Mainnet forking with 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.

Prompt your AI

Use these three in order. Each builds on the one before.

1. Basics & terminology

In one paragraph, explain what mainnet forking is and why protocol engineers rely on it.

2. Why it works (the mechanism)

Walk me through what Anvil actually does when I run `--fork-url`. Where does state come from, and what happens when I call a contract method?

3. Advanced — application & what's next

Design a test suite for a protocol that's sensitive to MEV. How would you simulate a sandwich attack against your protocol using a mainnet fork?

References

• Foundry Book — Forking
• Anvil docs