diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index c25314eab..fdd595892 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -205,6 +205,7 @@ async def _handle_stateful_request(self, scope: Scope, receive: Receive, send: S
         if request_mcp_session_id is None:
             # New session case
             logger.debug("Creating new transport")
+            is_session_migration = False
             async with self._session_creation_lock:
                 new_session_id = uuid4().hex
                 http_transport = StreamableHTTPServerTransport(
@@ -214,6 +215,32 @@ async def _handle_stateful_request(self, scope: Scope, receive: Receive, send: S
                     security_settings=self.security_settings,
                     retry_interval=self.retry_interval,
                 )
+        else:
+            # Unknown or expired session ID - server likely restarted.
+            # Create a transport with the client's session ID and mark as a
+            # session migration so the transport starts already initialized.
+            # This allows clients to reconnect transparently without sending
+            # a new initialize request.
+            if request_mcp_session_id in self._server_instances:
+                # Should not happen: already handled above, but check under lock
+                transport = self._server_instances[request_mcp_session_id]
+                await transport.handle_request(scope, receive, send)
+                return
+            logger.info(f"Unknown session {request_mcp_session_id}, reusing client session ID")
+            is_session_migration = True
+            async with self._session_creation_lock:
+                if request_mcp_session_id in self._server_instances:
+                    transport = self._server_instances[request_mcp_session_id]
+                    await transport.handle_request(scope, receive, send)
+                    return
+                new_session_id = request_mcp_session_id
+                http_transport = StreamableHTTPServerTransport(
+                    mcp_session_id=new_session_id,
+                    is_json_response_enabled=self.json_response,
+                    event_store=self.event_store,
+                    security_settings=self.security_settings,
+                    retry_interval=self.retry_interval,
+                )
 
                 assert http_transport.mcp_session_id is not None
                 self._server_instances[http_transport.mcp_session_id] = http_transport
@@ -235,11 +262,15 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                                 http_transport.idle_scope = idle_scope
 
                             with idle_scope:
+                                # For session migration (server restart), use stateless mode
+                                # so the session starts already initialized. Otherwise the
+                                # client receives "Received request before initialization"
+                                # for any non-initialize request sent with the old session ID.
                                 await self.app.run(
                                     read_stream,
                                     write_stream,
                                     self.app.create_initialization_options(),
-                                    stateless=False,
+                                    stateless=is_session_migration,
                                 )
 
                             if idle_scope.cancelled_caught:
@@ -268,22 +299,6 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
 
                 # Handle the HTTP request and return the response
                 await http_transport.handle_request(scope, receive, send)
-        else:
-            # Unknown or expired session ID - return 404 per MCP spec
-            # TODO: Align error code once spec clarifies
-            # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
-            logger.info(f"Rejected request with unknown or expired session ID: {request_mcp_session_id[:64]}")
-            error_response = JSONRPCError(
-                jsonrpc="2.0",
-                id=None,
-                error=ErrorData(code=INVALID_REQUEST, message="Session not found"),
-            )
-            response = Response(
-                content=error_response.model_dump_json(by_alias=True, exclude_unset=True),
-                status_code=HTTPStatus.NOT_FOUND,
-                media_type="application/json",
-            )
-            await response(scope, receive, send)
 
 
 class StreamableHTTPASGIApp: