Subgraphs

A subgraph is a graph that is used as a node in another graph — this is the concept of encapsulation applied to LangGraph. Subgraphs allow you to build complex systems with multiple components that are themselves graphs.

Some reasons for using subgraphs are:

• building multi-agent systems
• when you want to reuse a set of nodes in multiple graphs
• when you want different teams to work on different parts of the graph independently, you can define each part as a subgraph, and as long as the subgraph interface (the input and output schemas) is respected, the parent graph can be built without knowing any details of the subgraph

The main question when adding subgraphs is how the parent graph and subgraph communicate, i.e. how they pass the state between each other during the graph execution. There are two scenarios:

• parent and subgraph have shared state keys in their state schemas. In this case, you can include the subgraph as a node in the parent graph

  from langgraph.graph import StateGraph, MessagesState, START
  
  # Subgraph
  
  def call_model(state: MessagesState):
      response = model.invoke(state["messages"])
      return {"messages": response}
  
  subgraph_builder = StateGraph(State)
  subgraph_builder.add_node(call_model)
  ...
  subgraph = subgraph_builder.compile()
  
  # Parent graph
  
  builder = StateGraph(State)
  builder.add_node("subgraph_node", subgraph)
  builder.add_edge(START, "subgraph_node")
  graph = builder.compile()
  ...
  graph.invoke({"messages": [{"role": "user", "content": "hi!"}]})
  

• parent graph and subgraph have different schemas (no shared state keys in their state schemas). In this case, you have to call the subgraph from inside a node in the parent graph: this is useful when the parent graph and the subgraph have different state schemas and you need to transform state before or after calling the subgraph

  from typing_extensions import TypedDict, Annotated
  from langchain_core.messages import AnyMessage
  from langgraph.graph import StateGraph, MessagesState, START
  from langgraph.graph.message import add_messages
  
  class SubgraphMessagesState(TypedDict):
      subgraph_messages: Annotated[list[AnyMessage], add_messages]
  
  # Subgraph
  
  def call_model(state: SubgraphMessagesState):
      response = model.invoke(state["subgraph_messages"])
      return {"subgraph_messages": response}
  
  subgraph_builder = StateGraph(State)
  subgraph_builder.add_node(call_model)
  ...
  subgraph = subgraph_builder.compile()
  
  # Parent graph
  
  def call_subgraph(state: MessagesState):
      response = subgraph.invoke({"subgraph_messages": state["messages"]})
      return {"messages": response["subgraph_messages"]}
  
  builder = StateGraph(State)
  builder.add_node("subgraph_node", call_subgraph)
  builder.add_edge(START, "subgraph_node")
  graph = builder.compile()
  ...
  graph.invoke({"messages": [{"role": "user", "content": "hi!"}]})