LangGraph Subgraphs and Composition (Python)

Overview

A parent StateGraph invokes a compiled child subgraph as a node. The child node writes state["answer"] = "42" and returns. The parent's next node reads state["answer"] and gets None. No error, no warning, no deprecation notice — just a silent None that surfaces as a wrong answer three nodes later when the router picks the "couldn't find it" branch.

The cause is pain-catalog entry P21: LangGraph subgraphs run on an independent state schema. Only keys declared in both the parent's TypedDict and the child's TypedDict propagate across the subgraph boundary. answer existed in the child schema but not the parent schema, so it was discarded on return. The fix is to declare answer in both schemas (with matching reducers, if the field is a list) or to use explicit Command(graph=ParentGraph, update={"answer": "42"}) to bubble it up.