Trace design and reproducibility

This lesson covers designing the trace schema that makes every failure type measurable. The difference between v0 instrumentation, where logs exist but tell nothing, and v1 instrumentation, where retrieval recall can be proven to have dropped from 0.89 to 0.67 in the last 24 hours, is enormous. Consider that difference. v0 shows "success: true" but gives zero insight into which stage actually failed. v1 captures every stage separately with its own success indicators and latency measurements. That's the gap this lesson closes. This is about designing what the system records so there's evidence to make ship decisions. The fields chosen for capture determine which questions can be answered later. If the SQL query isn't logged, SQL correctness can't be measured. If retrieval document IDs aren't logged, retrieval precision can't be computed. What gets recorded isn't just about clean data; it's about building the evidence foundation that enables confident ship decisions.