Your AI agent's smallest diffs are its most dangerous

Last month, an AI coding agent handed me a beautiful fix. Five lines. Elegant. It reused an existing helper, matched the codebase style, compiled on the first try. Exactly the kind of diff we've all learned to praise since "make the agent write less code" became the standard advice. It was also completely untested, and it sat on a password-recovery path. That diff taught me something I now consider the central problem of AI-assisted coding in 2026: we've spent a year teaching agents to write less code, and almost no time teaching them to prove the code they kept actually holds. The two failure modes Every AI coding agent fails in one of two directions. Failure mode #1: the over-build. You ask for a date comparison; you get a new dependency, a ValidationService class, and a config layer. This one is well known — it's why minimal-code prompts and skills became popular, and they genuinely work on it. Failure mode #2: the confidently small diff. Minimal, clean, written after reading half the flow, verified never — dropped onto a path that handles money, auth, or user data. It compiles. It demos. It detonates in week three. Here's the uncomfortable part: fixing #1 aggressively makes #2 more likely. When the objective function is "shortest diff," the first things to quietly disappear are edge-case handling, failure-path tests, and the guard clause that looked optional. The diff gets smaller. The blast radius doesn't. A five-line change to a payment path is more dangerous than a four-hundred-line internal script that runs once. Code size is not risk. Blast radius is risk. Yet almost every skill and prompt in this category optimizes for size alone. What a guard does differently This is why I built Guardsman 💂 — an open-source skill that behaves less like a minimalist and more like the royal guard in front of the palace: nothing passes the post unchallenged, and the level of challenge depends on what's behind the gate. Three duties, on every task: 1. Read the standing orders