Writing Manual Test Cases Before the Code — How the Origin of a Catalog Flipped in Seven Weeks and Which Eight Bugs It Found
In mid-September 2026, this blog published a number about the catalog of manual test cases in the DI² project: 27 of 32 test cases existed only because something had been broken before. Whether that share would fall as the practice matured was left open. A third data point was supposed to show it, a few months out at the earliest. Seven weeks later the catalog has 159 active test cases, and 31 of…
In mid-September 2026, a blog post revealed that out of 32 manual test cases in the DI² project, 27 were derived from bugs rather than requirements. This trend continued even after the project's maintainers decided to write test cases before coding. Within seven weeks, the catalog expanded to 159 active test cases, with 31 of them originating from bug fixes.
The share of bug-derived test cases decreased from 84% to 19.5%, not due to improved practices, but because test cases were now written earlier in the process and many features were added. The decision to write test cases before coding was made by the project's maintainer, who used an agent named Claude Code to create and monitor the test cases.
Over ten weeks, the catalog underwent 34 runs, with four additional bug-derived test cases added. The majority of test case failures were due to bugs in the newly built feature, not the test cases themselves. Some test cases failed because requirements changed during the run, while others passed despite errors or incorrect steps.
This case study demonstrates the impact of changing when test cases are created and how it affects bug discovery in a project with a fixed development workflow.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
