Streamable Http transport handle ping message

[Gujiawei-Edinburgh]

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

code in src/mcp/client/streamable_http.py:162 validates the payload from incoming SSE messages.
However, on the server side, EventSourceResponse is used, which periodically sends ping message. These ping messages will cause client side parsing message error.
When SSE is used to stream logs, these ping messages trigger unwanted behavior, such as flushing logs unnecessarily. Currently, there doesn’t appear to be a configuration option to disable the ping messages or adjust their interval.
As a potential improvement, the client could check if the incoming message has an empty data field before validation. If the data is empty, the client can treat it as a ping message and safely ignore it.

Example Code

async def _handle_sse_event(
        self,
        sse: ServerSentEvent,
        read_stream_writer: StreamWriter,
        original_request_id: RequestId | None = None,
        resumption_callback: Callable[[str], Awaitable[None]] | None = None,
        is_initialization: bool = False,
    ) -> bool:
        """Handle an SSE event, returning True if the response is complete."""
        if sse.event == "message":
            try:
               # potential changes here
                if not sse.data:
                   logger.debug("Receiving a ping message")
                   # double check if directly return True is compatible or enough
                   return True
                message = JSONRPCMessage.model_validate_json(sse.data)
                logger.debug(f"SSE message: {message}")

Python & MCP Python SDK

python version: 3.13
MCP SDK version: 1.11.0

[raptium]

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format

The ping mesage is just a comment, so the client should ignore it at all.

[felixweinberger]

Hi @Gujiawei-Edinburgh thanks for submitting this issue, could you provide a snippet or minimal example that demonstrates the pings causing an issue? By default there shouldn't be anything broken by sending pings.

[meffmadd]

I am getting the exact same error! I have mcp version 1.22.0. Here is an example:

1. Start the everything MCP server: npx @modelcontextprotocol/server-everything@2025.11.25 streamableHttp
2. Run the following file:

import asyncio

from mcp import ClientSession
from mcp.client.streamable_http import streamablehttp_client


async def example():
    async with streamablehttp_client("http://localhost:3001/mcp", headers={"Content-Type": "application/json"}) as (
            read_stream,
            write_stream,
            _,
    ):
        async with ClientSession(read_stream, write_stream) as session:
            await session.initialize()

if __name__ == "__main__":
    asyncio.run(example())

3. See output:

Error parsing SSE message
Traceback (most recent call last):
  File "/path/to/.venv/lib/python3.12/site-packages/mcp/client/streamable_http.py", line 164, in _handle_sse_event
    message = JSONRPCMessage.model_validate_json(sse.data)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/path/to/.venv/lib/python3.12/site-packages/pydantic/main.py", line 744, in model_validate_json
    return cls.__pydantic_validator__.validate_json(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for JSONRPCMessage
  Invalid JSON: EOF while parsing a value at line 1 column 0 [type=json_invalid, input_value='', input_type=str]
    For further information visit https://errors.pydantic.dev/2.11/v/json_invalid

Process finished with exit code 0

This fails because sse.data is an empty string. Since the everything MCP Server is a reference implementation I would expect that this should not fail and that this is a standard and expected case.