LangGraph Tutor Skill

Expert guidance for building production AI agents with LangGraph.

Quick Start

from langgraph.graph import StateGraph, START, END
from typing import Annotated
from typing_extensions import TypedDict
from langgraph.graph.message import add_messages

class State(TypedDict):
    messages: Annotated[list, add_messages]

def chatbot(state: State):
    return {"messages": [llm.invoke(state["messages"])]}

graph = StateGraph(State)
graph.add_node("chatbot", chatbot)
graph.add_edge(START, "chatbot")
graph.add_edge("chatbot", END)
app = graph.compile()

Core Concepts

1. State Definition

State flows through the graph. Use TypedDict with optional reducers:

class State(TypedDict):
    messages: Annotated[list, add_messages]  # Accumulates
    context: str                              # Overwrites
    count: Annotated[int, operator.add]       # Sums

2. Nodes

Functions that receive state and return updates:

def my_node(state: State) -> dict:
    # Read from state
    messages = state["messages"]
    # Return updates (don't mutate!)
    return {"context": "new value"}

3. Edges

Connect nodes in the graph:

graph.add_edge(START, "node_a")      # Entry point
graph.add_edge("node_a", "node_b")   # Linear flow
graph.add_edge("node_b", END)        # Exit point

4. Conditional Routing

Route based on state:

def router(state: State) -> Literal["path_a", "path_b"]:
    if condition:
        return "path_a"
    return "path_b"

graph.add_conditional_edges("source", router)

5. Memory/Checkpointing

Persist state across invocations:

from langgraph.checkpoint.memory import InMemorySaver

memory = InMemorySaver()
app = graph.compile(checkpointer=memory)

config = {"configurable": {"thread_id": "user-123"}}
result = app.invoke({"messages": [msg]}, config)

6. Human-in-the-Loop

Pause for human input:

from langgraph.types import interrupt

def approval_node(state):
    response = interrupt({"question": "Approve?"})
    return {"approved": response["answer"]}

Common Patterns

ReAct Agent (Reasoning + Acting)

See references/patterns.md for full implementation.

Supervisor Multi-Agent

See references/multi-agent.md for patterns.

Tool Calling

See references/tools.md for integration.

Validation

Run the validation script to check your graph:

python scripts/validate_graph.py your_graph.py

Examples

Working examples in examples/:

simple_chatbot.py - Minimal chatbot
tool_agent.py - Agent with tools
multi_agent.py - Supervisor pattern

Common Errors

Error	Cause	Fix
"Node has no outgoing edge"	Missing edge	Add edge to END or next node
"State key not found"	Accessing undefined key	Check State TypedDict
"Cannot serialize"	Complex objects in state	Use JSON-serializable types

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