State
A shared snapshot. Nodes return partials. Per-key reducers merge them. Default is overwrite.
Not a new model
The loop is the agent.
LangGraph is a runtime for work a chain can't do: retry, branch, wait for a human, fan out, remember.
The problem
A chain is a list. Real work is not.
A list can only go forward. Support tickets retry. Research re-queries. Refunds wait for a human. Fan-out writes three jokes and joins them. The moment you need a cycle, a branch, or a pause, the list dies — and you start drawing a graph whether you meant to or not.
The model
Four beats, then you compile.
That is the whole API surface you actually have to hold in your head.
Nodes
Functions. They read state and write an update — or a Command. They do not call the next step.
Edges
Static, conditional, or dynamic (Send / Command). Edges decide next. Don’t mix mechanisms on one node.
Compile
Validates the graph and binds a checkpointer. You invoke the compiled graph, not the builder.
Two doors