How to beginner · 3 min read

How to add edges to LangGraph

Q: How to add edges to LangGraph

To add edges in LangGraph, use the add_edge method on a StateGraph instance, specifying the source node and the target node or END. Edges define transitions between states in the graph.

Quick answer

To add edges in LangGraph, use the add_edge method on a StateGraph instance, specifying the source node and the target node or END. Edges define transitions between states in the graph.

PREREQUISITES

Python 3.8+
pip install langgraph
Basic Python knowledge

Setup

Install the langgraph package via pip and import the necessary classes to create and manipulate a state graph.

bash

pip install langgraph

output

Collecting langgraph
  Downloading langgraph-0.1.0-py3-none-any.whl (10 kB)
Installing collected packages: langgraph
Successfully installed langgraph-0.1.0

Step by step

Create a StateGraph with a typed state dictionary, add nodes with functions, then add edges between nodes using add_edge. Use END to mark terminal states.

python

from langgraph.graph import StateGraph, END
from typing import TypedDict

class State(TypedDict):
    messages: list[str]

def node1(state: State) -> State:
    return {"messages": state["messages"] + ["Hello from node1"]}

def node2(state: State) -> State:
    return {"messages": state["messages"] + ["Hello from node2"]}

# Create the graph
graph = StateGraph(State)

# Add nodes
graph.add_node("node1", node1)
graph.add_node("node2", node2)

# Add edges
graph.add_edge("node1", "node2")
graph.add_edge("node2", END)

# Compile and invoke
app = graph.compile()
result = app.invoke({"messages": []})
print(result["messages"])

output

['Hello from node1', 'Hello from node2']

Common variations

You can add multiple edges from one node to several nodes to create branching logic.
Use END to mark terminal nodes where the graph stops.
Edges can be added dynamically after node creation to modify graph flow.

python

graph.add_edge("node1", "node2")
graph.add_edge("node1", END)  # node1 can lead to node2 or end

# Recompile after adding edges
app = graph.compile()
result = app.invoke({"messages": []})
print(result["messages"])

output

['Hello from node1', 'Hello from node2']

Troubleshooting

If you get a KeyError when adding edges, ensure the source and target nodes exist in the graph.
Calling invoke before compile will raise errors; always compile after adding edges.
Use END from langgraph.graph to mark terminal edges correctly.

✅

Key Takeaways

Use add_edge(source, target) on StateGraph to define transitions.
Edges can point to other nodes or END to mark graph termination.
Always compile the graph with compile() before invoking.
Add edges after nodes are registered to avoid errors.
Multiple edges from one node enable branching workflows.

Verified 2026-04

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