fix: discounts on update

[johnyeocx]

Summary by cubic

Preserves existing Stripe discounts across subscription updates and plan switches by sending the correct params, and ensures checkout sessions only use coupon/promo code. Prevents new discount creation and duration resets.

• Bug Fixes

  • Map discounts for updates using {discount: id} when present, else {promotion_code}/{coupon}; add stripeDiscountsToCheckoutParams and wire it into buildStripeCheckoutSessionAction.
  • Preserve discount identity and original start/end across immediate upgrades (including multi-hop), scheduled switches and phase transitions, cancel/uncancel, and downgrades; handle promo-code and deleted-coupon rollover cases.

• Tests

  • Immediate switch: discount ID and end date stay the same across upgrades, including pro→premium→ultra.
  • Scheduled switch: discounts persist through schedule creation, cycle advance, replacements, and multiple discounts; upgrade-cancelled schedules keep discounts.
  • Edge cases: repeating coupon duration preserved, deleted-coupon downgrade succeeds, and checkout-applied promo codes persist when downgrading to free.
  • Update flow: cancel then uncancel keeps the same discount ID and end date.

^{Written for commit 50f0257. Summary will update on new commits.}

Greptile Summary

This PR fixes a critical bug in discount handling during subscription updates. Previously, when customers upgraded/downgraded between products or performed cancel/uncancel operations, their discount durations were being reset instead of preserved.

Key Changes

Bug fixes:

• Fixed discount parameter mapping to preserve existing discount IDs and durations during subscription updates by using { discount: id } instead of { coupon: id } for existing discounts
• Separated checkout session discount handling (which doesn't support discount IDs) into a new stripeDiscountsToCheckoutParams function

Improvements:

• Added comprehensive test coverage for discount preservation across multiple scenarios:
  • Immediate product switches (pro → premium)
  • Scheduled product switches with phase transitions
  • Cancel/uncancel operations
  • Edge cases including deleted coupons and repeating coupon duration preservation

The fix ensures that if a customer has a 3-month discount and uses 1 month, they retain only 2 months remaining after product changes - rather than getting a fresh 3-month discount.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The fix is well-isolated, logically sound, and addresses a clear bug in discount handling. The implementation correctly differentiates between subscription updates (which support preserving existing discounts via { discount: id }) and checkout sessions (which only support { coupon } or { promotion_code }). Type safety ensures source.coupon always exists on discount objects, and comprehensive test coverage validates all critical scenarios including edge cases like deleted coupons and phase transitions.
• No files require special attention

Important Files Changed

Filename	Overview
server/src/internal/billing/v2/providers/stripe/utils/discounts/stripeDiscountsToParams.ts	Split discount parameter mapping into two functions - stripeDiscountsToParams for subscription updates (preserves existing discount IDs) and stripeDiscountsToCheckoutParams for checkout sessions (only supports coupon/promo code). Core fix that prevents discount duration from being reset.
server/src/internal/billing/v2/providers/stripe/actionBuilders/buildStripeCheckoutSessionAction.ts	Updated to use stripeDiscountsToCheckoutParams instead of stripeDiscountsToParams, correctly handling checkout session discount format restrictions.
server/tests/integration/billing/attach/immediate-switch/immediate-switch-discounts.test.ts	New test file covering discount preservation during immediate product upgrades (pro to premium). Verifies discount ID and end timestamp remain unchanged.
server/tests/integration/billing/attach/scheduled-switch/discounts/scheduled-switch-discounts-edge.test.ts	New test file covering edge cases: repeating coupon duration preservation across phase transitions, handling deleted coupons, and promo code preservation during downgrades.

_{Last reviewed commit: 50f0257}

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Project	Deployment	Actions	Updated (UTC)
autumn-vite	Ready	Preview, Comment	Mar 2, 2026 7:47pm

[cubic-dev-ai]

No issues found across 8 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.