Skip empty SSE data to avoid parsing errors (#1753)

Co-authored-by: ARJUN-TS1 <arjun.ts1@ibm.com>
Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

src/mcp/client/sse.py

@@ -105,6 +105,9 @@ async def sse_client(
                                         task_status.started(endpoint_url)
 
                                     case "message":
+                                        # Skip empty data (keep-alive pings)
+                                        if not sse.data:
+                                            continue
                                         try:
                                             message = types.JSONRPCMessage.model_validate_json(  # noqa: E501
                                                 sse.data

tests/shared/test_sse.py

@@ -4,12 +4,13 @@ import socket
 import time
 from collections.abc import AsyncGenerator, Generator
 from typing import Any
-from unittest.mock import Mock
+from unittest.mock import AsyncMock, MagicMock, Mock, patch
 
 import anyio
 import httpx
 import pytest
 import uvicorn
+from httpx_sse import ServerSentEvent
 from inline_snapshot import snapshot
 from pydantic import AnyUrl
 from starlette.applications import Starlette
@@ -28,8 +29,11 @@ from mcp.shared.exceptions import McpError
 from mcp.types import (
     EmptyResult,
     ErrorData,
+    Implementation,
     InitializeResult,
+    JSONRPCResponse,
     ReadResourceResult,
+    ServerCapabilities,
     TextContent,
     TextResourceContents,
     Tool,
@@ -532,3 +536,69 @@ def test_sse_server_transport_endpoint_validation(endpoint: str, expected_result
         sse = SseServerTransport(endpoint)
         assert sse._endpoint == expected_result
         assert sse._endpoint.startswith("/")
+
+
+# ResourceWarning filter: When mocking aconnect_sse, the sse_client's internal task
+# group doesn't receive proper cancellation signals, so the sse_reader task's finally
+# block (which closes read_stream_writer) doesn't execute. This is a test artifact -
+# the actual code path (`if not sse.data: continue`) IS exercised and works correctly.
+# Production code with real SSE connections cleans up properly.
+@pytest.mark.filterwarnings("ignore::ResourceWarning")
+@pytest.mark.anyio
+async def test_sse_client_handles_empty_keepalive_pings() -> None:
+    """Test that SSE client properly handles empty data lines (keep-alive pings).
+
+    Per the MCP spec (Streamable HTTP transport): "The server SHOULD immediately
+    send an SSE event consisting of an event ID and an empty data field in order
+    to prime the client to reconnect."
+
+    This test mocks the SSE event stream to include empty "message" events and
+    verifies the client skips them without crashing.
+    """
+    # Build a proper JSON-RPC response using types (not hardcoded strings)
+    init_result = InitializeResult(
+        protocolVersion="2024-11-05",
+        capabilities=ServerCapabilities(),
+        serverInfo=Implementation(name="test", version="1.0"),
+    )
+    response = JSONRPCResponse(
+        jsonrpc="2.0",
+        id=1,
+        result=init_result.model_dump(by_alias=True, exclude_none=True),
+    )
+    response_json = response.model_dump_json(by_alias=True, exclude_none=True)
+
+    # Create mock SSE events using httpx_sse's ServerSentEvent
+    async def mock_aiter_sse() -> AsyncGenerator[ServerSentEvent, None]:
+        # First: endpoint event
+        yield ServerSentEvent(event="endpoint", data="/messages/?session_id=abc123")
+        # Empty data keep-alive ping - this is what we're testing
+        yield ServerSentEvent(event="message", data="")
+        # Real JSON-RPC response
+        yield ServerSentEvent(event="message", data=response_json)
+
+    mock_event_source = MagicMock()
+    mock_event_source.aiter_sse.return_value = mock_aiter_sse()
+    mock_event_source.response = MagicMock()
+    mock_event_source.response.raise_for_status = MagicMock()
+
+    mock_aconnect_sse = MagicMock()
+    mock_aconnect_sse.__aenter__ = AsyncMock(return_value=mock_event_source)
+    mock_aconnect_sse.__aexit__ = AsyncMock(return_value=None)
+
+    mock_client = MagicMock()
+    mock_client.__aenter__ = AsyncMock(return_value=mock_client)
+    mock_client.__aexit__ = AsyncMock(return_value=None)
+    mock_client.post = AsyncMock(return_value=MagicMock(status_code=200, raise_for_status=MagicMock()))
+
+    with (
+        patch("mcp.client.sse.create_mcp_http_client", return_value=mock_client),
+        patch("mcp.client.sse.aconnect_sse", return_value=mock_aconnect_sse),
+    ):
+        async with sse_client("http://test/sse") as (read_stream, _):
+            # Read the message - should skip the empty one and get the real response
+            msg = await read_stream.receive()
+            # If we get here without error, the empty message was skipped successfully
+            assert not isinstance(msg, Exception)
+            assert isinstance(msg.message.root, types.JSONRPCResponse)
+            assert msg.message.root.id == 1