Hardhat 3

This skill covers Hardhat 3 itself. The toolbox layer (clients, contract interaction, ecosystem-specific assertions) lives in companion skills — load whichever matches the project:

• @nomicfoundation/hardhat-toolbox-viem → also load the hardhat-toolbox-viem skill.
• @nomicfoundation/hardhat-toolbox-mocha-ethers → also load the hardhat-toolbox-mocha-ethers skill.

Test organization

Hardhat 3 supports two distinct test layers:

Solidity tests (.t.sol files in contracts/, or any .sol file in test/) are the default choice for unit tests on individual contracts. They run directly in the EVM, compile-check against the real ABI, and have access to cheatcodes for EVM state manipulation. Any public function whose name starts with test is executed as a test case.

TypeScript tests (.ts files in test/) are the right choice when a test requires off-chain orchestration: multi-contract interactions, fixture reuse across a large suite, assertions about gas or ETH balances from the outside, or integration scenarios driven by external state.

Reach for TypeScript only when Solidity isn't enough. Solidity tests cover all contract logic; TypeScript tests cover the end-to-end flow as a user or script would experience it.