Paymaster API

The Paymaster API signs paymasterAndData for your ERC-4337 UserOperations. It exposes both an ERC-7677-compliant JSON-RPC surface and a thin REST surface.

Authentication

Authorization: Bearer sk_live_...

All endpoints accept either a sk_live_* server key or a publishable pmc_* key (REST only, with origin allowlisting).

Base URLs

Env	URL
Production	https://api.stablecoinx.com
Sandbox	https://api.sandbox.stablecoinx.com

REST

POST /v1/paymaster/sign

Sign paymasterAndData for a UserOperation.

Body

Field	Type	Required	Description
userOp	UserOperation	yes	The unsigned UserOp
mode	"sponsor" | "user_pays"	no	Override project default
chain	string	yes	e.g. base, arbitrum
policyKey	string	no	Named per-sender policy

Response

{
  "paymasterAndData": "0x...",
  "preVerificationGas": "0x...",
  "verificationGasLimit": "0x...",
  "callGasLimit": "0x..."
}

GET /v1/paymaster/info

Returns paymaster metadata for the calling project.

{
  "contracts": {
    "base":     "0xPaymaster...",
    "arbitrum": "0xPaymaster...",
    "optimism": "0xPaymaster...",
    "ethereum": "0xPaymaster..."
  },
  "supportedAssets": ["USDe", "USDC"],
  "modesEnabled": ["sponsor", "user_pays"],
  "limits": {
    "sponsorBudgetRemaining": "100.00",
    "perSenderDailyCap": "10.00"
  }
}

POST /v1/paymaster/policies

Create or update a per-sender policy.

{
  "name": "vip-users",
  "senders": ["0xabc...", "0xdef..."],
  "mode": "sponsor",
  "dailyCap": "5.00",
  "monthlyCap": "100.00"
}

JSON-RPC (ERC-7677)

Endpoint: POST https://api.stablecoinx.com/v1/paymaster/rpc

pm_getPaymasterStubData

Returns dummy paymaster data for gas estimation.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "pm_getPaymasterStubData",
  "params": [
    { /* UserOperation */ },
    "0xEntryPoint",
    "0x2105",
    { "mode": "sponsor" }
  ]
}

pm_getPaymasterData

Returns signed paymaster data ready to submit.

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "pm_getPaymasterData",
  "params": [ /* same shape */ ]
}

Errors

HTTP	Code	Meaning
400	invalid_userop	UserOp failed schema validation
401	unauthorized	Missing or invalid key
402	sponsor_budget_exhausted	Daily or monthly sponsor cap hit
403	sender_not_allowed	Sender outside the active policy
422	unsupported_chain	chain not enabled for this project
429	rate_limited	Retry after Retry-After

SDK

import { Paymaster } from "@stablecoinx/sdk";

const pm = new Paymaster({ apiKey: process.env.STABLECOINX_KEY! });

const { paymasterAndData } = await pm.sign({
  userOp,
  mode: "user_pays",
  chain: "base",
});