Lazy import sse_starlette to reduce import time

src/mcp/server/sse.py

@@ -46,7 +46,6 @@ async def handle_sse(request):
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import ValidationError
-from sse_starlette import EventSourceResponse
 from starlette.requests import Request
 from starlette.responses import Response
 from starlette.types import Receive, Scope, Send
@@ -185,6 +184,8 @@ async def response_wrapper(scope: Scope, receive: Receive, send: Send):
                 In this case we close our side of the streams to signal the client that
                 the connection has been closed.
                 """
+                from sse_starlette import EventSourceResponse
+
                 await EventSourceResponse(content=sse_stream_reader, data_sender_callable=sse_writer)(
                     scope, receive, send
                 )

src/mcp/server/streamable_http.py

@@ -20,7 +20,6 @@
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import ValidationError
-from sse_starlette import EventSourceResponse
 from starlette.requests import Request
 from starlette.responses import Response
 from starlette.types import Receive, Scope, Send
@@ -441,7 +440,7 @@ async def _validate_accept_header(self, request: Request, scope: Scope, send: Se
             return False
         return True
 
-    async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:
+    async def _handle_post_request(self, scope: Scope, request: Request, receive: Receive, send: Send) -> None:  # noqa: PLR0915
         """Handle POST requests containing JSON-RPC messages."""
         writer = self._read_stream_writer
         if writer is None:  # pragma: no cover
@@ -613,6 +612,8 @@ async def sse_writer():
                         self._sse_stream_writers.pop(request_id, None)
                         await self._clean_up_memory_streams(request_id)
 
+                from sse_starlette import EventSourceResponse
+
                 # Create and start EventSourceResponse
                 # SSE stream mode (original behavior)
                 # Set up headers
@@ -730,6 +731,8 @@ async def standalone_sse_writer():
                 logger.debug("Closing standalone SSE writer")
                 await self._clean_up_memory_streams(GET_STREAM_KEY)
 
+        from sse_starlette import EventSourceResponse
+
         # Create and start EventSourceResponse
         response = EventSourceResponse(
             content=sse_stream_reader,
@@ -934,6 +937,8 @@ async def send_event(event_message: EventMessage) -> None:
                 except Exception:
                     logger.exception("Error in replay sender")
 
+            from sse_starlette import EventSourceResponse
+
             # Create and start EventSourceResponse
             response = EventSourceResponse(
                 content=sse_stream_reader,