LangChain Common Errors (Python)

Overview

The same twelve-plus LangChain 1.0 / LangGraph 1.0 tracebacks show up every week in production: ImportError: cannot import name 'ChatOpenAI' from 'langchain.chat_models', AttributeError: 'list' object has no attribute 'lower', GraphRecursionError: Recursion limit of 25 reached, TypeError: Object of type datetime is not JSON serializable, KeyError: 'question' deep in LCEL internals. Stack traces are ambiguous, docs sprawl across 0.2 / 0.3 / 1.0 eras, and engineers lose 30–90 minutes per incident re-deriving the fix.

This skill is a paste-match catalog of 14 entries (E01–E14) grouped into 3 reference files by category, plus a triage decision tree. Every entry opens with the exact exception class and message string you see in your terminal, names the cause in one sentence with a pain-catalog code, and gives a one-line fix or a reference-file pointer. Pinned to langchain-core 1.0.x, langchain 1.0.x, langgraph 1.0.x, verified 2026-04-21.

Pain-catalog anchors: P02, P06, P09, P10, P16, P17, P38, P39, P40, P41, P42, P55, P56, P57, P66.