Claude Opus 4.6 did produce a functional C compiler — a 100,000-line Rust codebase that compiles Linux 6.9, passes 99% of GCC's torture tests, and builds major projects like FFmpeg, Redis, and PostgreSQL. However, the claim omits important context: the compiler relies on GCC's assembler and linker for critical steps, independent testers found reliability issues with basic programs, it was built by 16 parallel AI agents (not one instance) with human oversight, and it cost ~$20,000 in API usage. It works, but with significant caveats.