[SaladDay]

A microsandbox resume that failed before native restoration (missing or damaged snapshot, failed qualification, or a helper that cannot read the snapshot format) left its durable restore admission open. Every later attempt returned unconfirmed, so the allocation stayed in restoring until retention expiry, and the Session's pending input waited until its public deadline. Seen on mx on 2026-10-22.

The Core–Sandbox Provider protocol gains the Resume outcome ErrCheckpointUnusable (node wire code checkpoint_unusable): the adapter durably closed the exact attempt before native dispatch, and the snapshot cannot be restored. A later ObserveOnly reports it again, and it is never returned once native restoration was dispatched.

• Core: on this outcome, Core ends the allocation's retention at once with ExpireUnusableCheckpoint (running, restoring, same compute_revision), and the ordinary expiry cleanup runs from there. A qualified Session then continues through the existing cold replacement. Capacity-pressure expiry still applies only to suspended allocations.
• microsandbox helper:
  • The fresh dispatch that created the admission closes it on any failure before RestoreSandbox.
  • Snapshot defects are also recorded as unusable: not found, identity or integrity failure, or a failed resource, workspace or runtime-image qualification.
  • Other failures are only closed, so Core retries with a new operation ID through the existing RestoreAttemptClosed path.
• Docs: docs/sandbox-provider.md, contracts/agents-api/node-generation-protocol.md and their Chinese mirrors, and the helper README.

Checks:

• python3 scripts/build-microsandbox-provider.py test
• go test with -run filters on the restore, compute, retention and pressure tests in internal/execution, internal/deployment and deploymentpg, plus ./internal/sandbox/...
• go vet
• make check-docs

An independent review found two problems, both fixed: a failed target lookup left the admission open, and the helper README was not updated.

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