> ## 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. # Build onboarding transaction > Build the sponsor-funded onboarding transaction for a classic G-address: creates the account if needed and establishes the USDC trustline. The facilitator returns the unsigned transaction XDR for the user to sign and submit via `POST /stellar/onboard/submit`. If the account is already fully onboarded, `nothingToDo: true` is returned and no XDR needs to be signed. ## OpenAPI ````yaml /api-reference/openapi.json post /stellar/onboard/build 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: /stellar/onboard/build: post: tags: - Onboarding summary: Build onboarding transaction description: >- Build the sponsor-funded onboarding transaction for a classic G-address: creates the account if needed and establishes the USDC trustline. The facilitator returns the unsigned transaction XDR for the user to sign and submit via `POST /stellar/onboard/submit`. If the account is already fully onboarded, `nothingToDo: true` is returned and no XDR needs to be signed. operationId: postStellarOnboardBuild requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/OnboardBuildRequest' responses: '200': description: Onboarding transaction built or account already onboarded. content: application/json: schema: $ref: '#/components/schemas/OnboardBuildResponse' '400': description: Could not build the onboarding transaction. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' example: error: build_failed '401': description: Invalid or missing API key. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '502': description: Facilitator could not reach Stellar Horizon. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' components: schemas: OnboardBuildRequest: type: object required: - publicKey - chain properties: publicKey: allOf: - $ref: '#/components/schemas/StellarPublicKey' description: G-address of the account to onboard. chain: $ref: '#/components/schemas/FacilitatorChain' OnboardBuildResponse: type: object properties: ok: type: boolean description: Optional status flag. unsignedTxXdr: type: string description: >- Unsigned onboarding transaction as base64 XDR. Present when the account is not yet fully onboarded. Sign with the user's key and submit via `POST /stellar/onboard/submit`. nothingToDo: type: boolean description: >- When `true`, the account is already onboarded. No XDR needs to be signed or submitted. description: Response from `POST /stellar/onboard/build`. ErrorResponse: type: object required: - error properties: error: type: string description: >- Machine-readable error code. SDK error codes (e.g. `RELAYER_REJECTED`, `SESSION_POLICY_VIOLATION`) or raw facilitator codes (e.g. `auth_expired`, `simulation_failed`) depending on context. message: type: string description: >- Optional human-readable detail. Never log raw upstream messages - they may contain addresses or sensitive information. details: type: object description: >- Optional structured validation errors (present on BFF-level schema failures). description: >- Error body returned on non-2xx responses. The `error` field maps to a `BuckspayErrorCode` or a raw facilitator error string (see `mapFacilitatorError` in `@buckspay/relayer`). StellarPublicKey: type: string pattern: ^G[A-Z2-7]{55}$ description: Stellar classic public key (G-address, 56-character StrKey). FacilitatorChain: type: string enum: - stellar-testnet - stellar-pubnet description: Stellar network identifier used in facilitator requests. 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. ````