Im trying to create a dynamic nft by following the tutorial by alchemy, the NFT should update when performUpkeep is called which should get a random number using VRF v2 and change the NFT token URI.
However, when performUpkeep is called the VRF fails to generate a random number and gives the reason Callback gas limit set too low

To reproduce the problem you can use remix with this code:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/utils/Counters.sol";
import "@chainlink/contracts/src/v0.8/VRFConsumerBase.sol";

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

import "@chainlink/contracts/src/v0.8/KeeperCompatible.sol";

import "@chainlink/contracts/src/v0.8/interfaces/VRFCoordinatorV2Interface.sol";
import "@chainlink/contracts/src/v0.8/VRFConsumerBaseV2.sol";

contract BullBear is VRFConsumerBaseV2, ERC721, ERC721Enumerable, ERC721URIStorage, Ownable, KeeperCompatibleInterface {
    using Counters for Counters.Counter;
    VRFCoordinatorV2Interface COORDINATOR;

    Counters.Counter private _tokenIdCounter;
    uint public /*immutable*/ interval;
    uint public lastTimeStamp;

    AggregatorV3Interface public priceFeed;
    int256 public currentPrice;

    string[] bullUrisIpfs = [

    string[] bearUrisIpfs = [


    enum MarketTrend{BULL, BEAR} // Create Enum
    MarketTrend public currentMarketTrend = MarketTrend.BULL; 

    uint64 s_subscriptionId;
    address vrfCoordinator = 0x6168499c0cFfCaCD319c818142124B7A15E857ab;
    bytes32 keyHash = 0xd89b2bf150e3b9e13446986e571fb9cab24b13cea0a43ea20a6049a85cc807cc;
    uint32 callbackGasLimit = 100000;
    uint16 requestConfirmations = 3;
    uint32 numWords =  2;

    uint256[] public s_randomWords;
    uint256 public s_requestId;
    address s_owner;

    event TokensUpdated(string marketTrend);

    constructor(uint256 updateInterval, address _priceFeed, uint64 subscriptionId) 
        ERC721("Bull&Bear", "BBTK")
        interval = updateInterval;
        lastTimeStamp = block.timestamp;

        priceFeed = AggregatorV3Interface(_priceFeed);
        currentPrice = getLatestPrice();  
        COORDINATOR = VRFCoordinatorV2Interface(vrfCoordinator);
        s_subscriptionId = subscriptionId;

    function requestRandomness() internal {
        s_requestId = COORDINATOR.requestRandomWords(

    function fulfillRandomWords(uint256, /* requestId */ uint256[] memory randomWords) internal override {
        s_randomWords = randomWords;

        string[] memory urisForTrend = currentMarketTrend == MarketTrend.BULL ? bullUrisIpfs : bearUrisIpfs;
        uint256 index = randomWords[0] % urisForTrend.length;

        for (uint i = 0; i < _tokenIdCounter.current(); i++) {
            _setTokenURI(i, urisForTrend[index]);
        string memory trend = currentMarketTrend == MarketTrend.BULL ? "bullish" : "bearish";
        emit TokensUpdated(trend);

    function safeMint(address to) public onlyOwner {
        uint256 tokenId = _tokenIdCounter.current();
        _safeMint(to, tokenId);

        string memory defaultUri = bullUrisIpfs[0];
        _setTokenURI(tokenId, defaultUri);

    function checkUpkeep(bytes calldata /*checkData*/) external view override returns(bool upkeepNeeded, bytes memory /*performData*/) {
        upkeepNeeded = (block.timestamp - lastTimeStamp) > interval;

    function performUpkeep(bytes calldata /*perfromData*/) external override {
        if ((block.timestamp - lastTimeStamp) > interval) {
            lastTimeStamp = block.timestamp;
            int latestPrice = getLatestPrice();

            if(latestPrice == currentPrice) {
            if(latestPrice < currentPrice) {
                // bear
                currentMarketTrend = MarketTrend.BEAR;
            } else {
                // bull
                currentMarketTrend = MarketTrend.BULL;
            // Call function for randomness

            currentPrice = latestPrice;

    function getLatestPrice() public view returns (int256) {
        (,int price,,,)=priceFeed.latestRoundData();

        return price;

    function setInterval(uint256 newInterval) public onlyOwner {
        interval = newInterval;

    function setPriceFeed(address newFeed) public onlyOwner {
        priceFeed = AggregatorV3Interface(newFeed);

    function compareStrings(string memory a, string memory b) internal pure returns (bool) {
        return (keccak256(abi.encodePacked(a)) == keccak256(abi.encodePacked(b)));

    // The following functions are overrides required by Solidity.

    function _beforeTokenTransfer(address from, address to, uint256 tokenId)
        override(ERC721, ERC721Enumerable)
        super._beforeTokenTransfer(from, to, tokenId);

    function _burn(uint256 tokenId) internal override(ERC721, ERC721URIStorage) {

    function tokenURI(uint256 tokenId)
        override(ERC721, ERC721URIStorage)
        returns (string memory)
        return super.tokenURI(tokenId);

    function supportsInterface(bytes4 interfaceId)
        override(ERC721, ERC721Enumerable)
        returns (bool)
        return super.supportsInterface(interfaceId);
  1. Deploy on Injected Provider - Metamask Rinkeby fuacet
  2. Deploy with inputs 1, "0xECe365B379E1dD183B20fc5f022230C044d51404", and subscription id for chainlink VRF subscription.
  3. Once deployed i added my contract as consumer to chainlink VRF subscription and registered with Keepers and fund with link.
  4. safeMint to my address
  5. performUpKeep (input value of [])

Now if you check s_randomWords with input 1, a random number should return but instead i get the error Error: Returned error: execution reverted

And a snippet of [etherscan] of the VRF call:

VRF Failed Etherscan transaction Etherscan logs


1 Answer 1


You should increase the uint32 callbackGasLimit storage variable, which is currently set to 100000, to a higher value.

  • Thanks i gave this a go and no success, same issue
    – abdulqgg
    Aug 30, 2022 at 16:54

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