An engineer describes building Kept, a self-hostable e-commerce support agent, using an observability-first design approach. Rather than implementing the agent loop first, they designed a comprehensive tracing layer beforehand, treating Langfuse as a human-facing sink and maintaining an in-process trace model as the authoritative source for machine evaluation. This approach, guided by OpenTelemetry conventions and semantic standards, led to insights about span hierarchy, taxonomy design, and metadata selection optimized for different consumers.