Migrate a legacy chain pipeline to a LangGraph state machine

TL;DR: Sequential LLMChain-style pipelines become StateGraph nodes with typed state. Each chain step is a node, the state schema replaces the implicit dict passing, and conditional edges replace the branching logic people used to hide in callbacks.

Chain to graph migration

The mapping

• Each chain step -> one node function that takes state and returns a dict of state updates.
• The implicit dict passed between chains -> an explicit TypedDict state schema.
• Branching inside chain code -> conditional edges.
• LLMChain internals (prompt + model + parser) -> LCEL inside the node (prompt | model | parser), or a model call plus parsing code.

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

class State(TypedDict):
    topic: str
    draft: str
    approved: bool

def write(state: State):
    return {"draft": model.invoke(f"Write about {state['topic']}").content}

def review(state: State):
    ...

builder = StateGraph(State)
builder.add_node("write", write)
builder.add_node("review", review)
builder.add_edge(START, "write")
builder.add_edge("write", "review")
builder.add_conditional_edges("review", lambda s: "write" if not s["approved"] else END)
graph = builder.compile(checkpointer=checkpointer)

Rules

• Nodes return dicts on every path. See the InvalidUpdateError diagnostic.
• Add the checkpointer at compile time if any step needs retries, human approval, or time travel. Retrofitting persistence later means touching every invoke call site.
• Keep node functions small and single-purpose; a node that does three things is a chain hiding inside a graph.
• State keys written by parallel branches need reducers (Annotated[list, operator.add]). Design the schema before the edges.

When to use this

• This covers exactly what the title says: Migrate a legacy chain pipeline to a LangGraph state machine.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with Migrate; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Compatibility

• Not pinned to a specific version; follows current Migrate behavior.