Simon Willison · 2026-07-05 · notable
Simon Willison — sqlite-utils 4.0rc2, mostly written by Claude Fable
Simon Willison shipped sqlite-utils 4.0rc2 with Claude Fable doing most of the work — 37 prompts, 34 commits, 30 files, $149.25 at unsubsidized API rates — and caught a data-loss bug in delete_where() during pre-release review.
37 prompts, 34 commits, one data-loss bug caught — Simon Willison's field report on shipping a real library with Claude Fable.
What is it?
sqlite-utils 4.0rc2 is the release candidate for the next major version of Simon Willison's widely-used Python library for building and querying SQLite databases. In this post he documents how much of the pre-release work — code review, refactors, docs, and the release notes themselves — was done by Claude Fable rather than by hand.
How does it work?
Across 37 conversation turns, Claude Fable ran the full test suite, hunted for missing transaction guards, and landed 34 commits across 30 files. One of those commits fixed a bug in delete_where() where a failed batch could silently drop rows outside its own transaction — a data-loss issue Simon Willison had not spotted in his own review.
Why does it matter?
$149.25 at unsubsidized Anthropic API rates is a concrete price tag on 'have an agent review your library before you cut a release'. For maintainers deciding whether to spend a weekend on the same work themselves, this post is a rare public data point — with the prompts, the diffs, and the resulting cost all shown.
Who is it for?
Open-source maintainers, Python developers, teams evaluating coding-agent spend
Try it
pip install sqlite-utils==4.0rc2