langchain-common-errors

# LangChain Common Errors ## Overview Quick reference for the most frequent LangChain errors with exact error messages, root causes, and copy-paste fixes. ## Import Errors ### `Cannot find module '@langchain/openai'` ```bash # Provider package not installed npm install @langchain/openai # Also: @langchain/anthropic, @langchain/google-genai, @langchain/community ``` ### `Cannot import name 'ChatOpenAI' from 'langchain'` (Python) ```python # Old import path (pre-0.2). Use provider packages: # OLD: from langchain.chat_models import ChatOpenAI # NEW: from langchain_openai import ChatOpenAI ``` ### `@langchain/core version mismatch` ```bash # All @langchain/* packages must share the same minor version npm ls @langchain/core # Fix: update all together npm install @langchain/core@latest @langchain/openai@latest @langchain/anthropic@latest ``` ## Authentication Errors ### `AuthenticationError: Incorrect API key provided` ```typescript // Key not set or wrong format // Check: console.log("Key present:", !!process.env.OPENAI_API_KEY); console.log("Key prefix:", process.env.OPENAI_API_KEY?.slice(0, 7)); // Should be "sk-..." for OpenAI, "sk-ant-..." for Anthropic // Fix: ensure dotenv is loaded BEFORE imports import "dotenv/config"; import { ChatOpenAI } from "@langchain/openai"; ``` ### `Error: OPENAI_API_KEY is not set` ```typescript // Model constructor can't find the key // Option 1: environment variable process.env.OPENAI_API_KEY = "sk-..."; // Option 2: pass direct...