> ## Documentation Index > Fetch the complete documentation index at: https://docs.buckspay.xyz/llms.txt > Use this file to discover all available pages before exploring further. # Quote a gas-in-token fee > Request a fee quote for paying Soroban gas in a token instead of XLM. The facilitator simulates the calls, prices the XLM fee as a token amount, and returns the quote including the `FeeForwarder` contract address and a ledger-based expiry. The SDK uses this quote to build the `FeeForwarder.forward()` authorization entry before calling `POST /relay` with `feeToken` set. ## OpenAPI ````yaml /api-reference/openapi.json post /fee/quote openapi: 3.1.0 info: title: Buckspay Facilitator & BFF API version: 0.2.0 description: >- HTTP wire contract for the Buckspay SDK. The SDK calls these endpoints internally via `@buckspay/relayer`; this reference is for teams building their own BFF relay route or calling the facilitator from server-side code. All facilitator endpoints require the `x-api-key` header. **The API key is a server-side secret - it must only appear in BFF or server-side code and must never be shipped to a browser bundle.** Use `createRelayRoute` and `createSignerProxyRoute` from `@buckspay/nextjs` to build compliant BFF handlers. servers: - url: '{facilitatorUrl}' description: >- Buckspay facilitator (all endpoints). Set via BUCKSPAY_FACILITATOR_URL on the server. variables: facilitatorUrl: default: https://facilitator.buckspay.xyz description: Base URL of the Buckspay facilitator service. security: - FacilitatorApiKey: [] tags: - name: Relay description: >- Submit a signed Soroban authorization entry for gasless relaying to the Stellar network. - name: Fee description: >- Quote the fee-token amount required for gas-in-token (token gas mode) before building the forward() invocation. - name: Account State description: Read on-chain state for classic G-addresses and contract C-addresses. - name: Onboarding description: >- Build and submit the sponsored onboarding transaction; deploy passkey smart-accounts and policy session-accounts. - name: Auth description: >- Signer-proxy for social and email authentication. The BFF forwards these requests to the facilitator; provider secrets remain server-side. - name: Swap description: >- Quote a gasless EVM token swap. Requires `swapChain` to be configured in the relayer. paths: /fee/quote: post: tags: - Fee summary: Quote a gas-in-token fee description: >- Request a fee quote for paying Soroban gas in a token instead of XLM. The facilitator simulates the calls, prices the XLM fee as a token amount, and returns the quote including the `FeeForwarder` contract address and a ledger-based expiry. The SDK uses this quote to build the `FeeForwarder.forward()` authorization entry before calling `POST /relay` with `feeToken` set. operationId: postFeeQuote requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/FeeQuoteRequest' responses: '200': description: Fee quote computed successfully. content: application/json: schema: $ref: '#/components/schemas/FeeQuote' '400': description: The calls could not be simulated or the token is not supported. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' example: error: simulation_failed message: could not simulate the provided calls '401': description: Invalid or missing API key. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' example: error: unauthorized '502': description: Facilitator could not reach Stellar Horizon. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' example: error: horizon_unreachable components: schemas: FeeQuoteRequest: type: object required: - chain - from - token - calls properties: chain: $ref: '#/components/schemas/FacilitatorChain' from: allOf: - $ref: '#/components/schemas/StellarAddress' description: Payer address (G or C) whose account the fee will be deducted from. token: allOf: - $ref: '#/components/schemas/StellarContractAddress' description: Token contract address (USDC SAC) used to pay the gas fee. calls: type: array minItems: 1 description: Calls to simulate for fee estimation. items: type: object required: - contract - fn - args properties: contract: allOf: - $ref: '#/components/schemas/StellarContractAddress' description: Target contract address. fn: type: string description: Contract function name (e.g. `transfer`). args: type: array description: Soroban `ScVal` arguments serialized as base64 XDR strings. items: allOf: - $ref: '#/components/schemas/Base64' description: One `ScVal` argument as base64 XDR. description: >- Request body for `POST /fee/quote`. Calls are simulated server-side to price the XLM fee as a token amount. FeeQuote: type: object required: - forwarder - collector - token - estimatedXlmFee - tokenAmount - expiresAtLedger properties: forwarder: allOf: - $ref: '#/components/schemas/StellarContractAddress' description: >- `FeeForwarder` contract address. The SDK uses this to build the `forward()` authorization entry. collector: allOf: - $ref: '#/components/schemas/StellarAddress' description: >- Address that collects the fee (G or C). Embedded in the `forward()` invocation. token: allOf: - $ref: '#/components/schemas/StellarContractAddress' description: Token used to pay the fee. Echoes the request `token`. estimatedXlmFee: allOf: - $ref: '#/components/schemas/Stroops' description: Estimated XLM fee in stroops. tokenAmount: allOf: - $ref: '#/components/schemas/Stroops' description: >- Fee expressed in the requested token's smallest unit (USDC stroops for USDC). The SDK passes this as the `fee_amount` argument in the `forward()` invocation. expiresAtLedger: type: integer minimum: 1 description: >- Ledger sequence after which this quote expires. The SDK refuses to build a relay with an expired quote. description: >- Fee quote returned by `POST /fee/quote`. Mirrors the SDK `FeeQuote` type from `@buckspay/core`. ErrorResponse: type: object required: - error properties: error: type: string description: >- Machine-readable error code. SDK error codes (e.g. `RELAYER_REJECTED`, `SESSION_POLICY_VIOLATION`) or raw facilitator codes (e.g. `auth_expired`, `simulation_failed`) depending on context. message: type: string description: >- Optional human-readable detail. Never log raw upstream messages - they may contain addresses or sensitive information. details: type: object description: >- Optional structured validation errors (present on BFF-level schema failures). description: >- Error body returned on non-2xx responses. The `error` field maps to a `BuckspayErrorCode` or a raw facilitator error string (see `mapFacilitatorError` in `@buckspay/relayer`). FacilitatorChain: type: string enum: - stellar-testnet - stellar-pubnet description: Stellar network identifier used in facilitator requests. StellarAddress: type: string pattern: ^[GC][A-Z2-7]{55}$ description: 'Stellar address: either a classic G-address or a contract C-address.' StellarContractAddress: type: string pattern: ^C[A-Z2-7]{55}$ description: Stellar contract address (C-address, 56-character StrKey). Base64: type: string pattern: ^[A-Za-z0-9+/]+=*$ minLength: 1 description: Standard base64-encoded bytes. Stroops: type: string pattern: ^\d+$ description: >- A non-negative integer encoded as a decimal string to preserve precision. For Stellar token amounts this is the smallest unit with the asset's implied decimals (USDC = 7, i.e. stroops); the same string encoding also carries fees, nonces, and EVM swap amounts (which use their own token's decimals). securitySchemes: FacilitatorApiKey: type: apiKey in: header name: x-api-key description: >- Server-held API key. Injected by the BFF (`createRelayRoute` / `createSignerProxyRoute` from `@buckspay/nextjs`). This key must only appear in server-side code and must never be shipped to a browser bundle. ````