Building agents taught me the model is rarely the problem. What's your hard-won lesson?

I've been building agents lately and made pretty much every mistake possible. Here's the stuff that actually helped, in case it saves you some time: Tool design beats model choice. Most of my "why is the agent dumb" moments were just badly described tools. Clearer names and tighter schemas fixed more than swapping models ever did. Small loops beat giant prompts. A few narrow steps worked way better than one big "do everything" prompt. Log what the agent sees, not just what it says. Most bugs were bad context, not bad reasoning. Add guardrails early. Anything that sends, deletes, or spends money needs a confirm step before you trust the loop. Tiny evals go a long way. Even a handful of test cases caught stuff I'd never have noticed by eye. What's the thing you learned the hard way?