Update an existing checkout session's contract, customer details, expiration, currency, redirect URLs, attribution, or custom data.

Common use cases:

• Plan changes: Switch between subscription plans or products
• Quantity adjustments: Change seat counts or usage tiers
• Billing date changes: Adjust contract start date
• Customer information correction: Fix details for guest checkout
• Configuration updates: Change URLs, expiry, or add tracking data

Rules and validations:

• Only sessions in 'open' status and with payment_status 'not_started' can be updated
• Contract updates support both plan-based and product-based formats
• Currency must match between checkout session and contract if both provided
• URL fields must be valid, fully-qualified URLs with https scheme
• Customer details can only be updated for guest checkout sessions
• Customer identity (customer_id) cannot be changed after creation

System behavior:

• Contract changes automatically trigger recalculation of pricing estimates
• All updates are recorded in an audit trail under custom_data
• Last activity timestamp is updated to track checkout progression
• Validation errors will prevent any changes and return specific error codes

Related endpoints:

• POST /checkout_session: Creates the initial session that can be updated
• GET /checkout_session/{id}: Check current session state before updating
• POST /checkout_session/{id}/confirm: Process payment after updates are complete
• POST /checkout_session/summary: Preview pricing changes without creating a session