Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add UnboundCompute/security-agent-skills --skill auditing-payment-state-machine-and-idempotencygit clone --depth 1 https://github.com/UnboundCompute/security-agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-payment-state-machine-and-idempotency)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-payment-state-machine-and-idempotency"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-payment-state-machine-and-idempotency/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-payment-state-machine-and-idempotency"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-payment-state-machine-and-idempotency.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00175 | $0.02109 |
| Opus 5 | $0.00088 | $0.01055 |
| Sonnet 5 | $0.00035 | $0.00422 |
| Haiku 4.5 | $0.00017 | $0.00211 |
Grade A, and why
auditing-payment-state-machine-and-idempotency scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 12d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 140 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Auditing payment state machine and idempotency: value must not move faster than settlement
A checkout is a state machine, and money moves through it in a strict order: an order is placed, a charge is authorized, the charge settles, and only then are goods fulfilled; refunds and cancels run the sequence backward. The bugs are transitions that break the order or repeat for value. An order marked paid before the charge is actually confirmed lets fulfillment run against a payment that never settles. A step that can be skipped, so fulfillment triggers without a completed charge, or repeated, so one payment yields multiple fulfillments, releases value the sequence did not authorize. A charge or fulfillment endpoint that is not idempotent double-processes when a request is retried or deliberately replayed, so a network retry or an attacker's repeat becomes two charges or two shipments. And a refund or cancel that returns value while the charge stays captured pays out twice. The audit walks the state machine and tries to move value ahead of, or more than once per, settlement. You audit this by driving transitions out of order and replaying the ones that release value.
When to use
- A purchase moves through ordered payment states (placed, authorized, settled, fulfilled, refunded).
- Fulfillment may trigger before a charge is confirmed, or a paid state may be set ahead of settlement.
- Charge, fulfillment, or refund endpoints may not be idempotent under retry or replay.
Scope check
Test payment flows only against systems you own or are authorized to assess, on non-production or sandbox payment paths. Driving transitions and replays moves real value on a live system, so use sandbox payment credentials and test accounts and never trigger a real charge, refund, or fulfillment that is not yours. If you can't name the authorization, stop.
The loop
- Establish the intended state machine first. Map the legal states and the only transitions allowed between them, and which transition each value-releasing action (fulfillment, refund, payout) requires as its precondition. This is the false-positive killer: a flow where fulfillment requires a confirmed settled charge, every transition validates its precondition server-side, and every value-releasing handler is idempotent is correct. Name the intended sequence, then test deviations.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 12d ago First seen · 140 lines · 175 tokens per session scan A b4778d7cd990
auditing-payment-state-machine-and-idempotency is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 4d ago), licensed MIT. It adds 175 tokens to every session and 2,109 once invoked, about $0.0009 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
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