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. 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.

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.

Key topics discussed:

Why AI agents will always try to cheat on your tests
The union-vs-intersection bug an LLM introduced silently
Spec-driven development to keep LLMs on track
How to separate the AI that verifies from the one that fixes
Skipper: a compiler-like closed-loop coding agent
Sound, incremental TypeScript built for AI-speed iteration
Hot-reloading state without restarting — in milliseconds
Why developers are all becoming tech leads

Timestamps:

(00:00:00) Trailer & Intro
(00:02:34) How Did Julien Create the Hack Programming Language at Facebook?
(00:05:53) Does Static Typing Make Your Code More Secure?
(00:09:54) How Did You Convince Facebook Engineers to Adopt Hack at Scale?
(00:17:15) How Can Engineers Overcome Skepticism Toward AI Coding Tools?
(00:22:44) Should Junior Engineers Trust AI-Generated Code?
(00:29:44) How Do You Build Reliable Guardrails for LLM-Generated Code?
(00:42:15) What Validation Strategies Prevent AI Agents From Cheating on Tests?
(00:45:54) What Is Skipper and How Does a Closed-Loop Coding Agent Work?
(00:54:59) How Does Skipper Compare to Claude Code in Terms of Correctness?
(00:58:27) How Do You Get Started With Skipper and What Does the Output Look Like?
(01:04:50) How Will the Software Developer Role Change in an AI-First World?
(01:09:06) 3 Tech Lead Wisdom

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Julien Verlaguet’s Bio
Julien Verlaguet is a programming language designer and the Founder and CEO of SkipLabs. He is best known as the creator of Hack—the gradually typed language he built at Facebook that currently powers over 100 million lines of the company’s production code. After creating the open-source reactive framework Skip, Julien founded SkipLabs in 2022. His company recently launched Skipper, a closed-loop coding agent that takes a single prompt from a developer and returns a running, validated service.

Follow Julien:

LinkedIn – linkedin.com/in/julien-verlaguet-b5710a20
X – x.com/JulienVerlaguet
SkipLabs - skiplabs.io
Skipper - skipperai.dev
Skipper’s Discord – discord.gg/bsnXyw2F9P

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