> ## Documentation Index > Fetch the complete documentation index at: https://docs.chance.cc/llms.txt > Use this file to discover all available pages before exploring further. # Buy credits with x402 > Pays USDC on Base (eip155:8453) over x402 v2. Name the account with an API key or with `?account=`. Without a payment the response is 402 with a PAYMENT-REQUIRED header. Sign the EIP-3009 authorization it describes and retry with PAYMENT-SIGNATURE. Credits are added after settlement, once per authorization. A plain USDC transfer credits nothing. ## OpenAPI ````yaml /openapi.json post /api/v1/credits/topup openapi: 3.0.3 info: title: Chance API version: 1.0.0 description: >- Verify an agent's proposed action against its rules, manage escrow wallets, and buy credits. Every verification returns a signed, hash-chained receipt. servers: - url: https://harness.chance.cc description: Production security: - ApiKeyAuth: [] - BearerAuth: [] tags: - name: Verification description: Check an action against its rules and fetch past verdicts. - name: Credits description: Balance and x402 top-ups. - name: Escrow wallets description: Wallets that only move funds through a verified proposal. - name: Trader description: Liveness reports from trader containers. - name: Gas sponsorship description: Sponsored gas for trader instances on Polygon. - name: Status description: Public deployment facts for checking receipts. paths: /api/v1/credits/topup: post: tags: - Credits summary: Buy credits with x402 description: >- Pays USDC on Base (eip155:8453) over x402 v2. Name the account with an API key or with `?account=`. Without a payment the response is 402 with a PAYMENT-REQUIRED header. Sign the EIP-3009 authorization it describes and retry with PAYMENT-SIGNATURE. Credits are added after settlement, once per authorization. A plain USDC transfer credits nothing. operationId: topupCredits parameters: - name: credits in: query required: true description: Credits to buy. Integer, 20 to 10,000 by default. 1 credit = $0.05. schema: type: integer minimum: 20 maximum: 10000 - name: account in: query required: false description: >- Public top-up code (`chance_topup_…`). Use it instead of an API key to pay for another account. Ignored when an API key is sent. schema: type: string responses: '200': description: Settled and credited. PAYMENT-RESPONSE carries the settlement. headers: PAYMENT-RESPONSE: description: Base64 x402 v2 settlement response. schema: type: string content: application/json: schema: $ref: '#/components/schemas/TopupReceipt' example: credited: 200 usd: '10.00' creditsRemaining: 203 txHash: 0x… payer: 0x… replayed: false '400': description: '`credits` is missing, not an integer, or out of range.' content: application/json: schema: $ref: '#/components/schemas/Error' example: error: invalid_credits message: credits must be between 20 and 10000 … '401': description: No account named, or the API key is unknown or revoked. content: application/json: schema: $ref: '#/components/schemas/Error' examples: e0: summary: missing_account value: error: missing_account message: >- identify the account to credit: send an x-api-key header, or ?account= code e1: summary: invalid or revoked API key value: error: invalid or revoked API key '402': description: >- Payment required, or the attached payment failed. The PAYMENT-REQUIRED header always carries fresh terms. After a settlement failure, PAYMENT-RESPONSE is also set with `success: false`. headers: PAYMENT-REQUIRED: description: >- Base64 x402 v2 PaymentRequired: scheme `exact`, network eip155:8453, USDC, amount = credits × $0.05. schema: type: string PAYMENT-RESPONSE: description: Only after a settlement failure. schema: type: string content: application/json: schema: $ref: '#/components/schemas/TopupPaymentRequired' examples: required: summary: No payment attached value: error: payment_required message: 'Payment required: $10.00 USDC on Base for 200 credits. …' x402Version: 2 credits: 200 usd: '10.00' v1: summary: x402 v1 X-PAYMENT header sent value: error: x402 v1 payments are not supported … message: … x402Version: 2 credits: 200 usd: '10.00' delegated: summary: Smart or delegated payer rejected value: error: This wallet (0x…) can't complete the payment … message: … x402Version: 2 credits: 200 usd: '10.00' code: wallet_unsupported '404': description: Unknown account code. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: invalid_account message: unknown top-up account code '409': description: This authorization was already submitted for a different account. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: payment_claimed message: >- this payment authorization was already submitted for a different account '502': description: >- The facilitator is unavailable. Retry with the same PAYMENT-SIGNATURE; crediting is idempotent. headers: Retry-After: description: Seconds to wait. schema: type: integer example: 5 content: application/json: schema: $ref: '#/components/schemas/Error' example: error: facilitator_unavailable message: … retry with the same payment '503': description: Top-ups are not enabled on this deployment. content: application/json: schema: $ref: '#/components/schemas/Error' example: error: topup_not_configured message: Credit top-ups are not enabled on this deployment. security: - ApiKeyAuth: [] - BearerAuth: [] - {} components: schemas: TopupReceipt: type: object properties: credited: type: integer example: 200 usd: type: string example: '10.00' creditsRemaining: type: integer description: >- New balance. Only returned when the request authenticated with an API key. example: 203 txHash: type: string nullable: true description: Base settlement transaction. payer: type: string description: Address that signed the payment. replayed: type: boolean description: >- True when this payment was already credited and the original result is returned. Error: type: object required: - error properties: error: type: string description: >- An error code (for example `out_of_credits`) or a short sentence (for example `intent is required`). message: type: string description: Detail, when present. TopupPaymentRequired: type: object properties: error: type: string description: >- `payment_required` when no payment was attached; otherwise the failure message. message: type: string x402Version: type: integer enum: - 2 credits: type: integer usd: type: string code: type: string enum: - wallet_unsupported description: >- Set when the payer is a smart or EIP-7702-delegated account whose signature was rejected. Retrying from that wallet will not help. securitySchemes: ApiKeyAuth: type: apiKey in: header name: x-api-key description: An API key (`chance_sk_live_…`) from https://harness.chance.cc/keys. BearerAuth: type: http scheme: bearer description: >- The same API key sent as `Authorization: Bearer `. If both headers are sent, `x-api-key` is used. ````