Billing Review (Polar)

The handle is the diff. The evidence is where the change breaks a billing invariant the
team enforces in review.

Not a general code review. The other lenses know Polar's conventions, contract and deploy
shape. This one knows how the billing domain is supposed to behave.

Distilled from ~210 review comments François (frankie567) left on billing PRs between
February and August 2026, plus domain invariants raised by pieterbeulque, psincraian,
joebon and Yopi on the same PRs. Each rule cites its PR.

Read .agents/skills/polar-billing.md first. That is the map of the domain — entities,
services, tasks, the cycle flow, proration, dunning, the ledger. This file is the checklist
that assumes it.

Scope

polar/{subscription,order,billing_entry,meter,event,discount,checkout,checkout_link}/
polar/{payment,payment_method,refund,dispute,payout,payout_account,wallet,transaction}/
polar/{invoice,receipt,tax,product,customer_seat,account}/  ·  polar/benefit/grant/
polar/models/{subscription,order,order_item,product,product_price,discount,checkout,
              payment,billing_entry,refund,dispute,wallet,transaction}.py
migrations/ and server/scripts/ when they touch those tables

Owned elsewhere: schema and SDK impact → api-surface-review; migration and actor deploy
safety → ship-safety; lazy="raise" and repository conventions → conventions-check;
ADR-0006 / ADR-0007 → adr-check; existing helpers → reuse-check.

For a domain question you cannot settle from the diff, ask it rather than assert a defect.
That is how this team reviews.

Checks

1. Money comes from billing entries, not derived state

BillingEntry is the ledger tying an invoice line back to the events that caused it. Anything
computing an amount from a summary or a recomputation loses that trace.

• Invoicing metered usage reads billing entries, never the customer meter's computed balance.
  "Billing Entries are the source of truth… Otherwise, we lose the ability to track billing
  down to the events." (#12510)
• If consumption happened, the entries already exist. New code that recomputes usage instead
  of picking up pending entries is a smell. (#12510)
• Order creation is what consumes pending entries. The question for a new billing trigger is
  "should this create an order", not "should this compute an amount".

2. Reuse the lifecycle, do not hand-roll the transition

The cycle already moves periods, writes events, and grants or revokes benefits.

• Changing plan during a trial: call update_trial so periods and events are right, then let
  it cycle naturally because the new period is already past — same as ending a trial
  immediately. (#11898)
• New periodic behaviour gets a scheduler modelled on subscription/scheduler.py, not a
  bespoke loop. (#12990)
• One cycle, one invoice. Overages from a meter cycle that coincides with a billing cycle
  belong on the renewal invoice, not a second order with a second payment. (#12510)

3. Dunning decisions live in the dunning entry point

• The retry decision belongs in _handle_first_dunning_attempt, not the caller. Check the
  latest payment's decline code, set a retry date if recoverable, leave it None otherwise.
  Everything else (past due, benefit enqueue) is untouched. (#9744)
• Never schedule a retry for a non-recoverable decline. (#9744)
• Decline-code knowledge lives on the Payment model as a @property
  (UNRECOVERABLE_DECLINE_CODES), guarded by if self.processor == PaymentProcessor.stripe
  so a second processor does not inherit Stripe's codes. (#9744)
• Retry-exhaustion arithmetic is a classic off-by-one. Check the boundary. (#11905)

4. Payment locks release on one path

• The webhook handler for the payment outcome (handle_payment_failure) releases the lock. A
  caller that also releases it on its own error path is duplicated logic — delete it and its
  test. (#10653)
• Never release a payment lock on success. release_on_success was removed from the
  codebase for this reason. A diff reintroducing it is a regression. (#10653)
• with_for_update belongs in the task that owns the unit of work, not inside
  transfer_stripe. "This logic should be independent from Stripe behavior." (#12097)
• A wedged lock breaks dunning silently. (#13272)

5. Keep processor state in sync, keep processor names out of the domain

• Domain statuses mirror the processor's vocabulary. accepted was rejected as a dispute
  status because Stripe uses lost either way, and diverging breaks the sync. (#12713)
• A merchant action with a processor counterpart makes the processor call in the same flow.
  "The risk is way too high to forget… and let the dispute expire." (#12713)
• Name things after the domain: "Payout Account", not "Connect". (#12097)

6. Crons and batch jobs

• Catch-up loops hide missed runs. A while walking forward through skipped cycles
  computes credits wrong for the periods it skipped. Prefer an invariant alert that the cycle
  did not run. (psincraian, #12510)
• Use repository.stream rather than a keyset loop over UUIDs. (#12749)
• Loading every row in a script or sweep is a review stop. "Won't that blow up in memory?"
  (#11728, #13687)

7. Discounts

• Redemption counting is the whole game. A failed payment should not count. A fully
  refunded order is an open question the team has not settled. (pieterbeulque, #13328)
• Concurrent redemption of the same code needs a customer lock, and the caller acquires it —
  say so at the call site. (joebon, #13328)
• max_redemptions and max_redemptions_per_customer are separate limits; a guard checking
  one usually needs both. (#13394)
• Expiry is checked in cycle but has been missed elsewhere. New paths that apply a discount
  check it too — prefer extracting the shared check. (psincraian, #12510)
• Multi-line discounts apply as a waterfall, not proportionally. Questioned in review as
  unusual for invoices; a change to allocation is a merchant-visible invoice change. (joebon,
  #12172)

8. Amounts, currency and tax

• Fee values are basis points: 4% is 400. Check the unit before trusting arithmetic.
• Per-jurisdiction tax comes straight from the Numeral API response. Recomputing it from rates
  and polar_round introduces fractional-cent drift. (#11211)
• Tax sits on transactions of type payment, which are on the Polar side and not linked
  to the merchant's account. Transactions reach an organization through account_id;
  payment_organization_id is a Pledge-era leftover that does not mean what it looks like.
  (#12204)
• On imports, tax-inclusive versus exclusive must mirror the source provider, or the merchant
  loses money. (#12502)
• Voiding or reversing an order credits back what was applied (applied_balance_amount), not
  the customer's current balance. (#11637)

9. Free and zero-amount paths

These keep breaking because new code assumes a payment exists.

• Keep ProductPrice.is_free / Checkout.is_free_product_price checks when reworking price
  handling. (#12225)
• A new charge path supports a free price by skipping the payment step, not rejecting it.
  (#12089)
• Renewal emails, invoices and receipts all have free-subscription branches. (#9291)

10. Billing-specific additions to rules owned elsewhere

Short pointers only — the owning lens reports the general rule.

• Money tables are busy tables. orders, subscriptions, payments, customers,
  events. Heavy backfills go in a script; indexes go in concurrently. → ship-safety
• Order foreign keys get ondelete="restrict". "A DELETE statement is easy to spawn."
  (#11206) → ship-safety
• Lazy loads in money paths are stuck jobs, not just 500s. assert order.customer does
  not protect you — it raises the lazy-load error itself (#11900). When a denormalized column
  lands, the matching joinedload usually becomes dead; remove it (#12162). →
  conventions-check
• Customers cannot choose proration behaviour, so it does not belong in the customer
  portal API. (#13095) → api-surface-review

Output

## Billing

### 🔴 Blocking
- `file:line` — <invariant broken>. Fix: <fix>

### 🟠 Should fix
- `file:line` — <claim>. Fix: <fix>

### 🟡 Question
- `file:line` — <question>

### Notes
- domain context the author may not have: <one line, or omit>

### Verdict
✅ Clean  |  ❌ n blocking, n should-fix