on a few other mcp servers written in the official python mcp sdk.

Additional context

It works using the SSE transport just fine against these other MCP servers. Additionally, it is successful using MCP servers created with this go-sdk like the hello example. Tested on v0.1.0-0.2.0 on windows/amd

[findleyr]

This is my bad -- in my haste to add a streamable client for testing, I did not implement support for plain JSON responses (our server only returns event streams). The real mistake was not failing the client request on an unexpected content type. Mailed a CL to fail loudly. (I'm out, but @jba and @samthanawalla can look into adding this support).

[wagnerjt]

Ah I see -- nice catch! I did not take a moment to dive further into the cause. Appreciate the help!