LangGraph Subgraphs and Composition (Python) Overview A parent invokes a compiled child subgraph as a node. The child node writes and returns. The parent's next node reads and gets . No error, no warning, no deprecation notice — just a silent 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 and the child's propagate across the subgraph boundary. existed in the child schema but not the parent schema, so it w…