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 RevenueCat/play-billing-skills --skill securitygit clone --depth 1 https://github.com/RevenueCat/play-billing-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/revenuecat/play-billing-skills/security)<a href="https://agentmods.dev/skills/revenuecat/play-billing-skills/security"><img src="https://agentmods.dev/badge/skills/revenuecat/play-billing-skills/security/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/revenuecat/play-billing-skills/security"><img src="https://agentmods.dev/badge/skills/revenuecat/play-billing-skills/security.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 | $0.00041 | $0.01783 |
| Opus 5 | $0.00020 | $0.00892 |
| Sonnet 5 | $0.00008 | $0.00357 |
| Haiku 4.5 | $0.00004 | $0.00178 |
Grade A, and why
security 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 11d 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 — 139 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Securing Google Play Billing
Anything running on the user device can be tampered with. If your trust model lives in the client, you have already lost. This skill shows you how to move trust to your backend and defend against the attack patterns you will actually see in production.
Phase 1: Assess
Before you write a line of defensive code, answer these questions about your current integration:
- Where does purchase verification happen today? Client, backend, or both?
- Does your database enforce a unique constraint on
purchaseToken? - Do you set
obfuscatedAccountIdandobfuscatedProfileIdon every purchase flow? - Do you poll the Voided Purchases API on a schedule?
- Do any access decisions rely on
System.currentTimeMillis()from the device?
Any "no" answer maps to a layer you are missing below.
Phase 2: Plan the Layered Defense
Fraud prevention is not a single check. It is a stack of controls where each layer catches a different attack class. Plan all of them, even if you ship them incrementally.
| Layer | Purpose | Where it runs |
|---|---|---|
| 1. Backend receipt verification | Confirm the purchase exists in Google's records | Your server, via purchases.products.get / purchases.subscriptionsv2.get |
| 2. Response field validation | Catch product swaps and cross app token reuse | Your server (check packageName, productId, orderId) |
| 3. Purchase state check | Reject PENDING and expired purchases |
Your server |
| 4. Replay protection | Block reuse of valid tokens | Unique constraint on purchase token column |
| 5. Token to user binding | Detect token sharing across accounts | Your server (store and compare userId on duplicates) |
| 6. Acknowledgement | Prevent auto refund at 3 day mark | Your server, after entitlement grant |
| 7. Voided purchase sync | Revoke access on refunds and chargebacks | Daily batch job against purchases.voidedpurchases.list |
| 8. Device and profile signals | Give Google and yourself correlation data | Client sets obfuscatedAccountId / obfuscatedProfileId before launching the flow |
| 9. Integrity and pinning | Raise the cost of modified APKs | Play Integrity API plus TLS certificate pinning |
| 10. Server time of record | Defeat timezone manipulation | All time decisions use server clock |
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.
- 11d ago First seen · 139 lines · 41 tokens per session scan A ea013b99e027
security is a skill published in the GitHub repository RevenueCat/play-billing-skills (53 stars, last pushed 3mo ago), licensed Apache-2.0. It adds 41 tokens to every session and 1,783 once invoked, about $0.0002 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 2026-08-30.
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