This article explains LangGraph’s two state‑update modes—overwrite and merge—shows how to use Annotated with custom reducers such as operator.add or add_messages, demonstrates when reducers run, and provides full Python examples, including persistence with checkpointers and custom merge functions.
LangGraph’s state management hinges on three core concepts—Channel as the storage unit, Annotation as the declarative API for Channels, and Reducer as the pure function that merges updates—understanding their interactions resolves common bugs, enables custom state schemas, and ensures correct concurrent node updates.
This article dissects LangGraph's core execution engine, showing how it transforms LLM calls into a state‑machine workflow with mutable State, Nodes, Edges, Reducers, a scheduler loop, conditional branching, and parallel fan‑out/fan‑in execution.
This article introduces LangGraph 1.0 as a programming‑language‑style framework for AI agents, explains its core abstractions—State, Node, Edge, Reducer, and Human‑in‑the‑Loop—shows how to define state and node functions, builds simple and parallel graphs with static, conditional, and MapReduce edges, and demonstrates conflict‑resolution using built‑in and custom reducers.
