Run FHEVM tests in Hardhat

In this section, you'll find everything you need to test your FHEVM smart contracts in your Hardhat project.

FHEVM Runtime Modes

The FHEVM Hardhat plugin provides three FHEVM runtime modes tailored for different stages of contract development and testing. Each mode offers a trade-off between speed, encryption, and persistence.

1. The Hardhat (In-Memory) default network: 🧪 Uses mock encryption. Ideal for regular tests, CI test coverage, and fast feedback during early contract development. No real encryption is used.

2. The Hardhat Node (Local Server) network: 🧪 Uses mock encryption. Ideal when you need persistent state - for example, when testing frontend interactions, simulating user flows, or validating deployments in a realistic local environment. Still uses mock encryption.

3. The Sepolia Testnet network: 🔐 Uses real encryption. Use this mode once your contract logic is stable and validated locally. This is the only mode that runs on the full FHEVM stack with real encrypted values. It simulates real-world production conditions but is slower and requires Sepolia ETH.

Summary

Mode	Encryption	Persistent	Chain	Speed	Usage
Hardhat (default)	🧪 Mock	❌ No	In-Memory	⚡⚡ Very Fast	Fast local testing and coverage
Hardhat Node	🧪 Mock	✅ Yes	Server	⚡ Fast	Frontend integration and local persistent testing
Sepolia Testnet	🔐 Real Encryption	✅ Yes	Server	🐢 Slow	Full-stack validation with real encrypted data

The FHEVM Hardhat Template

To demonstrate the three available testing modes, we'll use the fhevm-hardhat-template, which comes with the FHEVM Hardhat Plugin pre-installed, a basic FHECounter smart contract, and ready-to-use tasks for interacting with a deployed instance of this contract.

Run on Hardhat (default)

To run your tests in-memory using FHEVM mock values, simply run the following:

npx hardhat test --network hardhat

Run on Hardhat Node

You can also run your tests against a local Hardhat node, allowing you to deploy contract instances and interact with them in a persistent environment.

1

Launch the Hardhat Node server:

• Open a new terminal window.

• From the root project directory, run the following:

npx hardhat node

2

Run your test suite (optional):

From the root project directory:

npx hardhat test --network localhost

3

Deploy the FHECounter smart contract on Hardhat Node

From the root project directory:

npx hardhat deploy --network localhost

Check the deployed contract FHEVM configuration:

npx hardhat fhevm check-fhevm-compatibility --network localhost --address <deployed contract address>

4

Interact with the deployed FHECounter smart contract

From the root project directory:

1. Decrypt the current counter value:

npx hardhat --network localhost task:decrypt-count

1. Increment the counter by 1:

npx hardhat --network localhost task:increment --value 1

1. Decrypt the new counter value:

npx hardhat --network localhost task:decrypt-count

Run on Sepolia Ethereum Testnet

To test your FHEVM smart contract using real encrypted values, you can run your tests on the Sepolia Testnet.

1

Rebuild the project for Sepolia

From the root project directory:

npx hardhat clean
npx hardhat compile --network sepolia

2

Deploy the FHECounter smart contract on Sepolia

npx hardhat deploy --network sepolia

3

Check the deployed FHECounter contract FHEVM configuration

From the root project directory:

npx hardhat fhevm check-fhevm-compatibility --network sepolia --address <deployed contract address>

If an internal exception is raised, it likely means the contract was not properly compiled for the Sepolia network.

4

Interact with the deployed FHECounter contract

From the root project directory:

1. Decrypt the current counter value (⏳ wait...):

npx hardhat --network sepolia task:decrypt-count

1. Increment the counter by 1 (⏳ wait...):

npx hardhat --network sepolia task:increment --value 1

1. Decrypt the new counter value (⏳ wait...):

npx hardhat --network sepolia task:decrypt-count