From 41% to 86% coverage: generating tests that catch real bugs

Quanta Cloud had a familiar problem: a critical billing service with thin test coverage and no one willing to touch it. Writing tests by hand kept losing to roadmap work.

Starting with the riskiest paths

The team pointed CodeCraft at the functions with the most production incidents first. For each one, it generated unit tests matched to their existing Vitest setup, including the edge cases that had caused past outages.

Reviewing tests like code

Every generated test arrived as a diff, so engineers reviewed intent rather than syntax. Tests that encoded wrong assumptions were caught in review and corrected in minutes.

Results after one month

Coverage on the billing service climbed from 41% to 86%, and two latent rounding bugs surfaced during the process — before a customer ever saw them.