JSON Parsing Fails & Client Gets Stuck in Waiting When Tool Returns Results with Special Unicode Characters

[Hugh488]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

During my usage, I noticed that if the tool's returned results contain special Unicode characters, it will cause JSON parsing to abort. What's more critical is that when a JSON parsing error occurs on the client side, the parsing function sends the exception directly to the tool-invoking function without wrapping the exception in the format agreed upon by both parties. This prevents the tool-invoking function from properly capturing the exception, leading it to get stuck in a state of continuous waiting.

In mcp.client.sse:

case "message":
    try:
        message = types.JSONRPCMessage.model_validate_json(  # noqa: E501
            sse.data
        )
        logger.debug(f"Received server message: {message}")
    except Exception as exc:
        logger.exception("Error parsing server message")
        await read_stream_writer.send(exc)
        continue

    session_message = SessionMessage(message)
    await read_stream_writer.send(session_message)

Should the exc be encapsulated before executing await read_stream_writer.send(exc)?
Currently, it seems that when an exception is sent, the message cannot be received at line 272 in mcp.shared.session, getting stuck in a wait state.

Example Code

When there is `\u2028` in the tool's results.

Python & MCP Python SDK

python3.13.7
MCP Python SDK:1.13.1

[pja-ant]

Thanks for reporting, I can reproduce this.

From my investigation so far, it looks like the httpx parser is incorrectly treating \u2028 as a line separator, which goes against what the SSE spec mandates: https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream

This is where SSE parsing happens, it uses aiter_lines to iterate the stream, which internally uses LineDecoder with a set of line separators that doesn't match what SSE specifies.

[pja-ant]

And yeah, the exception handling here is bad also for the reasons mentioned.

[pja-ant]

Reported issue to httpx-sse: florimondmanca/httpx-sse#34