Originally published on Loop & Retry — field notes on building LLM agents that survive production.
Search for "what does idempotent mean" and you'll find a definition, but not always one you can use. Here's the version that matters: idempotent means doing the same thing twice gives you the same result as doing it once.
This is essential infrastructure vocabulary. It's the difference between "safe to retry" and "dangerous to retry," and that difference determines whether your system charges customers twice, sends duplicate messages, corrupts data, or works reliably when the network fails.
Part 1: The Core Idea
Imagine you're playing chess. You tell someone: "Move my pawn to e4." They do it. If you say it again, does the pawn move twice? No — it stays on e4. That's idempotent. The board state is the same whether you made that request once, twice, or a hundred times.
