How to Call a Subgraph from a Node in LangGraph ?

In the previous article, we learned what subgraphs are and why they’re useful.

What Are Subgraphs in LangGraph?

We’ll begin with the first way to connect a subgraph to a parent graph. This approach is used when the parent graph and the subgraph have different state schemas.

Take a Getting Ready for Office routine. One of the steps is Making Tea. But making tea is not a single action — it involves several steps, such as boiling water, adding tea leaves, milk, sugar, and masala.

Since Making Tea is a complete workflow on its own, it makes sense to build it as a subgraph instead of a single node. The parent graph only needs to know when the tea is ready. It doesn’t need to know how the tea was made.

Let’s build the Tea Making subgraph and connect it to the Getting Ready for Office parent graph.

The Subgraph

class TeaState(TypedDict):
    water_ml: int
    milk_ml: int
    tea_leaves_tsp: int
    sugar_tsp: int
    tea_masala_tsp: int
    tea_ready: str

def boil_water(state: TeaState):
    return {"tea_ready": f"Boiled {state['water_ml']}ml water"}

def add_tea_leaves(state: TeaState):
    return {"tea_ready": state["tea_ready"] + f" -> added…