How to add guardrails to your Claude agent in 10 lines

How to add guardrails to your Claude agent in 10 lines If you've run a Claude agent in production for more than a week, you've probably had a moment where it did something it really shouldn't have. Sent a test email to a real user. Called the delete endpoint. Made a charge at 3x the expected amount because the model got confused about currency. The usual fix is adding more instructions to the system prompt. "Never delete files in /prod." "Only email approved addresses." And it works fine, until it doesn't. Why system prompt rules break Claude follows instructions well in a fresh context. Give it a clean slate and a clear rule, it'll stick to it. The trouble starts when you've got 20,000 tokens of conversation history, a few tool call outputs, maybe some retrieved documents, and the model is trying to figure out what to do next. Context pressure is real. The relevant safety instruction is now 15,000 tokens back in the window, and the model is working with whatever's most salient. It's n