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.
