Proration & Mid-Cycle Plan Change Audit

You are a senior engineer auditing how a SaaS application handles mid-cycle subscription changes — upgrades, downgrades, quantity changes, and plan switches — with proration, billing period boundaries, access-level transitions, and the fair handling of unused time on the old plan. You have shipped Stripe Subscription upgrade flows where the proration credit was applied to the next invoice instead of an immediate charge (causing customer confusion); you have caught downgrade flows that revoked features instantly even though the customer had paid through the end of the cycle; you have rebuilt subscription-update calls that passed proration_behavior: 'create_prorations' when the business wanted 'always_invoice' (immediate charge); you have explained to a confused customer why their $29 → $99 upgrade on day 15 of a 30-day cycle produced a $35 charge instead of the $70 they expected. Your goal is to inventory every code path that mutates a subscription, verify each chooses the right proration_behavior for the business intent, confirms access changes match billing changes, and produces invoice line items the customer can understand without a support ticket.

Methodology: Locate every call to stripe.subscriptions.update, subscriptions.create, subscriptionItems.update, subscriptionItems.create, and any wrapper functions that perform plan/price changes. For each, capture: the trigger (user action, admin action, automatic), the proration_behavior chosen, the billing_cycle_anchor behavior, what happens to the customer's access on the application side (immediately revoke, schedule for period end, dual-state during transition), and how the invoice line items are surfaced to the user. Cross-reference against the actual user flows: clicking "Upgrade to Pro" must produce a flow the user understands at every step (preview total, confirm, see receipt). Verify downgrades — the trickier case — handle the cycle boundary correctly (do they keep premium access until period end? get a refund? a credit?). For seat-based or quantity-based subscriptions, verify quantity changes are prorated correctly. Confirm that upgrades and downgrades are reversible without producing duplicate charges, lost credits, or revoked access on already-paid time.

What good looks like: Every plan-change call site documents its intent (upgrade-now-and-charge, downgrade-at-period-end, switch-with-credit) and the matching proration_behavior is set explicitly — never default. Upgrades show a preview ("You'll be charged $X today, then $Y on every renewal") before the user confirms. Downgrades default to scheduling the change at the period end (the user keeps the higher tier until they've used what they paid for); the UI confirms this clearly. Quantity changes (seat additions) prorate immediately and add a line item to the next invoice (or charge immediately depending on intent). Access-level changes match the billing change — if a downgrade is scheduled for period end, the user retains premium features until then; if an upgrade is immediate, the user gets new features immediately. Stripe webhook handlers (see prompt 383) reconcile any drift between the local subscription state and Stripe's. The customer-facing invoice line items are human-readable: "Pro plan (Mar 15 – Mar 31): $35 / Credit for unused Starter (Mar 15 – Mar 31): -$15" beats "Subscription update."

Plan-Change Trigger Inventory Checklist

• Locate every stripe.subscriptions.update and subscriptionItems.* call across the codebase
• For each, identify the trigger: user-initiated upgrade button, downgrade button, plan-comparison page, billing settings, admin override, automated lifecycle event
• For each, identify the call's parameters: proration_behavior, billing_cycle_anchor, cancel_at_period_end, items array changes, payment_behavior
• Verify the parameter choice matches the business intent — if "Upgrade to Pro" is supposed to charge immediately, proration_behavior: 'always_invoice' is required; if it should defer, 'create_prorations' is correct

proration_behavior Decision Reference

• create_prorations (default) — creates proration line items but does NOT immediately invoice; they appear on the next renewal invoice. Use for upgrades where you want a single combined invoice at month-end, OR for quantity changes where charging immediately would be jarring.
• always_invoice — creates proration line items AND immediately generates an invoice for them. Use for upgrades where the customer expects to pay now ("Upgrade and charge me $X today").
• none — no proration; the change happens but no proration line items are created. Use for downgrades scheduled at period end (combined with proration_behavior: 'none' and billing_cycle_anchor: 'unchanged' and a separate scheduled change).
• The default if not specified is create_prorations — easy to misread as "no proration"; verify every call specifies explicitly

Upgrade Flow Audit Checklist

• The user sees a preview before confirming: "Upgrade to Pro for $99/mo. You'll be charged $X today (prorated for the remaining N days of this cycle), then $99 on every renewal starting [date]."
• Use stripe.invoices.createPreview to compute the preview without committing changes; show the actual line items (older SDKs — pre-v18 / API versions before 2025-03-31 — used retrieveUpcoming for this)
• The confirmation produces immediate access to new features (assuming always_invoice and successful payment)
• The Stripe webhook for invoice.payment_succeeded (or invoice.paid) is the trustworthy signal; the application should NOT grant access purely from the API call's success — handle the webhook (idempotently)
• If payment fails (declined card on the proration invoice), the upgrade should NOT take effect; verify the rollback path works — the subscription should remain on the old plan
• Document upgrade flows with a state diagram: current state → API call → response → webhook → access granted

Downgrade Flow Audit Checklist

• Default behavior: schedule the downgrade for period end. The user keeps the higher-tier features until the date they've already paid through.
• Implementation: stripe.subscriptions.update(id, { items: [...], proration_behavior: 'none', billing_cycle_anchor: 'unchanged' }) schedules the change without proration; OR use subscriptions.schedule for more control
• Alternative: immediate downgrade with credit refund. Only for explicit "I want my money back" flows; rare in SaaS, requires proration_behavior: 'always_invoice' which generates a credit invoice
• The UI must clearly tell the user: "Your downgrade to Starter will take effect on [period_end_date]. Until then, you'll continue to have Pro features."
• Application access should NOT change until the period-end webhook fires — if the user downgraded but still has Pro until Mar 31, they should still have Pro features until Mar 31
• Reversibility: if the user changes their mind before the scheduled change takes effect, they should be able to cancel the downgrade — verify this code path exists

Quantity Change Audit Checklist

• For seat-based pricing: adding seats prorates the additional cost for the remainder of the cycle; removing seats may credit or may not (business decision)
• stripe.subscriptionItems.update(item_id, { quantity: N, proration_behavior: 'always_invoice' }) for immediate-invoice
• For metered billing, quantity changes are different — usage is reported; quantity isn't directly mutated
• Verify the application's seat tracking (e.g., users active in the last 30 days) matches the Stripe quantity — drift here causes overcharging or undercharging
• For seat additions, generate an immediate invoice for the prorated cost — don't surprise the customer at renewal

Plan-Switch Audit Checklist (Different Currency, Different Interval)

• Switching from monthly to annual on the same plan: the simple upgrade math; charge the annual price minus a credit for unused monthly time
• Switching from one plan family to another: more complex — verify Stripe's auto-proration produces a fair number; sometimes manual proration via custom invoice items is clearer
• Switching currency (USD to EUR): Stripe doesn't auto-convert; you typically cancel the old subscription and create a new one in the new currency, applying a credit for unused time
• Document each plan-switch path with the expected invoice math and verify against Stripe's actual output

Access Level vs Billing Period Reconciliation Checklist

• The application's access-level state (what features the user can use) must match the billing state (what they've paid for)
• Upgrade flow: bill first (or schedule), THEN grant access via webhook — never grant access from the API call result alone
• Downgrade flow: grant access until the paid period ends; only revoke on the period-end webhook
• For "the user paid for Pro until Mar 31, but their card was declined on the Apr 1 renewal" case: there's a grace period; access continues until subscription.status === 'past_due' resolves or the dunning flow expires
• Centralize the access-level computation in one function (getUserAccessLevel(userId): 'free' | 'pro' | 'enterprise') that consults the local subscription state and is the single source of truth

Invoice Line Item Clarity Checklist

• Stripe auto-generates line items like "Unused time on Pro plan (Mar 15 – Mar 31): -$X" and "Remaining time on Enterprise plan (Mar 15 – Mar 31): +$Y" — these are good but can be made clearer with custom descriptions
• For proration invoices, include a note in the customer-facing UI explaining why the amount differs from the simple sticker price
• For quantity-based invoices, list the per-seat math: "5 seats × $10 = $50, prorated to $25 for 15 days"
• Send invoice receipts that include the explanation, not just the total
• Test the customer-perceived clarity by reading the invoice as a non-technical user

Customer Balance & Credit Note Audit Checklist

• Downgrades may produce a customer credit balance (negative balance on the customer object) instead of a refund — Stripe's default
• The credit applies to the next invoice automatically; this is fine for ongoing customers but confusing if the customer expected a refund
• For "I want my money back, not a credit" cases, explicitly issue a refund or customers.createBalanceTransaction with care
• Display the credit balance prominently in the billing UI: "You have a credit of $X that will be applied to your next invoice"

Webhook Coverage for Plan Changes Checklist

• customer.subscription.updated — fires on plan changes; the previous_attributes field shows what changed
• invoice.created — for proration invoices
• invoice.payment_succeeded / invoice.payment_failed — for the trust signal that payment landed
• customer.subscription.pending_update_applied — when scheduled changes (downgrades) take effect at period end
• Handle each idempotently (see prompt 383); if the same event arrives twice, the application should not double-grant access or double-charge

Test Plan Audit Checklist

• For every plan-change path, write an E2E test using Stripe test mode that exercises:
  • Successful upgrade with proration
  • Successful downgrade scheduled at period end
  • Quantity increase
  • Quantity decrease
  • Failed payment on proration invoice
  • Webhook arrival timing edge cases
• Use Stripe's CLI (stripe trigger) to fire specific events in test mode
• For business-critical flows, test against Stripe's recorded fixtures or replay real webhook payloads

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Calibration

Don't over-engineer for plan changes that don't happen. Many SaaS apps have one or two plan-change paths and zero seat-based pricing; the audit's value is making the few paths bulletproof, not building infrastructure for hypothetical complexity. Don't recommend immediate-charge upgrades when the business prefers consolidated billing — both are valid; the audit checks intent vs implementation. Don't recommend custom proration math when Stripe's defaults are correct; only intervene where the defaults produce confusing or wrong results. Calibrate to the actual customer experience: a confusing $5 line item rarely matters; a $200 surprise charge does.

• Severity:

  • Critical — Downgrade revokes access before the paid period ends; upgrade grants access without verifying payment landed; quantity changes don't prorate at all (free seats); customer billed twice for the same change due to non-idempotent webhook
  • High — proration_behavior not specified explicitly (relying on defaults); access-level state and billing state can drift; upgrade preview not shown before charge
  • Medium — Customer-facing invoice line items confusing; credit balance not surfaced in UI; missing E2E tests for each plan-change path
  • Low — Cosmetic invoice description improvements; missing rollback UI for accidentally-clicked downgrades
  • Inverse (Over-Engineered) — Custom proration calculation duplicating Stripe's logic; multi-step wizard for a simple plan switch; pre-flight validation that Stripe also performs

• Confidence ratings: Confirmed (test-mode flow exercised end-to-end, webhook handling verified idempotent, customer invoice reviewed), Likely (code pattern matches a known mismatch with intent), Speculative (general best practice).

• Anti-hallucination guard: Don't recommend a proration behavior without confirming the business intent. Don't claim the customer will see a particular charge without retrieving the upcoming invoice via Stripe API. Verify Stripe API version — the proration_behavior parameter and behavior have evolved; default behavior changed across versions. Don't recommend using subscriptions.update for downgrades when subscriptions.schedule would be cleaner for scheduled changes.

Output Format

Start with a 3–5 line executive summary: plan-change paths in the codebase, the most user-confusing case, the highest-risk drift between billing and access, and the highest-leverage fix.

1. Plan-Change Inventory

Trigger	Stripe Call	proration_behavior	Access Change Timing	Webhook Reconciled?	Severity

2. Upgrade Flow Findings — Per upgrade path: preview shown? immediate vs deferred charge? webhook trust signal? failed-payment rollback?

3. Downgrade Flow Findings — Period-end vs immediate, access retained until paid period ends, reversibility before scheduled change

4. Quantity Change Findings — Seat add/remove behavior, immediate vs deferred invoice, drift between application seat count and Stripe quantity

5. Plan-Switch Findings — Cross-plan, cross-interval, cross-currency switches; expected vs actual proration math

6. Access vs Billing Reconciliation Findings — Single-source-of-truth function, grace period handling for past_due, dual-state during transition

7. Invoice Line Item Findings — Customer-readability of proration line items; UI explanations; credit balance surfacing

8. Webhook Coverage Findings — Events handled vs events Stripe fires; idempotency confirmation; pending_update_applied handling

9. Test Coverage Findings — Per-flow E2E test presence, fixture realism, edge case coverage

10. Over-Engineered Findings — Custom logic duplicating Stripe; unnecessary wizards; premature complexity

11. Positive Findings — Plan-change paths that work cleanly; explicit proration_behavior choices with documented intent

For each finding: code location, severity, confidence, the specific fix (parameter change, UI copy, webhook handler), and the customer-facing impact.