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 backend-architecturegit 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/backend-architecture)<a href="https://agentmods.dev/skills/revenuecat/play-billing-skills/backend-architecture"><img src="https://agentmods.dev/badge/skills/revenuecat/play-billing-skills/backend-architecture/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/backend-architecture"><img src="https://agentmods.dev/badge/skills/revenuecat/play-billing-skills/backend-architecture.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 | $0.00047 | $0.02144 |
| Opus 5 | $0.00023 | $0.01072 |
| Sonnet 5 | $0.00009 | $0.00429 |
| Haiku 4.5 | $0.00005 | $0.00214 |
Grade A, and why
backend-architecture 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 — 230 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Backend Architecture for Play Billing
The client is not trustworthy. Signatures can be forged, APKs can be modified, and local purchase data can be faked. Your backend must treat the purchase token as an opaque value to verify with Google. This skill walks you through the verification pipeline, from receiving a token to persisting entitlement state.
Phase 1: Scope
Confirm the following before you write code.
- You know which product types you sell (subscriptions, consumables, non consumables). The API endpoint differs per type.
- You have a user identity model. Each purchase token must map to one user account on your server.
- You have (or can add) a database for purchase records. The schema uses
purchaseTokenas the primary key. - You have a plan for acknowledgement within 3 days. Unacknowledged purchases auto refund.
- You have a Google Play Console account and can access Setup > API access.
If any of these are missing, stop and resolve before continuing.
Phase 2: Plan
2.1 Configure Google Play Developer API access
Do these steps once per project.
- In Play Console, open Setup > API access and link a Google Cloud project.
- In Cloud Console, enable the Google Play Android Developer API.
- Create a service account (for example
play-billing-backend). No Cloud IAM roles are required. - Generate a JSON key from the service account and store it as a secret. Never commit it.
- Back in Play Console, grant the service account Financial data and Manage orders permissions on your app.
For containerized deployments on Google Cloud, prefer Workload Identity Federation over JSON key files. GoogleCredentials.getApplicationDefault() picks up the attached service account automatically.
2.2 Map the seven step verification flow
Every purchase follows the same pipeline. Plan each step in code before wiring them.
- Receive purchase token from client.
- Call Google Play Developer API to fetch purchase state.
- Validate response fields (product ID, package name).
- Check purchase state is active or purchased.
- Check the token is not already processed (idempotency).
- Persist record and grant entitlement.
- Acknowledge the purchase with Google.
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 · 230 lines · 47 tokens per session scan A 253167072506
backend-architecture is a skill published in the GitHub repository RevenueCat/play-billing-skills (53 stars, last pushed 3mo ago), licensed Apache-2.0. It adds 47 tokens to every session and 2,144 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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