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Foundry uses Solidity for tests, runs 10–100× faster than Hardhat, and has built-in fuzz and invariant testing — ideal for new projects. Hardhat uses JavaScript/TypeScript, has a larger ecosystem, and is better for complex deployment scripts — ideal for existing codebases and JS-first teams. Both can be used together.
Framework Comparison
| Factor | Hardhat | Foundry |
|---|---|---|
Language | JavaScript/TypeScript | Solidity (tests in Solidity) |
Test speed | Slower (JS overhead) | 10–100× faster (native Solidity) |
Fuzz testing | Limited | Built-in, powerful |
Invariant testing | Not built-in | Built-in |
Gas reporting | Plugin required | Built-in |
Debugging | console.log in Solidity | Forge console |
Scripting | JavaScript | Solidity scripts |
Deployment | JS deployment scripts | Foundry scripts |
Learning curve | Lower (JS familiar) | Higher (write tests in Solidity) |
Industry adoption | Declining for new projects | Growing standard |
Ecosystem | Large (many plugins) | Growing |
Our Recommendation: Foundry for New Projects
We write all new smart contract projects in Foundry. Reasons:
- Test speed: Foundry runs tests 10–100× faster than Hardhat. At 1,000 test cases (which production contracts should have): 10-minute Hardhat test suite = 6-second Foundry test suite. Faster tests → more test runs → better coverage.
- Fuzz testing built-in: `forge fuzz` runs thousands of random inputs against every test. Finding edge cases before auditors do is cheaper and faster.
- Solidity tests: Writing tests in the same language as the contracts removes context-switching. The test reads like a specification of the contract's behavior.
- Cheatcodes: `vm.prank(address)`, `vm.warp(timestamp)`, `vm.roll(blockNumber)` make testing time-dependent and access-controlled logic trivial.
When Hardhat Still Makes Sense
- Existing Hardhat codebases: Don't migrate working code for its own sake. If the team is productive in Hardhat and the project is ongoing, migration cost exceeds benefit.
- JavaScript-first teams: Teams with strong JS and weak Solidity may find Hardhat's JS testing environment more accessible initially.
- Complex deployment scripts: Hardhat Ignition and Hardhat Deploy have mature deployment orchestration that Foundry's scripting is still catching up to for complex multi-contract deployments.