Usage-based subscription flow does not materialize charges/invoice lines, blocking customer credit consumption

[pratham-ez]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.
• I am not looking for support or already pursued the available support channels without success.

OpenMeter Version

latest

Expected Behavior

Use case

I am integrating OpenMeter for usage-based billing with prepaid customer credits.

My target flow is:

1. Create a customer
2. Create metered features (token/call based usage)
3. Create a plan with usage-based (non-flat) prices
4. Subscribe the customer to that plan
5. Ingest usage events
6. Grant customer-level billing credits
7. Let billing/subscription sync generate billable artifacts
8. Consume credits as usage is billed

Goal: use customer credit balance as the source of truth for ongoing spend control and potential usage blocking logic.

---

Problem

In this flow, usage is ingested and meter queries show usage, but billing artifacts for that customer are not materialized:

• customer charges endpoint stays empty
• invoice creation fails with invoice_create_no_lines
• customer credits remain unchanged (funded only, no consumed)

This prevents validating the expected “usage -> charge/invoice -> credit deduction” lifecycle for non-flat pricing.

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Environment

• OpenMeter quickstart Docker setup
• API base: http://localhost:48888
• Credits config in running container:
  • credits.enabled: true
  • enable_credit_then_invoice: true

---

Actual Behavior

• GET /customers/{id}/charges returns empty (total: 0)
• POST /api/v1/billing/invoices/invoice returns 400:
  • invoice_create_no_lines
• Credits are not consumed:
  • balance unchanged
  • transactions contain funded, but no consumed

Steps To Reproduce

1. Create customer (with usage attribution subject key)
2. Create meter + feature
3. Create plan with non-flat usage-based rate card
4. Publish plan
5. Create subscription for customer + plan
6. Ingest event(s) for that customer subject
7. Create customer-level credit grant (/api/v3/openmeter/customers/{id}/credits/grants)
8. Trigger subscription sync / collect jobs
9. Check:
   • GET /api/v3/openmeter/customers/{id}/charges
   • POST /api/v1/billing/invoices/invoice with customerId
   • GET /api/v3/openmeter/customers/{id}/credits/balance
   • GET /api/v3/openmeter/customers/{id}/credits/transactions

Additional Information

Expected behavior

For a customer with active subscription, non-flat usage-based pricing, ingested usage, and available credits:

1. billable artifacts should be generated from usage (charges and/or invoice lines)
2. invoice should be creatable/advancable with non-zero billable lines
3. customer-level credits should be consumed according to settlement mode/config

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Code-level hint / suspicion

This validation indicates non-flat invoice items require featureKey:

openmeter/billing/gatheringinvoice.go

if i.Price.Type() != productcatalog.FlatPriceType && i.FeatureKey == "" {
    errs = append(errs, errors.New("feature key is required for non-flat prices"))
}