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. Subgraph 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)
...
# highlight-next-line
subgraph = subgraph_builder.compile()

# Parent graph

builder = StateGraph(State)
# highlight-next-line
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):
    # highlight-next-line
    subgraph_messages: Annotated[list[AnyMessage], add_messages]

# Subgraph

# highlight-next-line
def call_model(state: SubgraphMessagesState):
    response = model.invoke(state["subgraph_messages"])
    return {"subgraph_messages": response}

subgraph_builder = StateGraph(SubgraphMessagesState)
subgraph_builder.add_node("call_model_from_subgraph", call_model)
subgraph_builder.add_edge(START, "call_model_from_subgraph")
...
# highlight-next-line
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)
# highlight-next-line
builder.add_node("subgraph_node", call_subgraph)
builder.add_edge(START, "subgraph_node")
graph = builder.compile()
...
graph.invoke({"messages": [{"role": "user", "content": "hi!"}]})