Writing
Notes from systems meeting real devices.
Notes on payment systems, device software, transaction uncertainty, platform design, and the emerging role of AI in commerce. I write from the questions that appear in real operating conditions.
APPROVED, DECLINED, UNKNOWN: designing honest transaction status
A host can approve a payment while the terminal still shows UNKNOWN. That third state is not a bug — it's the whole point.
Reversal and retry: designing inverse operations that don't lie
Retry-safe means idempotency-key aware. Reversal-safe means preserving the original approval alongside the reversal. Neither is the default.
Reading EMV BER-TLV without a spec sheet
A short field guide to the structure that carries every EMV response — tags, lengths, primitives, constructed nesting — with a live parser.
Consent, mandates, and trust in autonomous agent payments
How an agent can pay on your behalf and audibly prove why. A short taxonomy of the guardrails, and where each is easy to get wrong.
Agent payment protocols: what's converging
Multiple industry proposals for how AI agents should initiate payments have emerged in 2025-2026. Where they agree and where they don't.
PSD3 and the merchant integration checklist
Payment Services Directive 3 and the Payment Services Regulation land in staggered dates through 2027. Here's what merchant integrators need to prepare for now.
Rotating device keys without downtime
Terminal key rotation is the operation most fleets avoid until they can't. A design for making it routine rather than dramatic.
Stablecoin corridors: cost and settlement
Stablecoin payment corridors have real cost advantages and specific operational trade-offs. A comparison against traditional rails.
Contactless CVM: floor limits after 2024
The contactless-with-CVM threshold quietly moved in most European markets during 2024. What the practical implications look like.
On-call rotations for global teams
On-call for a team spread across three or more time zones needs different shapes than a co-located team. The patterns that work.