> ## Documentation Index > Fetch the complete documentation index at: https://developers.circle.com/llms.txt > Use this file to discover all available pages before exploring further. # Reserve settlement advance credit > Holds credit so the maker can lock in a fee snapshot before requesting the settlement advance. Reservations expire after a short window (~15 minutes); after expiry the maker must reserve again. Replaying the same `idempotencyKey` returns the existing reservation. Submitting a different `idempotencyKey` while another reservation for the same currency is still active will be rejected — the maker must cancel the active reservation first before creating a new one. There is one active reservation allowed per currency. A reservation is automatically cancelled after a single settlement advance is made against it. Calling `/reserve` is optional; the maker may skip it and go straight to requesting the settlement advance. ## OpenAPI ````yaml openapi/stablefx.yaml post /v1/exchange/stablefx/settlementAdvance/reserve openapi: 3.1.0 info: title: StableFX API description: > The StableFX API provides endpoints for trading stablecoins. ## Authentication All API requests require authentication using an API key. ## Base URL The base URL for all API endpoints is: `https://api.circle.com/v1/exchange/stablefx` version: 1.0.0 servers: - url: https://api.circle.com description: StableFX API server security: - BearerAuth: [] tags: - name: Quotes description: Endpoints for creating and managing quotes - name: Trades description: Endpoints for creating and managing trades - name: Signatures description: Endpoints for retrieving presign typed data and registering signatures - name: Fees description: Endpoints for retrieving fees - name: Funding description: Endpoints for funding trades - name: Webhook Subscriptions description: Manage subscriptions to notifications - name: Settlement Advance description: Request, track, and repay settlement advances. paths: /v1/exchange/stablefx/settlementAdvance/reserve: post: tags: - Settlement Advance summary: Reserve settlement advance credit description: > Holds credit so the maker can lock in a fee snapshot before requesting the settlement advance. Reservations expire after a short window (~15 minutes); after expiry the maker must reserve again. Replaying the same `idempotencyKey` returns the existing reservation. Submitting a different `idempotencyKey` while another reservation for the same currency is still active will be rejected — the maker must cancel the active reservation first before creating a new one. There is one active reservation allowed per currency. A reservation is automatically cancelled after a single settlement advance is made against it. Calling `/reserve` is optional; the maker may skip it and go straight to requesting the settlement advance. operationId: reserveSettlementAdvance requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/ReserveSettlementAdvance' responses: '200': description: >- Reservation created or existing reservation returned on idempotent replay. headers: X-Request-Id: $ref: '#/components/headers/XRequestId' content: application/json: schema: $ref: '#/components/schemas/SettlementAdvanceReservation' '400': description: Invalid request. content: application/json: schema: $ref: '#/components/schemas/Error' '404': description: Credit line not found. content: application/json: schema: $ref: '#/components/schemas/Error' '409': description: >- An active reservation for the same currency already exists. Cancel the existing reservation before creating a new one. content: application/json: schema: $ref: '#/components/schemas/Error' security: - BearerAuth: [] components: schemas: ReserveSettlementAdvance: type: object description: Request body for reserving settlement advance credit. required: - idempotencyKey - advance properties: idempotencyKey: $ref: '#/components/schemas/IdempotencyKey' advance: $ref: '#/components/schemas/SettlementAdvanceCurrencyAmount' SettlementAdvanceReservation: type: object description: Settlement advance reservation. required: - id - amount - status - expirationDate properties: id: $ref: '#/components/schemas/Id' amount: $ref: '#/components/schemas/SettlementAdvanceCurrencyAmount' status: $ref: '#/components/schemas/ReservationStatus' expirationDate: $ref: '#/components/schemas/ExpireDate' createDate: $ref: '#/components/schemas/CreateDate' Error: title: Error type: object required: - code - message properties: code: $ref: '#/components/schemas/ErrorCode' message: type: string description: Human-readable message that describes the error. example: Unknown error occurred errors: type: array description: Array of detailed error descriptions items: $ref: '#/components/schemas/DescriptiveError' IdempotencyKey: type: string description: >- Universally unique identifier (UUID v4) idempotency key. This key is utilized to ensure exactly-once execution of mutating requests. To create a UUIDv4 go to [uuidgenerator.net](https://www.uuidgenerator.net). If the same key is reused, it will be treated as the same request and the original response will be returned. example: a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11 format: uuid SettlementAdvanceCurrencyAmount: type: object description: Currency and amount for settlement advance flows. required: - currency - amount properties: currency: $ref: '#/components/schemas/SettlementAdvanceCurrency' amount: $ref: '#/components/schemas/Amount' XRequestId: type: string description: >- A unique identifier, which can be helpful for identifying a request when communicating with Circle support. example: 2adba88e-9d63-44bc-b975-9b6ae3440dde format: uuid Id: type: string format: uuid description: System-generated unique identifier of the resource. example: c4d1da72-111e-4d52-bdbf-2e74a2d803d5 ReservationStatus: type: string description: > Status of a settlement advance reservation. - `active` -- reservation is live; holds available-balance capacity on the credit line. - `expired` -- reservation window elapsed without an origination claiming it; terminal state. - `consumed` -- origination claimed this reservation; terminal state. - `canceled` -- reservation voided by admin or system action; terminal state. enum: - active - expired - consumed - canceled example: active ExpireDate: type: string format: date-time description: Date and time when the resource expires example: '2023-01-01T12:04:05Z' CreateDate: type: string format: date-time description: Date and time when the resource was created example: '2023-01-01T12:04:05Z' ErrorCode: title: ErrorCode type: integer description: The code that corresponds to the error. enum: - -1 - 0 - 1 - 2 - 3 - 400 - 401 - 403 - 404 DescriptiveError: title: DescriptiveError type: object required: - error - message properties: error: $ref: '#/components/schemas/DescriptiveErrorType' location: type: - string - 'null' description: The key or path where the error occurred message: type: string description: Detailed description of the error SettlementAdvanceCurrency: type: string description: Currency code for settlement advance flows. example: USDC Amount: type: string pattern: ^\d+(?:\.\d{1,6})?$ description: Amount of currency, formatted as a string with up to six decimal places. example: '100.00' DescriptiveErrorType: title: DescriptiveErrorType type: string enum: - MISSING_OR_INVALID_FIELD description: Type of descriptive error headers: XRequestId: description: > Circle-generated universally unique identifier (UUID v4). Useful for identifying a specific request when communicating with Circle Support. schema: $ref: '#/components/schemas/XRequestId' securitySchemes: BearerAuth: type: http scheme: bearer bearerFormat: PREFIX:ID:SECRET description: >- Circle's API Keys are formatted in the following structure "PREFIX:ID:SECRET". All three parts are requred to make a successful request. ````