diff --git a/libs/langgraph/langgraph/agent/middleware/swarm.py b/libs/langgraph/langgraph/agent/middleware/swarm.py
index 82be98549..feed59d6c 100644
--- a/libs/langgraph/langgraph/agent/middleware/swarm.py
+++ b/libs/langgraph/langgraph/agent/middleware/swarm.py
@@ -24,6 +24,7 @@ class SwarmMiddleware(AgentMiddleware):
       * handoff tool naming / descriptions
     """
 
+    @dataclass
     class State(AgentMiddleware.State):
         active_agent: str | None = None