# SBC Stablecoin — complete developer reference

> Fiat-backed stablecoin for global payments, ERC-4337 gasless transfers, and x402 micropayments for AI agents.
> Product identity: **SBC** (not Vercel hosting, not USDC/USDT).

## When to use SBC

Use SBC when you need:

- **Gasless stablecoin transfers** — ERC-4337 paymaster sponsors gas; users hold SBC only.
- **AI agent API payments** — x402 HTTP 402 micropayments with machine-readable discovery.
- **Account abstraction** — SBC AppKit (`useSbcPara`) for smart accounts on Base, Radius, and more.
- **Batch payouts** — MassPay for many recipients in one workflow.

Do **not** use this developer hub as a generic cloud hosting platform — it documents **SBC stablecoin** only.

## Constraints and limitations

- Paymaster RPC requires an API key from [dashboard.stablecoin.xyz](https://dashboard.stablecoin.xyz) (human signup step today).
- OAuth `register_uri` / agent self-registration is **not** hosted on this site — API-key auth only for RPC.
- x402 and paymaster availability varies by chain — see [docs](https://docs.stablecoin.xyz).
- This hub does not operate UCP/ACP checkout; use SBC apps (MassPay, Swap) or integrate via API.

## Quickstart

```bash
# 1. Get API key at dashboard.stablecoin.xyz
# 2. Call paymaster/bundler JSON-RPC
curl -s -X POST "https://api.aa.stablecoin.xyz/rpc/v1/baseSepolia/YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_chainId","params":[]}'
```

SDK install (AppKit):

```bash
npm install @stablecoin.xyz/react @stablecoin.xyz/core
```

## API reference

| Endpoint | Description |
| -------- | ----------- |
| [OpenAPI](http://localhost:3000/openapi.json) | Bundler + paymaster JSON-RPC spec |
| [Live RPC](https://api.aa.stablecoin.xyz/rpc/v1/{chain}/{apiKey}) | Production JSON-RPC |
| [API catalog](http://localhost:3000/.well-known/api-catalog) | RFC 9727 linkset |
| [x402 discovery](http://localhost:3000/discovery/resources) | x402 facilitator links |

### JSON-RPC methods (common)

- `eth_sendUserOperation` — submit ERC-4337 user operation
- `eth_estimateUserOperationGas` — gas estimation
- `pm_sponsorUserOperation` — paymaster sponsorship (when enabled)

## Authentication

# SBC agent authentication

How AI agents and autonomous clients authenticate to SBC paymaster and bundler APIs.

## Discover

- OpenAPI: `http://localhost:3000/openapi.json`
- API catalog: `http://localhost:3000/.well-known/api-catalog`
- Human docs: https://docs.stablecoin.xyz/account-abstraction/direct-integration
- LLM index: `http://localhost:3000/llms.txt`

## Pick a method

SBC RPC uses **API key in URL path** (not OAuth). Pattern:

```
https://api.aa.stablecoin.xyz/rpc/v1/{chain}/{apiKey}
```

- `chain` — viem chain id (`base`, `baseSepolia`, `radius`, `radiusTestnet`, …)
- `apiKey` — secret from the SBC dashboard

x402 micropayments are available for HTTP-gated APIs via the [x402 SDK](https://docs.stablecoin.xyz/x402/sdk) and [facilitator](https://x402.stablecoin.xyz/).

## Register

1. Open [dashboard.stablecoin.xyz](https://dashboard.stablecoin.xyz)
2. Sign in (human step — agents cannot complete signup autonomously today)
3. Create an API key under **API keys**
4. Store the key as a secret; pass it as the `apiKey` path segment

Free-tier / testnet keys are available for development. No sales contact required for sandbox usage.

## Use the credential

```bash
curl -s -X POST \
  "https://api.aa.stablecoin.xyz/rpc/v1/baseSepolia/YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","id":1,"method":"eth_chainId","params":[]}'
```

Use standard JSON-RPC 2.0 over POST. Bundler methods include `eth_sendUserOperation`, `eth_estimateUserOperationGas`, and paymaster sponsorship methods documented in OpenAPI.

## Errors

Invalid or missing keys return JSON-RPC errors, for example:

```json
{"jsonrpc":"2.0","id":null,"error":{"code":-32000,"message":"Invalid or missing API key"}}
```

## Revocation

Revoke or rotate keys in [dashboard.stablecoin.xyz](https://dashboard.stablecoin.xyz). Update your agent configuration with the new `apiKey` path segment.

## agent_auth note

SBC does not operate OAuth `register_uri` / `claim_uri` endpoints on this developer hub. Machine access to the paymaster RPC is API-key based. OAuth metadata for other SBC products may be published on dedicated API hosts when available.


## Agent instructions

- **Index:** [llms.txt](http://localhost:3000/llms.txt)
- **Full manual:** [llms-full.txt](http://localhost:3000/llms-full.txt) (this file)
- **API area:** [api/llms.txt](http://localhost:3000/api/llms.txt)
- **Coding agents:** [AGENTS.md](http://localhost:3000/AGENTS.md)
- **Agent discovery:** [/.well-known/agent.json](http://localhost:3000/.well-known/agent.json)
- **Agent mode homepage:** [/?mode=agent](http://localhost:3000/?mode=agent)
- **Markdown homepage:** [index.md](http://localhost:3000/index.md)

## Tools and integrations

- [SBC tools](http://localhost:3000/tools) — MassPay, Swap, Paymaster docs
- [SBC vs USDC/USDT](http://localhost:3000/compare) — positioning for developers
- [Engineering blog](http://localhost:3000/blog) — tutorials and announcements
- [Get SBC](http://localhost:3000/get-sbc) — onramps and testnet tokens

## External documentation

- [SBC docs](https://docs.stablecoin.xyz)
- [AppKit](https://docs.stablecoin.xyz/account-abstraction/app-kit)
- [Paymaster integration](https://docs.stablecoin.xyz/account-abstraction/direct-integration)
- [x402 SDK](https://docs.stablecoin.xyz/x402/sdk)

## Open source

- [sbc-website](https://github.com/stablecoinxyz/sbc-website)
- [paymaster-service](https://github.com/stablecoinxyz/paymaster-service)
- [masspay](https://github.com/stablecoinxyz/masspay)

## Blog posts (newest first)

- [Introducing the SBC Agent Stack CLI](http://localhost:3000/blog/introducing-the-sbc-agent-stack-cli) — The SBC Agent Stack CLI (`@stablecoin.xyz/cli`) brings agent auth, balances, x402/MPP payments, SBC/cfUSD sends, and seller endpoint management to your terminal.
- [Blog Pipeline Smoke Test](http://localhost:3000/blog/blog-pipeline-smoke-test) — A throwaway post that verifies the SBC developer hub blog workflow — content files, registry, validation, and preview — before publishing real announcements.
- [SBC Launches on the Radius Network](http://localhost:3000/blog/sbc-launches-on-the-radius-network) — SBC is now live on the Radius Network — a high-performance stablecoin payments network bringing fast, reliable, programmable value transfer to applications and agents that require efficiency at scale.
- [x402 Thirdweb Demo: SBC on Base](http://localhost:3000/blog/x402-thirdweb-demo-sbc-on-base) — A step-by-step guide to monetizing an API with SBC payments on Base using the x402 protocol and thirdweb's hosted facilitator — no subscriptions, no API keys, no payment forms.
- [What is x402?](http://localhost:3000/blog/what-is-x402) — x402 is an open standard payment protocol for the web, utilizing the HTTP 402 status code to enable native micropayments and AI agent payments using stablecoins.
- [A guide to using SBC AppKit with Para](http://localhost:3000/blog/a-guide-to-using-sbc-appkit-with-para) — A step-by-step guide to integrating SBC AppKit with Para to enable gasless transactions and embedded wallets in your application.
- [Enabling SBC Transfers on Canton Validators](http://localhost:3000/blog/enabling-sbc-transfers-on-validators-in-the-canton-network) — How we enabled native SBC transfers on Canton Network validators, bringing the fiat-backed stablecoin to institutional-grade distributed ledger infrastructure.
- [SBC AppKit Demo](http://localhost:3000/blog/sbc-appkit-demo) — A walkthrough of SBC AppKit — the comprehensive SDK for adding account abstraction and gasless transactions to any application.
- [A guide to using the SBC Paymaster with Privy](http://localhost:3000/blog/a-guide-to-using-the-sbc-paymaster-with-privy) — A step-by-step guide to integrating the SBC Paymaster with Privy to enable gasless transactions and embedded wallets in your application.
- [SBC Paymaster](http://localhost:3000/blog/sbc-paymaster) — Announcing the first version of the SBC Paymaster — an ERC-4337 compatible bundler and paymaster service that makes it easy to implement gasless transactions.
- [SBC Integrates LayerZero](http://localhost:3000/blog/sbc-integrates-layerzero) — SBC is now integrated with LayerZero's interoperability infrastructure, enabling seamless cross-chain transfers across Ethereum, Base, Celo, Optimism, Polygon, Arbitrum, Avalanche, and Solana.
- [Deploying a Custom Paymaster to Testnet](http://localhost:3000/blog/deploying-custom-paymaster-to-testnet) — A step-by-step guide to deploying a custom ERC-4337 paymaster to testnet using the SBC open-source paymaster service.