How to correctly ABI-decode multicall results (ERC-20 decimals + symbol) in tests?

Question

I’m trying to understand ABI decoding of multicall RPC calls so I can create stubbed tests using WireMock for my Rust EVM application.

The multicall function signature is:

function aggregate(Call[] calldata calls) external payable 
    returns (uint256 blockNumber, bytes[] memory returnData);

The two ERC-20 functions I’m calling inside it are:

function symbol() public view returns (string);
function decimals() public view returns (uint8);

What I Tried

I attempted to manually construct the encoded response.

For three tokens I tried this:

0x
1 -  000000000000000000000000000000000000000000000000000000000378cc49 // block number
2 -  0000000000000000000000000000000000000000000000000000000000000040 // Offset to start of array
3 -  0000000000000000000000000000000000000000000000000000000000000006 // Length of array
4 -  0000000000000000000000000000000000000000000000000000000000000080 // Offset to string 1
5 -  00000000000000000000000000000000000000000000000000000000000000e0 // Offset to string 2
6 -  0000000000000000000000000000000000000000000000000000000000000140 // Offset to string 3
7 -  0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 1
8 -  0000000000000000000000000000000000000000000000000000000000000004 // Length of string 1
9 -  57424e4200000000000000000000000000000000000000000000000000000000 // String contents
10 - 0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 
11 - 0000000000000000000000000000000000000000000000000000000000000004 // Length of string
12 - 43616b6500000000000000000000000000000000000000000000000000000000 // String contents
13 - 0000000000000000000000000000000000000000000000000000000000000012 // Decimals for coin 
14 - 0000000000000000000000000000000000000000000000000000000000000004 // Length of string
15 - 5553444300000000000000000000000000000000000000000000000000000000 // String contents

Then this failed so I tried to do this for a single return:


"0x
000000000000000000000000000000000000000000000000000000000378cc49 // Block number
0000000000000000000000000000000000000000000000000000000000000040 // Offset to array
0000000000000000000000000000000000000000000000000000000000000002 // Length of array
0000000000000000000000000000000000000000000000000000000000000040 // Offset to string
0000000000000000000000000000000000000000000000000000000000000012 // Decimals
0000000000000000000000000000000000000000000000000000000000000004 // String Length
57424e4200000000000000000000000000000000000000000000000000000000" // String contents

But when I run these through WireMock and hit them from Rust, I get:

type check failed for "offset (usize)" with data: 0000000000000000000000000040000000000000000000000000000000000000

What Actually Happens

When I make a real multicall, the response looks like this:

aggregateReturn { blockNumber: 58256789, returnData: [ "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 57424e4200000000000000000000000000000000000000000000000000000000", "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 43616b6500000000000000000000000000000000000000000000000000000000", "0x0000000000000000000000000000000000000000000000000000000000000012", "0x0000000000000000000000000000000000000000000000000000000000000020 0000000000000000000000000000000000000000000000000000000000000004 5553444300000000000000000000000000000000000000000000000000000000" ] }

My Question

How should I correctly construct a stubbed multicall response for testing (e.g. for 3 tokens with decimals = 18 and symbols "BNB", "Cake", "USDC")?

Thanks in advance

Edit: I tried to follow this as a tutorial so really can't see what I am missing:

ABI examples

TokenDeployer1245 · Accepted Answer · 2025-08-28 20:10:59Z

0


import { encodeAbiParameters, decodeAbiParameters, parseAbiParameters } from 'viem'

const blockNumber = 58256789n
const returnData = [
  encodeAbiParameters(parseAbiParameters('string'), ['BNB']),
  encodeAbiParameters(parseAbiParameters('uint8'), [18]),
  encodeAbiParameters(parseAbiParameters('string'), ['Cake']),
  encodeAbiParameters(parseAbiParameters('uint8'), [18]),
  encodeAbiParameters(parseAbiParameters('string'), ['USDC']),
  encodeAbiParameters(parseAbiParameters('uint8'), [18])
]

const encoded = encodeAbiParameters(
  parseAbiParameters('uint256, bytes[]'), 
  [blockNumber, returnData]
)

console.log('Encoded for WireMock:', encoded)

const [decodedBlock, decodedData] = decodeAbiParameters(
  parseAbiParameters('uint256, bytes[]'),
  encoded
)

console.log('Block:', decodedBlock)

// Decode each return value
decodedData.forEach((data, i) => {
  if (i % 2 === 0) {
    const [symbol] = decodeAbiParameters(parseAbiParameters('string'), data)
    console.log(`Token ${i/2 + 1} symbol:`, symbol)
  } else {
    const [decimals] = decodeAbiParameters(parseAbiParameters('uint8'), data)
    console.log(`Token ${Math.floor(i/2) + 1} decimals:`, decimals)
  }
})

You can try this script with viem library:

https://viem.sh/docs/getting-started

edited Aug 28 at 20:10

answered Aug 26 at 10:56

TokenDeployer1245

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Community Aug 26 at 11:05

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