How to Use SPDX License Identifiers in Your Smart Contracts

Software licenses are essential because they protect the intellectual property of the software creator and dictate how the software can be used, distributed, and modified by others.

Smart contracts are no exception. In fact, Solidity contracts that use compiler version 0.6.8 or above will throw a warning when a license is not specified.

Licenses for smart contracts should use an SPDX license identifier. For example, an MIT license declaration would look like:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;

contract SayHowdy {
    string public greet = "Howdy!";
}

Or, when a license is not required:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.17;

contract SayHowdy {
    string public greet = "Howdy!";
}