Build LangChain Chains Once with Lazy Initialization

Build LangChain chains once with lazy initialization Build LangChain chains once, on demand. Guard with a None check, initialize related singletons together on the first request, and let bad config fail as a clear runtime error instead of a startup crash. Why this matters Building a LangChain chain is not free. Constructing a SQLDatabase opens a database connection, inspects schema, and may sample rows. Instantiating ChatOpenAI validates configuration and prepares client state. Calling create_sql_query_chain wires prompts, models, and parsers into an executable graph. In our NL2SQL agent, initialization added several hundred milliseconds. If you do that work at import time, a missing DB URL or API key can kill the process before health checks or structured logs help you diagnose it. If you rebuild everything per request, you pay the same setup cost on every query before the model processes a single token. Lazy initialization avoids both problems. The problem Without lazy initialization