A production rollout exposes a contract mismatch between the UI boundary and the owning service. The affected area is checkout flow.
Repair the checkout flow command handling implementation so it accepts first operations, recognizes safe repeats, and rejects key reuse with a different payload.
Evidence
- Affected surface: Full-stack frontend focus / checkout flow.
- Observed failure family: enforce idempotent operations.
- Scope policy: cohort must equal stable.