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docs: add Chain 138 and Atomic x402 examples #1

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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,45 @@ resp = client.fetch_paid(
print(resp.status_code, resp.text[:120])
```

## Custom EVM example: Chain 138

Custom EVM networks work well with this client because selection happens via
CAIP-2 network ids rather than a hardcoded chain registry.

One useful production-style example is **Chain 138** (`eip155:138`):

| Field | Value |
| --- | --- |
| Chain name | DeFi Oracle Meta Mainnet |
| RPC | `https://rpc-http-pub.d-bis.org` |
| Explorer | `https://explorer.d-bis.org` |
| x402 origin | `https://x402.d-bis.org` |
| Well-known | `https://x402.d-bis.org/.well-known/x402.json` |
| Example asset | `cUSDC_V2` |

```python
def on_payment(parsed):
accept = select_payment_method(
parsed.accepts,
preferred_chain="eip155:138",
max_amount_usd=0.05,
)
return build_payment_header(accept, MY_ADDRESS, sign)

resp = client.fetch_paid(
"https://x402.d-bis.org/api/catalog",
on_payment=on_payment,
)
```

When signing an ERC-3009 authorization for Chain 138 `cUSDC_V2`, the EIP-712
domain values are:

- `name = "USD Coin (Compliant V2)"`
- `version = "2"`
- `chainId = 138`
- `verifyingContract = <accept.asset>`

## API

| Symbol | Purpose |
Expand Down Expand Up @@ -70,6 +109,19 @@ The x402 spec is scheme-agnostic: `erc3009`, `exact`, and future schemes (Solana

A reference signer built on [`eth-account`](https://pypi.org/project/eth-account/) is ~30 lines and lives in the [examples/ folder](https://github.com/cryptomotifs/x402-python/tree/main/examples) of the repo.

## Atomic x402 compatibility

This client also works well with merchants that use x402 as the front door to
an **atomic intent** flow:

1. the buyer signs an ERC-3009 authorization,
2. another actor executes settlement or downstream fulfillment,
3. the merchant returns the paid resource only after proof is accepted.

Because signing is a callback, the package does not force any particular
execution model. That keeps it useful for both direct settlement and
intent-driven corridors.

## Reference implementation / live demo

- **Live x402 demo**: <https://cipher-x402.vercel.app>
Expand Down