An AI agent doesn't fill out a checkout form. It hits an endpoint, gets a 402 Payment Required challenge, signs an authorization, and expects a real resource back within seconds. We tested that exact path against our own public endpoint — not a sandbox, real USDC on Base mainnet — and found a real production bug in the process.
What we ran: a batch of GPU jobs through our public x402 endpoint, the same one an external agent would hit. Different GPU classes, different providers (Vast.ai and RunPod), each paid for and torn down automatically.
Results — 7 settled payments, every one with an on-chain hash:
GPU Provider Real cost Charged Time Tx hash
RTX A4000 Vast.ai $0.001215 $0.01 80.1s 0x1aacdc35...05072b
RTX 4090 Vast.ai ~$0.0023 $0.01 ~100s 0xd190f93e...baa5f4
RTX A4000 Vast.ai $0.001215 $0.01 106.1s 0x76318a7e...4630de
RTX 3090 Vast.ai $0.001852 $0.01 109.4s 0x2875dcf1...7b9b8b
A40 RunPod $0.005667 $0.01 63.1s 0x8aea8983...965902
H100 Vast.ai $0.028959 $0.05 109.9s 0x1e595040...65f05ca
A10 (batch inf.) RunPod $0.005667 $0.01 110.7s 0xcfbb0dda...d9514
What broke: 2 of 8 jobs failed on a 120-second payment authorization window. Root cause wasn't price lookup or payment logic — it was our OFAC sanctions-list cache going stale, triggering a live 5MB Treasury re-download on every payment with no timeout. When that download hung, it added up to two minutes inside the payment path.
What we fixed: removed the live download from the payment path entirely, moved refresh to an hourly background job, widened the authorization window from 120s to 600s as a second layer. Re-ran the exact failing config: settled in 18.8 seconds. 0xa7c7ae02e2c4382b748c6736616f5570d7fd72520cb962b4b41434f0fd6f859a.
Independent verification: pulled Vast.ai's own billing ledger directly — every job shows up by contract ID, charge amount, and our own dispatcher's job label embedded in their record. That check also surfaced a second bug: RunPod silently substituted RTX 4090s for two jobs requested as an A40 and an A10, with no disclosure. Currently fixing that one too.
A payment rail is only as good as what happens when it breaks. This is what happened when ours did.
