A Latent Upstream Bug, Found and Fixed in About 30 Minutes

Wed, 29 Jul 2026 00:00:00 +0000

I keep seeing arguments about AI coding assistants conducted entirely in the abstract, so here is a concrete one: a real change I made recently, start to finish, including the parts that went wrong. Everything below is a real MCUboot change; the PR numbers are live and you can check my work.

Summary

An automated review of an external contributor’s pull request flagged that the author had quietly disabled a test on one simulated device. Rather than stopping at “you disabled a test”, the model worked out why the test failed — a long-standing bug in MCUboot’s own test simulator that had been invisible for as long as it had existed, and only became observable because this PR raised a configuration limit. The fix was implemented, reviewed, pushed as PR #2808, and then empirically demonstrated to resolve the original author’s problem.

Cost: roughly 30 minutes of my attention, some of it spent doing other things while test suites ran. My honest estimate for doing this by hand is one to two days — most of it spent staring at trailer arithmetic trying to work out why an “oversized” image wasn’t oversized.

The tool was Claude Code running Opus 5.

Rust on Brownian Motion

A Latent Upstream Bug, Found and Fixed in About 30 Minutes

Summary