A Solo Dev Built a Scheme-to-WASM Compiler in 4 Days with Claude — Then Couldn't Explain Part of the Code

Matthew Phillips shipped Puppy Scheme — a Scheme-to-WebAssembly compiler — in roughly four days, using Claude as the primary development tool. The feature list is worth actually reading: WASM GC implementation, dead-code elimination for compact binaries, self-hosting capability (the compiler compiles its own source code), WASI 2 and Component Model compatibility, and a web-based interface running on Cloudflare Workers. Then he ran an overnight session. He told the AI to "grind on performance." By morning, compilation time had dropped from 3.5 minutes to 11 seconds. He went to sleep with a slow compiler and woke up with a fast one. That's the headline story. But the more interesting story is what came next. The Cognitive Debt Nobody Talks About Simon Willison documented a problem that's quietly spreading through teams doing agentic coding: you ship working code that you don't fully understand. He built a Rust word cloud generator using Claude Opus 4.6. The code worked. He read through i