> ## Documentation Index > Fetch the complete documentation index at: https://docs.withflex.com/llms.txt > Use this file to discover all available pages before exploring further. # Preview Refund Allocation > Computes how a refund would be allocated across the Checkout Session's payment methods, without executing any refund. Takes the same request body as the refund endpoint and returns the per-payment-method allocations so you can preview a split-cart refund before committing. ## OpenAPI ````yaml /openapi.json post /v1/checkout/sessions/{id}/refund_allocation openapi: 3.1.0 info: title: Flex API description: >- The Flex API powers HSA/FSA payment processing for healthcare commerce: checkout sessions, payment intents, subscriptions, customers, refunds, disputes, and eligibility. Authenticate with your partner API key as a Bearer token. All endpoints are scoped to your partner account, and test mode is fully supported via test API keys. version: 0.1.0 servers: - url: https://api.withflex.com security: - BearerAuth: [] tags: - name: Balance Transactions - name: Captures - name: Checkout Sessions - name: Coupons - name: Customers - name: Disputes - name: Events - name: Exports - name: Files - name: Invoices - name: Letters - name: Orders - name: Payment Intents - name: Payment Links - name: Payouts - name: Prices - name: Products - name: Promo Codes - name: Receipts - name: Refunds - name: Setup Intents - name: Shipping Rates - name: Subscriptions paths: /v1/checkout/sessions/{id}/refund_allocation: post: tags: - Checkout Sessions summary: Preview Refund Allocation description: >- Computes how a refund would be allocated across the Checkout Session's payment methods, without executing any refund. Takes the same request body as the refund endpoint and returns the per-payment-method allocations so you can preview a split-cart refund before committing. operationId: v1.checkout_sessions.preview_refund_allocation parameters: - in: path name: id required: true schema: examples: - fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA type: string style: simple example: fcs_01J9XR8M3K7VZ8N2YB4WJ6T0RA requestBody: description: An envelope wrapping a single checkout session object. content: application/json: schema: $ref: >- #/components/schemas/CheckoutSessionBody_for_RefundCheckoutSessionRequest example: checkout_session: line_items: [] required: true responses: '200': description: Success content: application/json: schema: $ref: '#/components/schemas/RefundAllocationResponse' example: allocations: - payment_method_id: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA last4: '4242' amount: 2500 '401': description: Unauthorized content: application/json: schema: $ref: '#/components/schemas/HttpErrorOut' example: code: string detail: string '403': description: Forbidden content: application/json: schema: $ref: '#/components/schemas/HttpErrorOut' example: code: string detail: string '404': description: Not Found content: application/json: schema: $ref: '#/components/schemas/HttpErrorOut' example: code: string detail: string '409': description: Conflict content: application/json: schema: $ref: '#/components/schemas/HttpErrorOut' example: code: string detail: string '422': description: Validation Error content: application/json: schema: $ref: '#/components/schemas/HTTPValidationError' example: detail: - loc: - string msg: string type: string '429': description: Too Many Requests content: application/json: schema: $ref: '#/components/schemas/HttpErrorOut' example: code: string detail: string security: - BearerAuth: - checkout_sessions:write components: schemas: CheckoutSessionBody_for_RefundCheckoutSessionRequest: description: An envelope wrapping a single checkout session object. type: object required: - checkout_session properties: checkout_session: $ref: '#/components/schemas/RefundCheckoutSessionRequest' description: The checkout session payload. example: checkout_session: line_items: [] RefundAllocationResponse: description: >- Preview of how a refund would be distributed across the session's payment methods. type: object required: - allocations properties: allocations: description: >- The computed refund amount allocated to each payment method. No refund is executed. type: array items: $ref: '#/components/schemas/RefundAllocation' example: allocations: - payment_method_id: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA last4: '4242' amount: 2500 HttpErrorOut: title: HttpError description: The error response returned when a request cannot be completed. type: object required: - code - detail properties: code: description: A short machine-readable error code identifying the type of error. type: string detail: description: A human-readable explanation of what went wrong. type: string example: code: string detail: string HTTPValidationError: description: The error response returned when request validation fails (HTTP 422). type: object required: - detail properties: detail: description: The list of validation errors, one entry per invalid field. type: array items: $ref: '#/components/schemas/ValidationErrorItem' example: detail: - loc: - string msg: string type: string RefundCheckoutSessionRequest: description: >- Parameters for refunding a Checkout Session. Specify what to refund either by `line_items` or by a total `amount`; omit both to refund the full remaining amount. type: object example: line_items: - {} properties: refund_metadata: description: >- Set of key-value pairs to set on the Checkout Session's `metadata` when the refund succeeds. `null` to leave the session metadata unchanged. type: - object - 'null' additionalProperties: type: string example: order_id: '8842' channel: shopify amount_tax: description: >- Portion of the refund attributable to tax, in the smallest currency unit (e.g., `175` = $1.75 USD). type: - integer - 'null' format: int32 example: 2500 amount_shipping: description: >- Portion of the refund attributable to shipping, in the smallest currency unit (e.g., `599` = $5.99 USD). type: - integer - 'null' format: int32 example: 2500 amount_discount: description: >- Portion of the refund attributable to discounts, in the smallest currency unit (e.g., `500` = $5.00 USD). type: - integer - 'null' format: int32 example: 2500 payment_methods: description: >- Explicit allocation to specific payment methods. When provided, bypasses automatic HSA-based allocation and refunds exact amounts to each payment method. This feature is currently in beta. Contact the Flex team to request access. type: - array - 'null' items: $ref: '#/components/schemas/PaymentMethodRefundRequest' line_items: description: The line items to refund, with the amount to refund for each. type: array items: $ref: '#/components/schemas/LineItemRefundRequest' amount: description: >- The total amount to refund, in the smallest currency unit (e.g., `2500` = $25.00 USD). type: integer format: int64 minimum: 0 maximum: 4294967295 RefundAllocation: description: How much of a refund is allocated to a specific payment method. type: object required: - amount - last4 - payment_method_id properties: payment_method_id: description: >- ID of the PaymentMethod this portion of the refund would be returned to. type: string example: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA last4: description: >- Last four digits of the payment method's card, for display. Falls back to `"unknown"` if it cannot be resolved. type: string example: '4242' amount: description: >- Amount that would be refunded to this payment method, in the smallest currency unit (e.g., `2500` = $25.00 USD). type: integer format: int64 example: 2500 example: payment_method_id: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA last4: '4242' amount: 2500 ValidationErrorItem: description: >- Validation errors have their own schema to provide context for invalid requests eg. mismatched types and out of bounds values. There may be any number of these per 422 UNPROCESSABLE ENTITY error. type: object required: - loc - msg - type properties: loc: description: >- The location as a [`Vec`] of [`String`]s -- often in the form `["body", "field_name"]`, `["query", "field_name"]`, etc. They may, however, be arbitarily deep. type: array items: type: string msg: description: The message accompanying the validation error item. type: string type: description: >- The type of error, often "type_error" or "value_error", but sometimes with more context like as "value_error.number.not_ge" type: string example: loc: - string msg: string type: string PaymentMethodRefundRequest: description: >- Request to refund a specific amount to a specific payment method. When multiple `PaymentMethodRefundRequest` entries have the same `payment_method_id`, their amounts are aggregated during validation. # Fields * `payment_method_id` - The payment method to refund to * `amount` - Amount to refund in cents (must be non-negative) type: object required: - amount - payment_method_id properties: payment_method_id: description: ID of the PaymentMethod to refund to. type: string example: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA amount: description: >- Amount to refund to this payment method, in the smallest currency unit (e.g., `2500` = $25.00 USD). Must be non-negative. type: integer format: int32 example: 2500 example: payment_method_id: fpm_01J9XR8M3K7VZ8N2YB4WJ6T0RA amount: 2500 LineItemRefundRequest: description: Identifies a single line item to refund and how much of it to refund. type: object example: product: fprod_01J9XR8M3K7VZ8N2YB4WJ6T0RA properties: amount_to_refund: description: >- The amount to refund for this line item, in the smallest currency unit (e.g., `2500` = $25.00 USD). Omit to refund the full amount for the item. type: - integer - 'null' format: int64 minimum: 0 maximum: 4294967295 example: 2500 product: type: string example: fprod_01J9XR8M3K7VZ8N2YB4WJ6T0RA description: ID of the Product identifying the line item. price: type: string example: fprice_01J9XR8M3K7VZ8N2YB4WJ6T0RA description: ID of the Price identifying the line item. securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: API Key description: Use a Bearer token to access this API. ````