> ## Documentation Index > Fetch the complete documentation index at: https://docs.httpayer.com/llms.txt > Use this file to discover all available pages before exploring further. # Simulate proxy payment request > Dry-run simulation endpoint that returns payment requirements and fee breakdown without executing payment. **Fee Model:** - 3% proxy fee (covers credits charged from account) ## OpenAPI ````yaml post /proxy/sim openapi: 3.0.0 info: title: HTTPayer Proxy API version: 1.0.0 description: >- HTTPayer is a payment router for 402 response codes and the x402 Protocol. Users can access APIs that use the x402 standard through the HTTPayer proxy. contact: name: HTTPayer Team url: https://httpayer.com servers: - url: https://api.httpayer.com description: Production server security: [] tags: - name: Relay description: Cross-chain payment relay endpoints - name: Account description: Account balance and limits endpoints - name: Webhooks description: Async task status and webhooks - name: LLM description: OpenAI-powered endpoints for translation and chat - name: Health description: Health check endpoints paths: /proxy/sim: post: tags: - Proxy summary: Simulate proxy payment request description: > Dry-run simulation endpoint that returns payment requirements and fee breakdown without executing payment. **Fee Model:** - 3% proxy fee (covers credits charged from account) parameters: [] requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/RelayRequest' responses: '200': description: Simulation completed successfully content: application/json: schema: oneOf: - type: object description: No payment required properties: requiresPayment: type: boolean example: false status: type: number example: 200 response: type: object - type: object description: Payment required with fee breakdown properties: requiresPayment: type: boolean example: true targetPaymentRequirements: $ref: '#/components/schemas/PaymentRequirements' proxyFeeBreakdown: type: object properties: targetAmount: type: number description: Amount required by target API in USDC example: 0.01 proxyFee: type: number description: HTTPayer proxy fee in USDC example: 0.0003 totalCreditsCharged: type: number description: >- Total credits that will be charged from account (1 credit = 0.001 USDC) example: 10.3 proxyFeePercentage: type: number description: Proxy fee percentage example: 3 proxyFeeMinUSDC: type: number description: Minimum proxy fee in USDC example: 0 feeModel: type: string description: Human-readable fee model description example: 3% with $0 minimum creditsPerUsdc: type: number description: Conversion rate from USDC to credits example: 1000 message: type: string example: >- Payment required - this is a dry-run simulation, no payment will be executed and no credits will be charged '400': description: Bad request content: application/json: schema: $ref: '#/components/schemas/Error' '401': description: Unauthorized - invalid API key '500': description: Internal server error content: application/json: schema: $ref: '#/components/schemas/Error' security: - ApiKeyAuth: [] components: schemas: RelayRequest: type: object properties: api_url: type: string description: The target API URL to call via the relay example: https://demo.httpayer.com/base-weather method: type: string enum: - GET - POST - PUT - DELETE - PATCH description: HTTP method for the target API example: GET json: type: object description: >- JSON body — serialized with Content-Type: application/json. Mirrors Python requests(json=...). data: description: >- Form-encoded body (object) or raw string body (string). Mirrors Python requests(data=...). oneOf: - type: object - type: string body: type: string description: >- Pre-serialized body string passed through unchanged — Content-Type is NOT set automatically. If you pass an object it will be JSON-stringified, but you must set Content-Type: application/json via headers yourself. Use json instead for automatic Content-Type handling. payload: type: object description: Deprecated alias for json. Use json instead. params: type: object description: Query parameters appended to the target URL headers: type: object description: Custom headers forwarded to the target API example: X-API-KEY: your-target-api-key Authorization: Bearer token123 auth: type: object description: >- Basic auth shorthand — converted to Authorization: Basic . Mirrors Python requests(auth=(...)). properties: username: type: string password: type: string cookies: type: object description: >- Cookie shorthand — merged into the Cookie header. Mirrors Python requests(cookies={...}). example: session: abc123 network: type: string enum: - base - base-sepolia - skale-base - skale-base-sepolia - solana - solana-mainnet-beta - solana-devnet description: Blockchain network to use for payment example: base required: - api_url - method PaymentRequirements: type: object properties: scheme: type: string example: exact network: type: string example: base payTo: type: string description: Recipient address asset: type: string description: Token contract address maxAmountRequired: type: string description: Amount in microUSDC resource: type: string description: Resource URL description: type: string mimeType: type: string example: application/json maxTimeoutSeconds: type: number example: 180 Error: type: object properties: error: type: string description: Error message details: type: string description: Additional error details securitySchemes: ApiKeyAuth: type: apiKey in: header name: x-api-key description: API key for authentication ````