fix: propagate pre-endpoint errors in sse_client instead of deadlocking

When sse_reader encounters an error before receiving the endpoint event,
the except handler tried to send the exception to read_stream_writer.
With a zero-buffer stream and no reader (the caller is still blocked in
tg.start() waiting for task_status.started()), send() blocks forever.

Track whether started() has fired. Before it, re-raise so the exception
propagates through tg.start(). After it, send to the stream as before.

This also adds a guard for the case where a server sends a message event
before the endpoint event, which would deadlock on the same send() call.

The dedicated SSEError handler from #975 is removed since the started
flag now handles all pre-endpoint exceptions uniformly.

Github-Issue: #447

Author: maxisbey

src/mcp/client/sse.py

@@ -9,7 +9,6 @@
 from anyio.abc import TaskStatus
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from httpx_sse import aconnect_sse
-from httpx_sse._exceptions import SSEError
 
 from mcp import types
 from mcp.shared._httpx_utils import McpHttpClientFactory, create_mcp_http_client
@@ -69,6 +68,12 @@ async def sse_client(
             write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 
             async def sse_reader(task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED):
+                # Before task_status.started() fires, the caller is blocked inside
+                # tg.start() and nobody reads from read_stream. Sending to the
+                # zero-buffer stream in that phase would deadlock, so errors must
+                # be raised instead. After started(), the caller has the streams
+                # and errors are delivered through read_stream.
+                started = False
                 try:
                     async for sse in event_source.aiter_sse():  # pragma: no branch
                         logger.debug(f"Received SSE event: {sse.event}")
@@ -79,27 +84,28 @@ async def sse_reader(task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED):
 
                                 url_parsed = urlparse(url)
                                 endpoint_parsed = urlparse(endpoint_url)
-                                if (  # pragma: no cover
+                                if (
                                     url_parsed.netloc != endpoint_parsed.netloc
                                     or url_parsed.scheme != endpoint_parsed.scheme
                                 ):
-                                    error_msg = (  # pragma: no cover
+                                    raise ValueError(
                                         f"Endpoint origin does not match connection origin: {endpoint_url}"
                                     )
-                                    logger.error(error_msg)  # pragma: no cover
-                                    raise ValueError(error_msg)  # pragma: no cover
 
                                 if on_session_created:
                                     session_id = _extract_session_id_from_endpoint(endpoint_url)
                                     if session_id:
                                         on_session_created(session_id)
 
                                 task_status.started(endpoint_url)
+                                started = True
 
                             case "message":
                                 # Skip empty data (keep-alive pings)
                                 if not sse.data:
                                     continue
+                                if not started:
+                                    raise RuntimeError("Received message event before endpoint event")
                                 try:
                                     message = types.jsonrpc_message_adapter.validate_json(sse.data, by_name=False)
                                     logger.debug(f"Received server message: {message}")
@@ -112,11 +118,10 @@ async def sse_reader(task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED):
                                 await read_stream_writer.send(session_message)
                             case _:  # pragma: no cover
                                 logger.warning(f"Unknown SSE event: {sse.event}")  # pragma: no cover
-                except SSEError as sse_exc:  # pragma: lax no cover
-                    logger.exception("Encountered SSE exception")
-                    raise sse_exc
-                except Exception as exc:  # pragma: lax no cover
+                except Exception as exc:
                     logger.exception("Error in sse_reader")
+                    if not started:
+                        raise
                     await read_stream_writer.send(exc)
                 finally:
                     await read_stream_writer.aclose()

tests/shared/test_sse.py

@@ -604,6 +604,102 @@ async def mock_aiter_sse() -> AsyncGenerator[ServerSentEvent, None]:
             assert msg.message.id == 1
 
 
+def _mock_sse_connection(aiter_sse: AsyncGenerator[ServerSentEvent, None]) -> Any:
+    """Patch sse_client's HTTP layer to yield the given SSE event stream."""
+    mock_event_source = MagicMock()
+    mock_event_source.aiter_sse.return_value = aiter_sse
+    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()))
+
+    return patch.multiple(
+        "mcp.client.sse",
+        create_mcp_http_client=Mock(return_value=mock_client),
+        aconnect_sse=Mock(return_value=mock_aconnect_sse),
+    )
+
+
+@pytest.mark.anyio
+async def test_sse_client_raises_on_endpoint_origin_mismatch() -> None:
+    """Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/447
+
+    When the server sends an endpoint URL with a different origin than the
+    connection URL, sse_client must raise promptly instead of deadlocking.
+    Before the fix, the ValueError was caught and sent to a zero-buffer stream
+    with no reader, hanging forever.
+    """
+
+    async def events() -> AsyncGenerator[ServerSentEvent, None]:
+        yield ServerSentEvent(event="endpoint", data="http://wrong-host:9999/messages?sessionId=abc")
+        await anyio.sleep_forever()
+
+    with _mock_sse_connection(events()), anyio.fail_after(5):
+        with pytest.RaisesGroup(pytest.RaisesExc(ValueError, match="Endpoint origin does not match")):
+            async with sse_client("http://test/sse"):
+                pytest.fail("sse_client should not yield on origin mismatch")
+
+
+@pytest.mark.anyio
+async def test_sse_client_raises_on_error_before_endpoint() -> None:
+    """Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/447
+
+    Any exception raised while waiting for the endpoint event must propagate
+    instead of deadlocking on the zero-buffer read stream.
+    """
+
+    async def events() -> AsyncGenerator[ServerSentEvent, None]:
+        raise ConnectionError("connection reset by peer")
+        yield  # pragma: no cover
+
+    with _mock_sse_connection(events()), anyio.fail_after(5):
+        with pytest.RaisesGroup(pytest.RaisesExc(ConnectionError, match="connection reset")):
+            async with sse_client("http://test/sse"):
+                pytest.fail("sse_client should not yield on pre-endpoint error")
+
+
+@pytest.mark.anyio
+async def test_sse_client_raises_on_message_before_endpoint() -> None:
+    """Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/447
+
+    If the server sends a message event before the endpoint event (protocol
+    violation), sse_client must raise rather than deadlock trying to send the
+    message to a stream nobody is reading yet.
+    """
+
+    async def events() -> AsyncGenerator[ServerSentEvent, None]:
+        yield ServerSentEvent(event="message", data='{"jsonrpc":"2.0","id":1,"result":{}}')
+        await anyio.sleep_forever()
+
+    with _mock_sse_connection(events()), anyio.fail_after(5):
+        with pytest.RaisesGroup(pytest.RaisesExc(RuntimeError, match="before endpoint event")):
+            async with sse_client("http://test/sse"):
+                pytest.fail("sse_client should not yield on protocol violation")
+
+
+@pytest.mark.anyio
+async def test_sse_client_delivers_post_endpoint_errors_via_stream() -> None:
+    """After the endpoint is received, errors in sse_reader are delivered on the
+    read stream so the session can handle them, rather than crashing the task group.
+    """
+
+    async def events() -> AsyncGenerator[ServerSentEvent, None]:
+        yield ServerSentEvent(event="endpoint", data="/messages/?session_id=abc")
+        raise ConnectionError("mid-stream failure")
+
+    with _mock_sse_connection(events()), anyio.fail_after(5):
+        async with sse_client("http://test/sse") as (read_stream, _):
+            received = await read_stream.receive()
+            assert isinstance(received, ConnectionError)
+            assert "mid-stream failure" in str(received)
+
+
 @pytest.mark.anyio
 async def test_sse_session_cleanup_on_disconnect(server: None, server_url: str) -> None:
     """Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/1227