[REBUILD] Payment State Machine with Compensating Transactions (x402 safety)

[Mosss-OS]

Rebuild: Payment State Machine with Compensating Transactions

What exists

x402 (HTTP 402 pay-per-request) payments are handled inline — charge then process. If the processor crashes after charging but before confirming, the payer has no recovery path.

What I'd build instead

A formal payment state machine with compensating transactions:

States: Pending → Authorized → Captured → Completed
                             ↘ Failed → Refunded (compensation)

• Every payment gets a UUID idempotency key (stored in Upstash Redis)
• Two-phase: authorize holds funds, capture settles only after successful processing
• On failure in any post-authorization step: automatic refund via compensating transaction
• Payment event log for audit trail (who paid what, when, and the outcome)

Why this matters

• Zero lost funds: no scenario where money leaves wallet without service rendered
• Idempotency: retrying a failed request doesn't double-charge
• Audit trail: you can prove every payment outcome
• Users trust the system when they see "Authorized — will only charge on completion"

Migration path

1. Extract payment logic into a standalone service with a state machine
2. Generate idempotency keys before any HTTP 402 interaction
3. Store payment state in Redis with TTL
4. Add a refund endpoint triggered by compensating events
5. Add idempotency check on all payment webhooks