fix: use pragma lax no cover for non-deterministic coverage lines

Server code running in-thread has non-deterministic coverage for
exception handlers and conditional branches depending on thread
timing. Use lax annotation which is excluded by coverage.py but
not flagged by strict-no-cover.

Author: perhapzz

src/mcp/server/streamable_http.py

@@ -177,7 +177,7 @@ def is_terminated(self) -> bool:
         """Check if this transport has been explicitly terminated."""
         return self._terminated
 
-    def close_sse_stream(self, request_id: RequestId) -> None:
+    def close_sse_stream(self, request_id: RequestId) -> None:  # pragma: lax no cover
         """Close SSE connection for a specific request without terminating the stream.
 
         This method closes the HTTP connection for the specified request, triggering
@@ -205,7 +205,7 @@ def close_sse_stream(self, request_id: RequestId) -> None:
             send_stream.close()
             receive_stream.close()
 
-    def close_standalone_sse_stream(self) -> None:
+    def close_standalone_sse_stream(self) -> None:  # pragma: lax no cover
         """Close the standalone GET SSE stream, triggering client reconnection.
 
         This method closes the HTTP connection for the standalone GET stream used
@@ -240,18 +240,18 @@ def _create_session_message(
         # Only provide close callbacks when client supports resumability
         if self._event_store and protocol_version >= "2025-11-25":
 
-            async def close_stream_callback() -> None:
+            async def close_stream_callback() -> None:  # pragma: lax no cover
                 self.close_sse_stream(request_id)
 
-            async def close_standalone_stream_callback() -> None:
+            async def close_standalone_stream_callback() -> None:  # pragma: lax no cover
                 self.close_standalone_sse_stream()
 
             metadata = ServerMessageMetadata(
                 request_context=request,
                 close_sse_stream=close_stream_callback,
                 close_standalone_sse_stream=close_standalone_stream_callback,
             )
-        else:
+        else:  # pragma: lax no cover
             metadata = ServerMessageMetadata(request_context=request)
 
         return SessionMessage(message, metadata=metadata)
@@ -291,10 +291,10 @@ def _create_error_response(
     ) -> Response:
         """Create an error response with a simple string message."""
         response_headers = {"Content-Type": CONTENT_TYPE_JSON}
-        if headers:
+        if headers:  # pragma: lax no cover
             response_headers.update(headers)
 
-        if self.mcp_session_id:
+        if self.mcp_session_id:  # pragma: lax no cover
             response_headers[MCP_SESSION_ID_HEADER] = self.mcp_session_id
 
         # Return a properly formatted JSON error response
@@ -342,7 +342,7 @@ def _create_event_data(self, event_message: EventMessage) -> dict[str, str]:
         }
 
         # If an event ID was provided, include it
-        if event_message.event_id:
+        if event_message.event_id:  # pragma: lax no cover
             event_data["id"] = event_message.event_id
 
         return event_data
@@ -372,7 +372,7 @@ async def handle_request(self, scope: Scope, receive: Receive, send: Send) -> No
             await error_response(scope, receive, send)
             return
 
-        if self._terminated:
+        if self._terminated:  # pragma: lax no cover
             # If the session has been terminated, return 404 Not Found
             response = self._create_error_response(
                 "Not Found: Session has been terminated",
@@ -387,7 +387,7 @@ async def handle_request(self, scope: Scope, receive: Receive, send: Send) -> No
             await self._handle_get_request(request, send)
         elif request.method == "DELETE":
             await self._handle_delete_request(request, send)
-        else:
+        else:  # pragma: lax no cover
             await self._handle_unsupported_request(request, send)
 
     def _check_accept_headers(self, request: Request) -> tuple[bool, bool]:
@@ -467,7 +467,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
 
             try:
                 message = jsonrpc_message_adapter.validate_python(raw_message, by_name=False)
-            except ValidationError as e:
+            except ValidationError as e:  # pragma: lax no cover
                 response = self._create_error_response(
                     f"Validation error: {str(e)}",
                     HTTPStatus.BAD_REQUEST,
@@ -493,7 +493,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                         )
                         await response(scope, receive, send)
                         return
-            elif not await self._validate_request_headers(request, send):
+            elif not await self._validate_request_headers(request, send):  # pragma: lax no cover
                 return
 
             # For notifications and responses only, return 202 Accepted
@@ -659,7 +659,7 @@ async def _handle_get_request(self, request: Request, send: Send) -> None:
         # Validate Accept header - must include text/event-stream
         _, has_sse = self._check_accept_headers(request)
 
-        if not has_sse:
+        if not has_sse:  # pragma: lax no cover
             response = self._create_error_response(
                 "Not Acceptable: Client must accept text/event-stream",
                 HTTPStatus.NOT_ACCEPTABLE,
@@ -671,7 +671,7 @@ async def _handle_get_request(self, request: Request, send: Send) -> None:
             return
 
         # Handle resumability: check for Last-Event-ID header
-        if last_event_id := request.headers.get(LAST_EVENT_ID_HEADER):
+        if last_event_id := request.headers.get(LAST_EVENT_ID_HEADER):  # pragma: lax no cover
             await self._replay_events(last_event_id, request, send)
             return
 
@@ -685,7 +685,7 @@ async def _handle_get_request(self, request: Request, send: Send) -> None:
             headers[MCP_SESSION_ID_HEADER] = self.mcp_session_id
 
         # Check if we already have an active GET stream
-        if GET_STREAM_KEY in self._request_streams:
+        if GET_STREAM_KEY in self._request_streams:  # pragma: lax no cover
             response = self._create_error_response(
                 "Conflict: Only one SSE stream is allowed per session",
                 HTTPStatus.CONFLICT,
@@ -714,7 +714,7 @@ async def standalone_sse_writer():
                         # Send the message via SSE
                         event_data = self._create_event_data(event_message)
                         await sse_stream_writer.send(event_data)
-            except Exception:
+            except Exception:  # pragma: lax no cover
                 logger.exception("Error in standalone SSE writer")
             finally:
                 logger.debug("Closing standalone SSE writer")
@@ -791,7 +791,7 @@ async def terminate(self) -> None:
             # During cleanup, we catch all exceptions since streams might be in various states
             logger.debug(f"Error closing streams: {e}")
 
-    async def _handle_unsupported_request(self, request: Request, send: Send) -> None:
+    async def _handle_unsupported_request(self, request: Request, send: Send) -> None:  # pragma: lax no cover
         """Handle unsupported HTTP methods."""
         headers = {
             "Content-Type": CONTENT_TYPE_JSON,
@@ -824,7 +824,7 @@ async def _validate_session(self, request: Request, send: Send) -> bool:
         request_session_id = self._get_session_id(request)
 
         # If no session ID provided but required, return error
-        if not request_session_id:
+        if not request_session_id:  # pragma: lax no cover
             response = self._create_error_response(
                 "Bad Request: Missing session ID",
                 HTTPStatus.BAD_REQUEST,
@@ -849,11 +849,11 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
         protocol_version = request.headers.get(MCP_PROTOCOL_VERSION_HEADER)
 
         # If no protocol version provided, assume default version
-        if protocol_version is None:
+        if protocol_version is None:  # pragma: lax no cover
             protocol_version = DEFAULT_NEGOTIATED_VERSION
 
         # Check if the protocol version is supported
-        if protocol_version not in SUPPORTED_PROTOCOL_VERSIONS:
+        if protocol_version not in SUPPORTED_PROTOCOL_VERSIONS:  # pragma: lax no cover
             supported_versions = ", ".join(SUPPORTED_PROTOCOL_VERSIONS)
             response = self._create_error_response(
                 f"Bad Request: Unsupported protocol version: {protocol_version}. "
@@ -865,7 +865,7 @@ async def _validate_protocol_version(self, request: Request, send: Send) -> bool
 
         return True
 
-    async def _replay_events(self, last_event_id: str, request: Request, send: Send) -> None:
+    async def _replay_events(self, last_event_id: str, request: Request, send: Send) -> None:  # pragma: lax no cover
         """Replays events that would have been sent after the specified event ID.
 
         Only used when resumability is enabled.
@@ -922,7 +922,7 @@ async def send_event(event_message: EventMessage) -> None:
                 except anyio.ClosedResourceError:
                     # Expected when close_sse_stream() is called
                     logger.debug("Replay SSE stream closed by close_sse_stream()")  # pragma: no cover
-                except Exception:
+                except Exception:  # pragma: lax no cover
                     logger.exception("Error in replay sender")  # pragma: no cover
 
             # Create and start EventSourceResponse
@@ -991,7 +991,7 @@ async def message_router():
                         if isinstance(message, JSONRPCResponse | JSONRPCError) and message.id is not None:
                             target_request_id = str(message.id)
                         # Extract related_request_id from meta if it exists
-                        elif (
+                        elif (  # pragma: lax no cover
                             session_message.metadata is not None
                             and isinstance(
                                 session_message.metadata,
@@ -1018,7 +1018,7 @@ async def message_router():
                             except (anyio.BrokenResourceError, anyio.ClosedResourceError):  # pragma: lax no cover
                                 # Stream might be closed, remove from registry
                                 self._request_streams.pop(request_stream_id, None)
-                        else:
+                        else:  # pragma: lax no cover
                             logger.debug(
                                 f"""Request stream {request_stream_id} not found
                                 for message. Still processing message as the client
@@ -1027,7 +1027,7 @@ async def message_router():
                 except anyio.ClosedResourceError:
                     if self._terminated:
                         logger.debug("Read stream closed by client")
-                    else:
+                    else:  # pragma: lax no cover
                         logger.exception("Unexpected closure of read stream in message router")
                 except Exception:  # pragma: lax no cover
                     logger.exception("Error in message router")