Simon Willison’s Weblog

I wonder if one of the reasons I'm finding LLMs so much more useful for coding than a lot of people that I see in online discussions is that effectively all of the code I work on has automated tests.

I've been trying to stay true to the idea of a Perfect Commit - one that bundles the implementation, tests and documentation in a single unit - for over five years now. As a result almost every piece of (non vibe-coding) code I work on has pretty comprehensive test coverage.

This massively derisks my use of LLMs. If an LLM writes weird, convoluted code that solves my problem I can prove that it works with tests - and then have it refactor the code until it looks good to me, keeping the tests green the whole time.

LLMs help write the tests, too. I finally have a 24/7 pair programmer who can remember how to use unittest.mock!

Next time someone complains that they've found LLMs to be more of a hindrance than a help in their programming work, I'm going to try to remember to ask after the health of their test suite.