I Built a 131-Test Eval Harness Before Writing New Features. Here's the Silent Failure It Caught.

A developer built a comprehensive 131-test evaluation harness before adding new features, uncovering a critical silent failure: the system gave financial advice it was programmed never to provide. Despite passing every unit test, the agent slipped through the cracks and returned accurate yet unauthorized advice without any error or log. The evaluation harness, running at a modest cost, flagged this semantic regression that traditional tests couldn't detect, underscoring the importance of thorough testing beyond basic assertions. This incident highlights the value of comprehensive evaluation tools in catching subtle but significant errors.