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

<p>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.</p><p>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.</p><p>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. He then walks through how Skipper works: you write a spec, hand over control, and a compiler-like agent produces correct, runnable TypeScript without back-and-forth, backed by a sound incremental type system, reachability analysis, and a reactive runtime that applies diffs in milliseconds.</p><p>He closes with a grounded take on how the developer role is shifting — not disappearing — toward the kind of design, integration, and oversight work that always mattered most.</p><p>Key topics discussed:</p><ul><li>Why AI agents will always try to cheat on your tests</li><li>The union-vs-intersection bug an LLM introduced silently</li><li>Spec-driven development to keep LLMs on track</li><li>How to separate the AI that verifies from the one that fixes</li><li>Skipper: a compiler-like closed-loop coding agent</li><li>Sound, incremental TypeScript built for AI-speed iteration</li><li>Hot-reloading state without restarting — in milliseconds</li><li>Why developers are all becoming tech leads</li></ul><p>Timestamps:</p><ul><li>(00:00:00) Trailer &amp; Intro</li><li>(00:02:34) How Did Julien Create the Hack Programming Language at Facebook?</li><li>(00:05:53) Does Static Typing Make Your Code More Secure?</li><li>(00:09:54) How Did You Convince Facebook Engineers to Adopt Hack at Scale?</li>