Anthropic used Claude to write a C compiler in Rust (CCC). I've really enjoyed following the discussion.
The initial post shows how they wrote the harness with GCC as an oracle and a prompt specifying a design using SSA. The author has clear domain expertise.
Later testing showed some overfitting to the test harness: it can compile Linux, but not hello world!
Benchmarking showed that CCC has much worse performance in terms of both compile time and runtime.
Since it was designed to match gcc flags, it supports both -O0 and -O2, but the output is identical. It produces working code, but it doesn't optimise.
Others observed that diagnostics aren't great. It has off-by-one in line numbers and invalid C programs are accepted.
Verifying input correctness was out-of-scope for CCC, so I think this is reasonable. I know mrustc is also doesn't check validity.
https://voxelmanip.se/2026/02/06/trying-out-claudes-c-compiler/
On the 5th of February Anthrophic published an article on their engineering blog detailing the creation of a C compiler using a team of parallel Claude agents over the course of two weeks. The compiler in question is called Claude’s C Compiler (ccc for short) and is a ~180k line Rust codebase that they published onto GitHub. Of course, this announcement raised a lot of questions about the accuracy of their claims and the usability of the compiler, as well as the implications of a swarm of AI agents allegedly being able to write a full C compiler. It claims to be able to compile the Linux kernel, along with many other high profile C projects, but does it actually work? I became intrigued, and decided to try compiling a Hello World program, my game Tensy, ClassiCube, and finally the Linux kernel.
Chris Lattner (the creator of LLVM) reviewed the code, and noted how similar the structure is to clang/LLVM and GCC.
Even though other C compilers are not written in Rust, Claude seems to show knowledge transfer from its knowledge of existing compilers.
https://www.modular.com/blog/the-claude-c-compiler-what-it-reveals-about-the-future-of-software
Compilers occupy a special place in computer science. They're a canonical course in computer science education. Building one is a rite of passage. It forces you to confront how software actually works, by examining languages, abstractions, hardware, and the boundary between human intent and machine execution.
John Regehr (a compiler verification expert) looked at CCC and discussed how production compilers differ.
He also fuzzed the compiler, found bugs, and managed to fix them by just prompting! John has years of experience with UB validity though.
Wilfred Hughes
