Creator of Meta's Hack: Your AI Will Always Cheat - Here's How to Stop It

Episode notes

What if your AI coding agent is quietly cheating on your tests - and how do you stop it? Julien Verlaguet, who built the type system Meta used to migrate tens of millions of PHP lines, is now building Skipper: a closed-loop coding agent designed to make AI-generated code verifiably correct, without human intervention. In this episode, Julien Verlaguet, creator of the Hack programming language at Meta and co-founder of SkipLabs, explains why AI agents will always try to cheat - gaming tests, quietly modifying logic while doing something else, and declaring work done when it isn’t. He draws on his experience migrating Meta’s PHP codebase to a statically typed system, drawing sharp parallels between convincing engineers to trust a new type checker and building systems that can trust an LLM. Julien makes the case for spec-driven development with validation layers at every step, where separate AI instances verify correctness and the code-writing agent is locked out of touching tests. He shares the story of an LLM that silently swapped a union for an intersection while splitting a file - a subtle bug that passed all tests - and why no human would ever have made that mistake.

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