Simon Willison’s Weblog

22nd February 2026

The Claude C Compiler: What It Reveals About the Future of Software. On February 5th Anthropic's Nicholas Carlini wrote about a project to use parallel Claudes to build a C compiler on top of the brand new Opus 4.6

Chris Lattner (Swift, LLVM, Clang, Mojo) knows more about C compilers than most. He just published this review of the code.

Some points that stood out to me:

• Good software depends on judgment, communication, and clear abstraction. AI has amplified this.
• AI coding is automation of implementation, so design and stewardship become more important.
• Manual rewrites and translation work are becoming AI-native tasks, automating a large category of engineering effort.

Chris is generally impressed with CCC (the Claude C Compiler):

Taken together, CCC looks less like an experimental research compiler and more like a competent textbook implementation, the sort of system a strong undergraduate team might build early in a project before years of refinement. That alone is remarkable.

It's a long way from being a production-ready compiler though:

Several design choices suggest optimization toward passing tests rather than building general abstractions like a human would. [...] These flaws are informative rather than surprising, suggesting that current AI systems excel at assembling known techniques and optimizing toward measurable success criteria, while struggling with the open-ended generalization required for production-quality systems.

The project also leads to deep open questions about how agentic engineering interacts with licensing and IP for both open source and proprietary code:

If AI systems trained on decades of publicly available code can reproduce familiar structures, patterns, and even specific implementations, where exactly is the boundary between learning and copying?