The null input that broke my production agent and what fixed it (opens in new tab)

The demo ran flawlessly for three weeks. Every test input parsed clean, every output routed correctly, and I thought we had a reliable system. Then a supplier sent a confirmation email with an empty subject line. The agent, which was supposed to extract order references and route them into a queue, got a null where it expected a string. It didn't crash. That would have been better. Instead it generated a plausible-looking order reference, routed it, and the downstream system processed it like...