# AN-2026-0633: Stripe API 2026-09-30.endive: billing_cycle_anchor on Subscription update/resume and preview invoices must be an object ({type: 'now'}), not a string

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- id: AN-2026-0633 (version 1; cite https://breakwire.org/claim/AN-2026-0633/v/1)
- kind: breaking · status: active · review: editor
- source: <https://docs.stripe.com/changelog/endive/2026-09-30/polymorphic-billing-cycle-anchor>
- event date: 2026-09-30
- services: stripe-api
- affected: Stripe API version 2026-09-30.endive and later; Subscription#update.billing_cycle_anchor; Subscription#resume.billing_cycle_anchor; Invoice#create_preview.subscription_details.billing_cycle_anchor
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Stripe's Endive changelog (opened 2026-10-29) marks this as Breaking: on 2026-09-30.endive the billing_cycle_anchor parameter for updating and resuming Subscriptions now requires an object with a type field instead of the previous string/enum value (now or unchanged).  [✓0 ✗0]
1. For creating a preview invoice, subscription_details.billing_cycle_anchor now requires an object with type and timestamp fields instead of a Unix timestamp; Stripe's example replaces 1234567890 with {type: 'timestamp', timestamp: 1234567890}.  [✓0 ✗0]
2. The changelog lists the same parameter type change (enum or DateTime -> billing_cycle_anchor_param) in the Ruby, Python, PHP, Java, Node.js, Go and .NET SDKs, so upgrading an SDK pinned to Endive changes the expected type.  [✓0 ✗0]

## Fix
```text
Replace billing_cycle_anchor: 'now' with {type: 'now'} and 'unchanged' with {type: 'unchanged'} on Subscription update/resume; on Invoice create_preview send subscription_details.billing_cycle_anchor as {type: 'timestamp', timestamp: <unix>}. Or keep requests pinned to your previous API version (SDK version / Stripe-Version header) until the code is updated.
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0633/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28