A head-to-head comparison of Foundry and Ethers.js — pricing, the features that actually differ, and which one fits which use case.
Blazing-fast Rust-based Solidity testing framework with built-in fuzzing and invariant testing
The long-dominant, complete JavaScript library for Ethereum
These are the 9 capabilities where the two tools genuinely diverge — the rest of their feature sets overlap.
| Capability | Foundry | Ethers.js |
|---|---|---|
| Local Node | Yes | No |
| Mainnet Forking | Yes | No |
| Language | Solidity (tests) | JavaScript / TypeScript |
| Test Framework | forge-std | No |
| Plugin Ecosystem | Growing (smaller than Hardhat) | Yes |
| Debugging | Yes | No |
| Deployment System | Forge scripts | No |
| Fuzz Testing | Yes | — |
| Invariant Testing | Yes | — |
| Feature | Foundry | Ethers.js |
|---|---|---|
| Local Node | ✓ | ✗ |
| Mainnet Forking | ✓ | ✗ |
| Language | Solidity (tests) | JavaScript / TypeScript |
| Test Framework | forge-std | ✗ |
| Plugin Ecosystem | Growing (smaller than Hardhat) | ✓ |
| Debugging | ✓ | ✗ |
| Deployment System | Forge scripts | ✗ |
| Open Source | ✓ | ✓ |
| Fuzz Testing | ✓ | — |
| Invariant Testing | ✓ | — |
| Coverage Reporting | — | ✗ |
| Gas Reporting | — | ✗ |
Both tools are free to use — costs come from network or usage fees.
Foundry
Free
Ethers.js
Free
Watch out: Smaller plugin ecosystem than Hardhat — some tooling requires custom solutions.
Watch out: New-project momentum shifted to Viem.
Foundry edges ahead on our editorial score (4.8/5), but these tools aren’t straight substitutes. Pick Foundry when 5–20x faster test execution than hardhat — critical for large suites and audit pipelines matters most to your workflow; pick Ethers.js when long-dominant, battle-tested library is the priority. The deciding factor is usually the trade-off you can least afford — Foundry means accepting that smaller plugin ecosystem than hardhat — some tooling requires custom solutions, while Ethers.js means new-project momentum shifted to viem.
Foundry carries the higher editorial rating (4.8/5 vs 4.6/5), but they solve different problems. Foundry is the stronger pick when you need 5–20x faster test execution than hardhat — critical for large suites and audit pipelines. Ethers.js wins when long-dominant, battle-tested library.
Both tools are free to use, with costs coming from network or usage fees rather than subscriptions.
Foundry's main limitation is that smaller plugin ecosystem than hardhat — some tooling requires custom solutions. For Ethers.js, new-project momentum shifted to viem. Weigh these against how you actually plan to use the tool.
In most cases yes — many teams run both, using each where it's strongest. Since Foundry leads on 5–20x faster test execution than hardhat — critical for large suites and audit pipelines and Ethers.js on long-dominant, battle-tested library, the two are often complementary rather than mutually exclusive.