swarm

libs/langgraph/langgraph/agent/__init__.py

@@ -144,6 +144,8 @@ def create_agent(
     first_node = (
         f"{middleware_w_before[0].__class__.__name__}.before_model"
         if middleware_w_before
+        else f"{middleware_w_modify_model_request[0].__class__.__name__}.modify_model_request"
+        if middleware_w_modify_model_request
         else "model_request"
     )
     last_node = (

libs/langgraph/langgraph/agent/middleware/swarm.py

@@ -1,13 +1,66 @@
-from langgraph.agent.types import AgentMiddleware, AgentState, ModelRequest
-from typing import Dict, Any, List, Optional, Union
-from langgraph.types import interrupt
+from langgraph.agent.types import AgentMiddleware, ModelRequest
+from langchain_core.tools import BaseTool, tool
+from langgraph.agent import create_agent
+from langchain_core.messages import AIMessage, ToolMessage
+from dataclasses import dataclass
+from typing import cast
+
+@dataclass
+class SwarmAgent:
+    name: str
+    system_prompt: str
+    tools: list[BaseTool]
 
 
-class SwarmMiddleWare(AgentMiddleware):
+class SwarmMiddleware(AgentMiddleware):
+    """Swarm middleware.
+    
+    TODOs:
+    * Support create_agent for handoffs
+    * Support handoff customization 
+      * do we want to include handoff messages / enable togglging
+      * default active agent
+      * handoff tool naming / descriptions
+    """
+    
+    class State(AgentMiddleware.State):
+        active_agent: str | None = None
 
-    def __init__(self, model_configs: dict[str, dict]):
-        super().__init__()
+    @staticmethod
+    def _create_handoff_tools(agents: list[SwarmAgent]) -> list[BaseTool]:
 
-    def modify_model_request(
-        self, request: ModelRequest, state: AgentState
-    ) -> ModelRequest:
+        def handoff_tool(agent_name: str) -> str:
+            return f"Handing off to {agent_name}"
+
+        return [
+            tool(f"handoff_to_{agent.name}", description=f"Handoff tool to trigger a handoff to {agent.name}")(handoff_tool)
+            for agent in agents
+        ]
+
+    def __init__(self, agents: list[SwarmAgent]):
+        self.agents: dict[str, SwarmAgent] = {agent.name: agent for agent in agents}
+        self.handoff_tools = self._create_handoff_tools(agents)
+
+    def modify_model_request(self, request: ModelRequest, state: State) -> ModelRequest:
+        if (active_agent := getattr(state, "active_agent", None)) is not None:
+            agent = self.agents[active_agent]
+            request.system_prompt = agent.system_prompt
+            request.tools = agent.tools
+
+        request.tools.extend(self.handoff_tools)
+        return request
+
+    def after_model(self, state) -> State | None:
+        # TODO: handle parallel handoffs
+
+        ai_msg: AIMessage = cast(AIMessage, state.messages[-1])
+        if ai_msg.tool_calls:
+            for call in ai_msg.tool_calls:
+                if call["name"].startswith("handoff_to_"):
+                    active_agent = call["args"]["agent_name"]
+                    return {
+                        "messages": [ToolMessage(name=call["name"], content=f"Successfully transferred to {active_agent}")],
+                        "active_agent": active_agent,
+                        "jump_to": "model",
+                    }
+            return None

libs/langgraph/langgraph/agent/types.py

@@ -21,16 +21,16 @@ JumpTo = Literal["tools", "model", "__end__"]
 class ModelRequest:
     model: BaseChatModel
     system_prompt: str
-    messages: Sequence[AnyMessage]  # excluding system prompt
+    messages: list[AnyMessage]  # excluding system prompt
     tool_choice: Any
-    tools: Sequence[BaseTool]
+    tools: list[BaseTool]
     response_format: ResponseFormat | None
 
 
 @dataclass
 class AgentState:
     messages: Annotated[list[AnyMessage], add_messages]
-    model_request: Annotated[ModelRequest | None, EphemeralValue]
+    model_request: Annotated[ModelRequest | None, EphemeralValue] = None
     jump_to: Annotated[JumpTo | None, EphemeralValue] = None
     response: dict | None = None
 

libs/prebuilt/langgraph/prebuilt/chat_agent_executor.py

@@ -38,7 +38,6 @@ from typing_extensions import Annotated, NotRequired, TypedDict
 
 from langgraph._internal._runnable import RunnableCallable, RunnableLike
 from langgraph._internal._typing import MISSING
-from langgraph.agent import create_agent
 from langgraph.agent.types import AgentMiddleware
 from langgraph.errors import ErrorCode, create_error_message
 from langgraph.graph import END, StateGraph
@@ -472,6 +471,9 @@ def create_react_agent(
         assert pre_model_hook is None
         assert post_model_hook is None
         assert state_schema is None
+
+        from langgraph.agent import create_agent
+
         return create_agent(
             model=model,
             tools=tools,