Reading the application prompts as signal

Here is the same skeleton delivered as a compressed ~90-second answer, on an anonymized usage-billing rebuild:

“The problem: our old biller only did flat monthly subscriptions, but sales had started signing usage-based deals with metered API calls, monthly commitments and overage, and finance was stitching invoices together by hand. Constraints: a hard quarter-end deadline, an audit coming, and just me plus one accountant, so ripping out the ERP was off the table. Architecture: I put a metering-and-rating service in front of the ERP to aggregate raw usage events into period totals, then a thin revenue subledger that applied the recognition rules before anything posted to the GL as summarized journal entries. Trade-offs: I wrote the rating service in code because the packaged billing module couldn't express tiered overage, but I kept recognition config-driven so accounting could change the rules without a deploy - which cost me a slower first release. Outcome: manual invoice prep dropped from days to under an hour a cycle and we tied out clean in the first audit. What broke: my first idempotency key let a retried usage batch double-count one customer's overage; I caught it in reconciliation, keyed dedup on the event id, and now I reconcile metered totals to invoiced totals every run.”