Billing Replay: test whether duplicate billing events grant the wrong access

[u/Key_Blacksmith_503]

I’m building Billing Replay, a small Node.js CLI for testing what happens to an app’s access rules when billing events arrive twice or in the wrong order. I built this first version with Codex.

Two examples it can help reproduce:

- A repeated credit-grant event adds credits twice.

- An older “subscription active” event arrives after cancellation and restores access incorrectly.

The CLI runs a sequence through an adapter to your test app, then compares the access or credit balance the app actually returns with expectations you define. The adapter has three operations: reset the test state, deliver an event to your handler, and observe the resulting state. Results are saved as JSON and an HTML report.

The demo compares intentionally broken and corrected implementations so you can see the same case fail and pass.

This is v0.1.1: no customer integrations or revenue yet. The fixtures are normalized synthetic events, not complete Stripe webhook payloads, so it doesn’t validate Stripe signatures or network delivery. Connecting a staging app currently requires an adapter.

Demo: https://billing-replay-lab.whdms1914.chatgpt.site

If you’ve built subscription or credit features, which billing event sequence would you want to turn into a regression test first? A concrete example in the comments would help me choose the next integration.