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Conditional Edges

A plain add_edge("a", "b") always routes from a to b. add_conditional_edges routes based on a function of the current state — this is how a graph loops an agent node against a tool node until the model stops requesting calls.

from typing import Literal
from langgraph.graph import StateGraph, START, END

def route(state: State) -> Literal["tools", "__end__"]:
    last = state["messages"][-1]
    if getattr(last, "tool_calls", None):
        return "tools"
    return END

builder = StateGraph(State)
builder.add_node("agent", call_model)
builder.add_node("tools", call_tools)
builder.add_edge(START, "agent")
builder.add_conditional_edges("agent", route)
builder.add_edge("tools", "agent")
graph = builder.compile()
danger

Always set a step cap. graph.invoke(input, {"recursion_limit": 25}) raises GraphRecursionError instead of running forever if the router never returns END — a router bug here is a billing incident, not a graceful failure. For a softer stop, track a RemainingSteps field in state and route to END when it runs low, so the graph returns its current state instead of erroring.

See also​