fix: stop infinite GET stream reconnection on repeated empty connections

Only reset the attempt counter when events were actually received
during the connection. Connections that close immediately without
delivering events now count toward MAX_RECONNECTION_ATTEMPTS.

Github-Issue:#1401

Author: heyhayes

src/mcp/client/streamable_http.py

@@ -214,7 +214,9 @@ async def handle_get_stream(self, client: httpx.AsyncClient, read_stream_writer:
                     event_source.response.raise_for_status()
                     logger.debug("GET SSE connection established")
 
+                    received_events = False
                     async for sse in event_source.aiter_sse():
+                        received_events = True
                         # Track last event ID for reconnection
                         if sse.id:
                             last_event_id = sse.id
@@ -224,14 +226,18 @@ async def handle_get_stream(self, client: httpx.AsyncClient, read_stream_writer:
 
                         await self._handle_sse_event(sse, read_stream_writer)
 
-                    # Stream ended normally (server closed) - reset attempt counter
-                    attempt = 0
+                    # Only reset attempts if we actually received events;
+                    # empty connections count toward MAX_RECONNECTION_ATTEMPTS
+                    if received_events:
+                        attempt = 0
+                    else:
+                        attempt += 1
 
             except Exception:
                 logger.debug("GET stream error", exc_info=True)
                 attempt += 1
 
-            if attempt >= MAX_RECONNECTION_ATTEMPTS:  # pragma: no cover
+            if attempt >= MAX_RECONNECTION_ATTEMPTS:
                 logger.debug(f"GET stream max reconnection attempts ({MAX_RECONNECTION_ATTEMPTS}) exceeded")
                 return
 

tests/client/test_streamable_http.py

@@ -748,3 +748,26 @@ async def test_resolving_an_abandoned_request_after_the_reader_closed_is_contain
                 _abandoned_request_context(http, send), "evt-7", None, MAX_RECONNECTION_ATTEMPTS
             )
     send.close()
+
+
+@pytest.mark.anyio
+async def test_get_stream_gives_up_after_repeated_empty_connections(monkeypatch: pytest.MonkeyPatch) -> None:
+    """A GET stream that keeps opening but closing with no events counts each empty connection toward
+    the reconnection budget, so the loop terminates instead of reconnecting forever."""
+    monkeypatch.setattr("mcp.client.streamable_http.DEFAULT_RECONNECTION_DELAY_MS", 0)
+    get_requests = 0
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        nonlocal get_requests
+        get_requests += 1
+        return httpx.Response(200, headers={"content-type": "text/event-stream"}, content=b"")
+
+    transport = StreamableHTTPTransport("http://test/mcp")
+    transport.session_id = "sess-1"
+    send, receive = create_context_streams[SessionMessage | Exception](1)
+    async with httpx.AsyncClient(transport=httpx.MockTransport(handler)) as http:
+        with anyio.fail_after(5):
+            await transport.handle_get_stream(http, send)
+    assert get_requests == MAX_RECONNECTION_ATTEMPTS
+    send.close()
+    receive.close()