Simon Willison shipped sqlite-utils 4.0 mostly written by Claude Fable, 37 prompts, $149

This is the case study I'll be pointing people to for months. Simon Willison shipped sqlite-utils 4.0rc2 on July 5, his first major version bump since 2020, with most of the code written by "Claude Fable" across 37 prompts and 34 commits, for about $149.25 total.

The number that made me sit up wasn't the cost. It was that Fable surfaced 5 issues Willison categorized as release blockers that he hadn't hit himself, including a silent transaction bug. On a backwards-compatibility-sensitive major release. From one of the most careful maintainers in the Python ecosystem.

That reframes the whole "agents are for greenfield scaffolding" assumption. Greenfield is the easy case. Nobody's callers break if your weekend prototype has a subtle bug. A SemVer major bump on a library thousands of projects depend on is the opposite, and it's exactly the unglamorous, high-stakes work people assumed agents couldn't touch. Willison proved them on it.

The reusable technique here is worth stealing directly. Before cutting a breaking release, run a single agent task scoped only to "enumerate everything that could break existing callers," completely separate from feature work. Not "review the code." Not "find bugs." Specifically: trace every path where an existing caller's behavior might change. Agents are willing to exhaustively walk edge cases a human maintainer skims past on the fiftieth read of their own code. That's the actual edge, and it's the opposite of the "vibe coding" reputation. This is agentic engineering with a review gate, and it caught a transaction bug that would've silently corrupted data.

$149 for a major release with a dedicated correctness audit built in. I've paid more for worse code reviews from humans. If you maintain anything with a public API, add a scoped "find the release blockers" pass to your release checklist before the next breaking change. It's the highest-leverage 20 minutes of agent time I've seen described this week.