SyntaxError: Cannot convert decimal string to BigInt in reduce accumulator

[mwelche]

Bug Description

When a swap produces 2+ ERC-20 transfer logs to the same recipient (e.g., partial fills, rebates, or multi-hop output tokens), parseSwap throws:

SyntaxError: Cannot convert 1044719.296572006383948274 to a BigInt
    at BigInt (<anonymous>)
    at t (dist/index.esm.js:1:7714)
    at Array.reduce (<anonymous>)

Root Cause

In the native token output branch, the reduce accumulator stores amount as a formatted decimal string (output of formatUnits), then on the next iteration attempts BigInt(acc.amount) on that decimal string — which throws because BigInt() cannot parse decimal values.

Deminified from index.esm.js:

const outputToken = logs
  .filter(u => u.to.toLowerCase() === senderAddress)
  .reduce((acc, log) => ({
    symbol: log.symbol,
    amount: formatUnits(BigInt(acc.amount) + parseUnits(log.amount, log.decimals), log.decimals),
    //                   ^^^^^^^^^^^^^^^^^^^
    //                   BUG: on 2nd+ iteration, acc.amount is a decimal string
    //                   like "1044719.296572006383948274" from the previous
    //                   formatUnits() call — BigInt() can't parse decimals
    address: log.address,
  }), { symbol: "", amount: "", address: "" });

What happens step by step:

1. 1st iteration: acc.amount is "" → BigInt("") returns 0n (or throws depending on engine). formatUnits(...) produces a decimal string like "1044719.296572006383948274".
2. 2nd iteration: acc.amount is now "1044719.296572006383948274" → BigInt("1044719.296572006383948274") throws SyntaxError.

Suggested Fix

Track the raw bigint value through the accumulator instead of round-tripping through formatUnits:

.reduce((acc, log) => {
  const newRaw = acc.amountRaw + parseUnits(log.amount, log.decimals);
  return {
    symbol: log.symbol,
    amountRaw: newRaw,
    amount: formatUnits(newRaw, log.decimals),
    address: log.address,
  };
}, { symbol: "", amount: "", amountRaw: 0n, address: "" });

Reproduction

This triggers when a swap transaction has multiple ERC-20 transfer logs directed to the sender address — for example partial fills or multi-hop swaps with rebates.

Environment

• @0x/0x-parser version: 2.14.0
• Sentry issue: MATCHA-V3-Q5Y