StreamableHttp - stateless server support (#554)

src/mcp/server/lowlevel/server.py

@@ -479,11 +479,21 @@ class Server(Generic[LifespanResultT]):
         # but also make tracing exceptions much easier during testing and when using
         # in-process servers.
         raise_exceptions: bool = False,
+        # When True, the server as stateless deployments where
+        # clients can perform initialization with any node. The client must still follow
+        # the initialization lifecycle, but can do so with any available node
+        # rather than requiring initialization for each connection.
+        stateless: bool = False,
     ):
         async with AsyncExitStack() as stack:
             lifespan_context = await stack.enter_async_context(self.lifespan(self))
             session = await stack.enter_async_context(
-                ServerSession(read_stream, write_stream, initialization_options)
+                ServerSession(
+                    read_stream,
+                    write_stream,
+                    initialization_options,
+                    stateless=stateless,
+                )
             )
 
             async with anyio.create_task_group() as tg:

src/mcp/server/session.py

@@ -85,11 +85,17 @@ class ServerSession(
         read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception],
         write_stream: MemoryObjectSendStream[types.JSONRPCMessage],
         init_options: InitializationOptions,
+        stateless: bool = False,
     ) -> None:
         super().__init__(
             read_stream, write_stream, types.ClientRequest, types.ClientNotification
         )
-        self._initialization_state = InitializationState.NotInitialized
+        self._initialization_state = (
+            InitializationState.Initialized
+            if stateless
+            else InitializationState.NotInitialized
+        )
+
         self._init_options = init_options
         self._incoming_message_stream_writer, self._incoming_message_stream_reader = (
             anyio.create_memory_object_stream[ServerRequestResponder](0)