Skip empty SSE data to avoid parsing errors

[felixweinberger]

Skip empty SSE data events (keep-alive pings) in the streamable HTTP client to avoid JSON parsing errors.

Motivation and Context

SSE servers may send empty data lines as keep-alive pings. The current implementation attempts to parse all SSE data as JSON, which fails for empty data, causing connection errors.

How Has This Been Tested?

• Added unit test for _handle_sse_event with empty SSE data
• Existing streamable HTTP integration tests pass

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This is a defensive fix to handle keep-alive pings gracefully.

[meffmadd]

I provided a reproducible example in the relevant issue for this PR: #1146 (comment)
The issue also occurs on the reference everything MCP server.

[meffmadd]

Patching the _handle_sse_event method as proposed by this PR fixes the error:

import asyncio
from functools import wraps
from unittest.mock import patch

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

def patch_mcp_sse_issue(view_func):
    @wraps(view_func)
    async def wrapper(*args, **kwargs):
        sse = args[1]
        if not sse.data:
            return False
        else:
            return await view_func(*args, **kwargs)

    return wrapper


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__":
    from mcp.client.streamable_http import StreamableHTTPTransport
    with patch("mcp.client.streamable_http.StreamableHTTPTransport._handle_sse_event",
               new=patch_mcp_sse_issue(StreamableHTTPTransport._handle_sse_event)):
        asyncio.run(example())