langgraph-fundamentals

To maintain stateful message history in LangGraph, define reducers for list-accumulating fields in your StateGraph. Reducers ensure that partial state updates from sequential nodes append to the history correctly rather than overwriting previous entries.

Use Command for conditional routing by bundling state updates and route selection together, while use Send for parallel worker fan-out to dispatch multiple asynchronous tasks simultaneously. Choose Send when you need concurrent execution across multiple nodes.

Your LangGraph workflow fails because the StateGraph requires compiling the graph before execution. Applying compile-before-invoke discipline establishes clear entry and exit points, reducing runtime surprises and ensuring the directed graph is safely executable.

The best way to structure parallel worker fan-out in LangGraph is by using the Send orchestration primitive. Send dispatches multiple asynchronous workers simultaneously, while defined reducers safely merge the resulting partial state updates back into the shared state.

Yes, LangGraph supports error handling and streaming for agent orchestration. By applying reliability patterns during StateGraph construction, you can safely stream stateful message histories and handle runtime exceptions within your directed graph workflows.