fix: reset retry counter only on forward progress, not unconditionally

The previous fix (attempt + 1 unconditionally) broke legitimate
multi-reconnection scenarios where the server checkpoints progress
between disconnections. Now we check whether new SSE events were
delivered (last_event_id changed) — if so, reset the counter; if not,
increment it toward MAX_RECONNECTION_ATTEMPTS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: chernistry

src/mcp/client/streamable_http.py

@@ -423,7 +423,13 @@ async def _handle_reconnection(
 
                 # Stream ended again without response - reconnect again
                 logger.info("SSE stream disconnected, reconnecting...")
-                await self._handle_reconnection(ctx, reconnect_last_event_id, reconnect_retry_ms, attempt + 1)
+                # Reset attempt counter only if the stream delivered new events
+                # (i.e. made forward progress). If no new events arrived, the
+                # server is connecting then dropping immediately — count that
+                # towards the retry budget to avoid infinite loops (#2393).
+                made_progress = reconnect_last_event_id != last_event_id
+                next_attempt = 0 if made_progress else attempt + 1
+                await self._handle_reconnection(ctx, reconnect_last_event_id, reconnect_retry_ms, next_attempt)
         except Exception as e:  # pragma: no cover
             logger.debug(f"Reconnection failed: {e}")
             # Try to reconnect again if we still have an event ID

tests/shared/test_streamable_http.py

@@ -2329,15 +2329,16 @@ async def test_streamable_http_client_preserves_custom_with_mcp_headers(
 
 @pytest.mark.anyio
 async def test_handle_reconnection_does_not_retry_infinitely():
-    """Reconnection must count TOTAL attempts, not reset on each successful connect.
+    """Reconnection must give up when no forward progress is made.
 
     Regression test for #2393: when a stream connects successfully but drops
     before delivering a response, the attempt counter was reset to 0 on the
     recursive call, allowing an infinite retry loop.
 
     This test simulates a stream that connects, yields one non-completing SSE
-    event, then ends — repeatedly. With MAX_RECONNECTION_ATTEMPTS the loop
-    must terminate.
+    event with the SAME event ID each time (no new data), then ends —
+    repeatedly. Without forward progress the loop must terminate within
+    MAX_RECONNECTION_ATTEMPTS.
     """
     transport = StreamableHTTPTransport(url="http://localhost:8000/mcp")
     transport.session_id = "test-session"
@@ -2354,13 +2355,15 @@ async def fake_aconnect_sse(*args: Any, **kwargs: Any) -> AsyncIterator[Any]:
         mock_response = MagicMock()
         mock_response.raise_for_status = MagicMock()
 
-        # Yield a single non-completing notification SSE event, then end the stream
-        # (simulating a server that drops the connection after partial delivery)
+        # Yield a single non-completing notification SSE event with the SAME
+        # event ID every time, then end the stream.  Because the ID never
+        # changes, the transport sees no forward progress and should count
+        # each reconnection towards the retry budget.
         async def aiter_sse() -> AsyncIterator[ServerSentEvent]:
             yield ServerSentEvent(
                 event="message",
                 data='{"jsonrpc":"2.0","method":"notifications/progress","params":{"progressToken":"tok","progress":1,"total":10}}',
-                id=f"evt-{connect_count}",
+                id="evt-static",
                 retry=None,
             )