# Schemes and networks

Check which schemes, chains and tokens a facilitator actually serves before you build against it.

Section: Understand x402
Source: https://www.roundhouse.studio/docs/x402/schemes-and-networks

---

The protocol is open about *what* can be paid and *where*. The served set is narrow, and the gap
between "the spec allows it" and "a facilitator serves it" causes a confusing class of failure.

## The `exact` scheme

One scheme is in production use: `exact`. It means an [EIP-3009](https://eips.ethereum.org/EIPS/eip-3009)
`TransferWithAuthorization` for a precise amount, signed over the token contract's EIP-712 domain.

Why EIP-3009 rather than `approve` + `transferFrom`:

- **Gasless for the payer.** The signature is data; someone else broadcasts it. An agent needs the
  payment asset only, not the chain's gas token.
- **Exact and bounded.** The amount, the recipient, and a validity window are all inside the signed
  payload. A facilitator cannot pay a different address or a different amount.
- **Single-use by construction.** The `nonce` is enforced by the token contract, so a captured
  authorization cannot be replayed.
- **Auditable.** The token emits `AuthorizationUsed`, which lets an indexer distinguish an
  x402 payment from an ordinary transfer.

The token must implement EIP-3009. USDC does; most ERC-20s do not. That constraint, not policy, is
why the ecosystem is USDC-shaped today.

## Networks

| Network | CAIP-2 | Also written as | Status |
| --- | --- | --- | --- |
| Base mainnet | `eip155:8453` | `base` | Live. Where real value moves |
| Base Sepolia | `eip155:84532` | `base-sepolia` | Testnet. Free to get wrong |

> [!IMPORTANT]
> Publishers label the same chain inconsistently — `base` and `eip155:8453` both
> appear in live catalog entries. A client should **accept both spellings** and
> normalise to the CAIP-2 form. Roundhouse does exactly this on ingest, which is
> why a service listed as `base` is still queryable as `eip155:8453`.

## Assets

USDC, at 6 decimals, on both networks.

| Network | USDC contract |
| --- | --- |
| Base | `0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913` |
| Base Sepolia | `0x036CbD53842c5426634e7929541eC2318f3dCF7e` |

Read the address from the `402` challenge's `asset` field or from a facilitator descriptor rather
than hard-coding it. The index stores per-chain token metadata rather than assuming one token per
chain, so a second asset is a configuration change and not a schema change — expect the set to
grow.

An amount is always atomic units in a string. `"10000"` is one cent. There is no decimal form
anywhere on the wire.

## Facilitator coverage is not universal

This is the trap worth internalising, because the error message actively misleads.

**`x402.org/facilitator` is EVM testnet only.** It serves `base-sepolia` / `eip155:84532` and has no
Base mainnet support. Asked to settle on a chain it does not serve, it returns a `500` that reads
like a payment failure — but the payment was fine and the configuration was not.

Always ask first:

```bash
curl -sL "https://x402.roundhouse.studio/supported" | jq
```

```json
{
  "kinds": [
    { "x402Version": 1, "scheme": "exact", "network": "base" },
    { "x402Version": 2, "scheme": "exact", "network": "eip155:8453" }
  ]
}
```

The Roundhouse facilitator serves **Base mainnet**, `exact`, USDC — both the v1 short name and the
v2 CAIP-2 form, so a client of either vintage works unmodified. For testnet work, point at
`x402.org/facilitator` instead. Details in [facilitators](https://www.roundhouse.studio/docs/x402/facilitators).

## Choosing a network

| You are | Use |
| --- | --- |
| Building and testing a client | Base Sepolia, with `x402.org/facilitator` |
| Charging real money | Base, with a mainnet facilitator |
| Wanting the payment indexed and public | Base — the index tracks mainnet settlements |

A testnet settlement is not indexed as economic activity, which is the point: your integration
tests should not accumulate a reputation.

## Next steps

- [Facilitators](https://www.roundhouse.studio/docs/x402/facilitators) — what they do, and picking one
- [Receipts and verification](https://www.roundhouse.studio/docs/x402/receipts-and-verification) — proving a payment happened
- [Charge for your API](https://www.roundhouse.studio/docs/guides/charge-for-your-api) — be the resource server

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Every page in these docs is available as markdown at its own URL plus `.md`.
Full index: https://www.roundhouse.studio/docs.md