A missing span should resolve to unknown, not success
One failure mode I do not see discussed enough in LLM observability is absence. If a trace contains no policy check, did the agent bypass it, did the instrumentation drop it, was it sampled out, or is the viewer not authorized to see it? Those are different operational states, but many dashboards render them as the same blank space.
We handle that boundary by treating observability as a contract rather than a bag of spans. For a completion claim, the contract names the evidence that must exist: the requested intent, contract and source versions, tool calls, validator results, and resulting authoritative state. A required surface can be present, explicitly not applicable, unavailable, or missing. Missing does not become a pass. The worker's trace is useful for diagnosis, but a separate verifier decides acceptance from the retained artifacts.
The same principle applies to the observability system itself. A sentinel flow should produce a known span or receipt. If that artifact disappears, the detector failed before anyone interprets silence in production. How are people modeling negative evidence? Can your system distinguish "the action did not happen" from "we cannot prove whether it happened"?