Writing Rust code that's fast by asking agents to make the code faster

In January 2025, I had a fun hypothesis for a blog post : can LLMs write better code if you keep asking them to "write better code"? That was prior to the advent of robust agentic coding, but Claude Sonnet 3.5 was still able to iteratively improve on algorithmic Python code. The "better" instruction turned out to be underspecified: Sonnet abused that ambiguity to instead add a ton of useless features but the code was indeed faster. Even with the rise of agentic LLMs specifically RLHF ed to handle solving common pass/fail coding problems, optimization is generally not a part of that suite.