Practice on a Copy of Mainnet (Foundry + Anvil)

Why this matters

Forking lets you point your toy at mainnet state — every contract, every storage slot, every account balance — and run your protocol against the real world without paying gas or risking funds. This is how protocol engineers get a realistic adversary: real MEV bots watching, real liquidations triggering, real oracle prices moving. A protocol that survives a forked simulation isn't proven, but a protocol that fails one definitely isn't ready.

Demo

Anvil with --fork-url boots a local node mirroring mainnet at a specific block. Foundry tests can deploy your toy into that state and call mainnet contracts (Uniswap, Aave, Chainlink) as if they were local. The integration cost drops to zero.

Try it yourself

1. Install Foundry (curl -L https://foundry.paradigm.xyz | bash && foundryup). Start anvil --fork-url $MAINNET_RPC_URL. Run cast balance vitalik.eth against the fork and verify it returns a real number.
2. Write the ForkedSwapTest above, run it, and confirm the quote is non-zero. Notice you wrote zero deployment scaffolding for Uniswap — that's the whole point.
3. Use --fork-block-number to pin to a specific block. Re-run your test. Results should be deterministic. Now you can write tests that assume specific market conditions.

References

• Foundry Book — Forking
• Anvil docs