Link: Building a C compiler with a team of parallel Claudes via Engineering at Anthropic

To stress test the limits of LLMs today, Nicholas Carlini used a team of Claude Opus 4.6 agents to build a Rust-based C compiler. The “agent team” approach (multiple Claude instances working in parallel) required him to design and build a harness of scaffolding, tests, environment, and feedback so they could work on a shared codebase without his intervention.

He tasked 16 agents with the project, and they built a C compiler for just under $20,000 in API costs. The compiler can build a bootable Linux 6.9 kernel and compile SQLite, Postgres, and Redis, but has limitations he wasn’t able to fix without breaking existing functionality.

The compiler, however, is not without limitations. These include:

• It lacks the 16-bit x86 compiler that is necessary to boot Linux out of real mode. For this, it calls out to GCC (the x86_32 and x86_64 compilers are its own).
• It does not have its own assembler and linker; these are the very last bits that Claude started automating and are still somewhat buggy. The demo video was produced with a GCC assembler and linker.
• The compiler successfully builds many projects, but not all. It’s not yet a drop-in replacement for a real compiler.
• The generated code is not very efficient. Even with all optimizations enabled, it outputs less efficient code than GCC with all optimizations disabled.
• The Rust code quality is reasonable, but is nowhere near the quality of what an expert Rust programmer might produce.

The resulting compiler has nearly reached the limits of Opus’s abilities. I tried (hard!) to fix several of the above limitations but wasn’t fully successful. New features and bugfixes frequently broke existing functionality.

Via: https://www.anthropic.com/engineering/building-c-compiler