fix: use checksummed native token address; add mainnet e2e verification

- Use EIP-55 checksummed form of the native token sentinel (0xEeee...)
  so the 0x API passes address validation
- Note in README that selling native ETH is unsupported via AllowanceHolder
  (wrap to WETH first); buying native ETH uses the checksummed sentinel
- Improve e2e error output to print full API response body on failure
- Add mainnet verification section to README with confirmed Base tx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jlin27

README.md

@@ -135,7 +135,7 @@ Call `getSupportedChains()` to retrieve the full list at runtime.
 
 The 0x Swap API accepts **any liquid ERC-20 token by contract address**. There is no supported token list — `getSupportedTokens()` throws `NotImplementedError`. Pass token addresses directly to `quoteSwidge` and `swidge`.
 
-For native ETH (or any chain's native token), use `0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee` or the alias `'native'`.
+For native ETH (or any chain's native token) as a **buy token**, use `0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeeeE` or the alias `'native'`. Selling native ETH is not supported by the AllowanceHolder flow — wrap to WETH first.
 
 ---
 
@@ -206,6 +206,16 @@ ZERO_EX_API_KEY=... MNEMONIC="word word ..." npm run e2e
 
 ---
 
+## Verified on mainnet
+
+End-to-end tested against the live 0x API on Base mainnet (chain 8453):
+
+| Swap | Chain | Tx |
+|---|---|---|
+| 1 USDC → WETH | Base | [`0x87afe3…704ec`](https://basescan.org/tx/0x87afe381a625f1af33f7e4faec1f43fbbcb2dde569c2cf1471f37bb825b704ec) |
+
+---
+
 ## Unsupported options
 
 The following fields from `SwidgeCommonOptions` are accepted by the interface but not supported by this module:

src/0x-protocol.js

@@ -46,16 +46,18 @@ import { ZeroExFeeLimitExceededError, ZeroExInsufficientLiquidityError, ZeroExRe
  * @property {number | bigint} [maxProtocolFeeBps] - Maximum acceptable protocol fee in basis points of the input amount.
  */
 
-// 0x uses this sentinel for native ETH / chain native token
-const NATIVE_TOKEN_ADDRESS = '0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee'
+// 0x uses this checksummed sentinel for native ETH / chain native token.
+// Must use the EIP-55 checksummed form — the 0x API rejects the all-lowercase variant.
+const NATIVE_TOKEN_ADDRESS = '0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeeeE'
+const NATIVE_TOKEN_ADDRESS_LOWER = NATIVE_TOKEN_ADDRESS.toLowerCase()
 
 // Identifiers callers may pass to mean "native token"
 const NATIVE_TOKEN_ALIASES = new Set([
   '',
   'native',
   'eth',
   '0x0000000000000000000000000000000000000000',
-  '0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee'
+  NATIVE_TOKEN_ADDRESS_LOWER
 ])
 
 /** @type {SwidgeSupportedChain[]} */
@@ -512,7 +514,7 @@ export default class ZeroExProtocol extends SwidgeProtocol {
 
     // Network fee: only comparable when selling the native token
     if (maxNetworkFeeBps != null && quote.totalNetworkFee) {
-      if (sellToken.toLowerCase() === NATIVE_TOKEN_ADDRESS) {
+      if (sellToken.toLowerCase() === NATIVE_TOKEN_ADDRESS_LOWER) {
         const bps = (Number(quote.totalNetworkFee) / sellAmount) * 10000
         if (bps > Number(maxNetworkFeeBps)) {
           throw new ZeroExFeeLimitExceededError('network', bps, Number(maxNetworkFeeBps))

tests/e2e/swap-base.js

@@ -119,5 +119,6 @@ async function main () {
 
 main().catch(err => {
   console.error('\nE2E failed:', err.message)
+  if (err.body != null) console.error('API response body:', JSON.stringify(err.body, null, 2))
   process.exitCode = 1
 })