LangChain LangGraph Basics (Python)

Overview

A conditional edge whose router returns a string that is not in path_map halts the graph without reaching END. No exception. No log line. The invocation just returns whatever state existed at the halt point — pain-catalog entry P56, and the single most common reason a newly wired StateGraph "almost works." The sibling pain: Command(update={"messages": [msg]}) wipes the prior message history because messages was declared as a plain list[AnyMessage] instead of Annotated[list[AnyMessage], add_messages] — the reducer is what turns update into "append" instead of "replace" (P18).

Two more gotchas this skill defuses: