Contract not found when forking, but is present on remote network

Question

I have forked polygon mumbai network in my local project so that I can solve the Ethernaut challenges by writing tests (to improve my understanding of ethers.js, testing, and solving the problems etc)

I thought that forking the network would create a clone with exactly the same state etc as that of the original, but I can't seem to confirm this...

I do this with hardhat.config.js:

networks: {
    hardhat: {
      accounts,
      forking: {
        url: "https://polygon-mumbai.g.alchemy.com/v2/<key>",
        blockNumber: 0,
      },
    },
}

And I run the fork locally by running:

npx hardhat node --fork https://polygon-mumbai.g.alchemy.com/v2/<key> --port 7545 --verbose

I have the ethernaut ABI, followed by the ethernaut address, and then my before hook:

const ETHERNAUT_ABI = [
  {
    anonymous: false,
    inputs: [
      {
        indexed: true,
        internalType: "address",
        name: "player",
        type: "address",
      },
      {
        indexed: false,
        internalType: "contract Level",
        name: "level",
        type: "address",
      },
    ],
    name: "LevelCompletedLog",
    type: "event",
    signature:
      "0x9dfdf7e3e630f506a3dfe38cdbe34e196353364235df33e5a3b588488d9a1e78",
  },
  {
    anonymous: false,
    inputs: [
      {
        indexed: true,
        internalType: "address",
        name: "player",
        type: "address",
      },
      {
        indexed: false,
        internalType: "address",
        name: "instance",
        type: "address",
      },
    ],
    name: "LevelInstanceCreatedLog",
    type: "event",
    signature:
      "0x7bf7f1ed7f75e83b76de0ff139966989aff81cb85aac26469c18978d86aac1c2",
  },
  {
    anonymous: false,
    inputs: [
      {
        indexed: true,
        internalType: "address",
        name: "previousOwner",
        type: "address",
      },
      {
        indexed: true,
        internalType: "address",
        name: "newOwner",
        type: "address",
      },
    ],
    name: "OwnershipTransferred",
    type: "event",
    signature:
      "0x8be0079c531659141344cd1fd0a4f28419497f9722a3daafe3b4186f6b6457e0",
  },
  {
    inputs: [
      {
        internalType: "contract Level",
        name: "_level",
        type: "address",
      },
    ],
    name: "createLevelInstance",
    outputs: [],
    stateMutability: "payable",
    type: "function",
    payable: true,
    signature: "0xdfc86b17",
  },
  {
    inputs: [],
    name: "owner",
    outputs: [
      {
        internalType: "address",
        name: "",
        type: "address",
      },
    ],
    stateMutability: "view",
    type: "function",
    constant: true,
    signature: "0x8da5cb5b",
  },
  {
    inputs: [
      {
        internalType: "contract Level",
        name: "_level",
        type: "address",
      },
    ],
    name: "registerLevel",
    outputs: [],
    stateMutability: "nonpayable",
    type: "function",
    signature: "0x202023d4",
  },
  {
    inputs: [],
    name: "renounceOwnership",
    outputs: [],
    stateMutability: "nonpayable",
    type: "function",
    signature: "0x715018a6",
  },
  {
    inputs: [
      {
        internalType: "address payable",
        name: "_instance",
        type: "address",
      },
    ],
    name: "submitLevelInstance",
    outputs: [],
    stateMutability: "nonpayable",
    type: "function",
    signature: "0xc882d7c2",
  },
  {
    inputs: [
      {
        internalType: "address",
        name: "newOwner",
        type: "address",
      },
    ],
    name: "transferOwnership",
    outputs: [],
    stateMutability: "nonpayable",
    type: "function",
    signature: "0xf2fde38b",
  },
];

ETHERNAUT_ADDRESS = `0xe04f955e4Cf9858F8f8d60C09aBf16DF23D4672b`;

before(async () => {
  const ethernaut = await ethers.getContractAt(
    ETHERNAUT_ABI,
    ETHERNAUT_ADDRESS
  );
}

However, I get the error:

0xe04f955e4Cf9858F8f8d60C09aBf16DF23D4672b is not a contract account.

This address most certainly is a contract account, as it is the Ethernaut contract on Mumbai, seen here:

https://mumbai.polygonscan.com/address/0xe04f955e4Cf9858F8f8d60C09aBf16DF23D4672b

Clearly this is a contract account on Mumbai testnet, so why isn't it on my fork of Mumbai testnet?

Answer 1

When you write:

      forking: {
        url: "https://polygon-mumbai.g.alchemy.com/v2/<key>",
        blockNumber: 0,
      },

blockNumber: 0 means you're forking at block number 0 (docs), so of course the contract isn't deployed yet.

I believe removing it should fix your problem:

      forking: {
        url: "https://polygon-mumbai.g.alchemy.com/v2/<key>",
      },