A fast swap is a token exchange on Ethereum that confirms in roughly 200 milliseconds — before the block that includes it has even been produced.

Standard Ethereum swaps work like this: you approve a token, submit a transaction, and wait for a block. That block takes 12 seconds. During that window, your transaction sits in a mempool, visible to searchers who may sandwich it, and you have no guarantee about when or where it will land.

Fast swaps eliminate the wait and the uncertainty.

How a fast swap works

1. Connect your wallet to Fast Protocol — any Ethereum wallet works
2. Select your tokens and amount — the interface shows you the expected output including mev rewards
3. Sign a Permit2 approval — a single signature authorizes the swap with no separate approval transaction and no extra gas
4. Your swap routes through FAST RPC to the mev-commit builder auction
5. A builder commits to inclusion — you receive confirmation in ~200ms
6. The swap settles on-chain in the next Ethereum block

From your perspective, the swap feels instant. You click, you sign, you see confirmation.

Permit2: no approval transactions

Traditional ERC-20 swaps require two transactions: an approval transaction (which costs gas and takes a full block) followed by the actual swap. Permit2 collapses this into a single off-chain signature.

How Permit2 works for swaps

• You sign a typed message that authorizes the exact token amount for this specific swap
• No on-chain approval transaction needed
• No unlimited allowance sitting in a smart contract
• One signature, one swap

This is both faster and more secure than the traditional approve-then-swap flow.

mev rewards and Miles

Many token swaps on Ethereum generate mev — the arbitrage and ordering opportunities created by your trade's price impact. On most platforms, searchers capture this value entirely.

On Fast Protocol, builders compete in an auction for the right to include your swap. This competition forces them to return value to you. At least 90% of the mev your swap generates comes back as an improved execution price. Swaps that generate mev also earn Miles.

Not every swap generates mev. Factors like pair volatility, liquidity depth, swap size, and market conditions determine whether extractable mev exists. A stablecoin swap in deep liquidity during calm markets may generate no mev — and therefore no mev rewards or Miles.

The earning is automatic when it occurs. You don't claim anything, stake anything, or interact with any additional contracts. The auction mechanism handles redistribution at the protocol level.

What tokens can I swap?

Fast Protocol supports swaps between major Ethereum tokens including ETH, WETH, USDC, USDT, DAI, and other liquid ERC-20 tokens. The available token list is displayed in the swap interface.

Swaps route through existing Ethereum liquidity — the protocol doesn't operate its own AMM or liquidity pool. Your trade executes against the deepest available liquidity on mainnet, with the builder optimizing routing as part of the auction.

Slippage and execution

Slippage is the maximum price movement you accept between quote and execution. If the actual output falls below your tolerance, the swap reverts instead of filling at a worse price.

Because fast swaps confirm in ~200ms, your window of price exposure is compressed from a full 12+ second block down to sub-second — so the tolerance you need is typically much smaller than on a standard DEX. Fast Protocol leans into that with an auto mode that keeps defaults tight.

Auto mode (default)

Auto mode picks the smallest tolerance that still lets the swap route reliably:

• 0.5% when you're selling ETH
• 1% when you're selling an ERC-20 token (the permit path carries slightly more routing overhead)

If the router reports that the route can't actually deliver within the base tolerance — for example, because gas has spiked or the pair is temporarily thin — auto mode bumps your tolerance to just above the observed routing shortfall, plus a small buffer. The buffer is there so a subsequent quote refresh reporting slightly higher shortfall doesn't immediately re-block you.

Concrete example: if the route would deliver 2.7% less than the quote, auto sets your slippage to 3.2% (2.7% rounded up + 0.5% buffer). If it would deliver 7.1% less, auto sets 7.6%. Auto will scale up to the 50% ceiling if that's what the route actually needs — the design goal is that auto should unblock the swap, not strand you at a static value that wasn't enough.

When an auto-bump happens, the confirmation screen shows a note: "Your slippage has been auto-adjusted to cover gas costs."

Custom mode

For volatile pairs, large trades, or tokens with transfer taxes, you can switch to Custom and set any value from the path floor (0.5% or 1%) up to 50% — the same ceiling Uniswap uses.

The >5% warning

Whenever the effective slippage — auto-bumped or custom — crosses 5%, an amber banner opens in the settings popover:

Slippage above 5% is unusual. You will earn more miles, but will likely receive less tokens.

That tradeoff is architectural, not advisory. On Fast Protocol, the settlement contract pays you exactly your slippage floor (the minimum you signed for) and retains everything above as surplus — which becomes your Miles. So a wider slippage means:

• Your floor (the tokens you actually receive) drops proportionally
• The retained surplus grows by the same amount
• Your Miles balance grows in lockstep with that surplus

At 0.5% slippage on a 1 ETH-equivalent trade, your floor is 0.995 of quote and any extra room becomes a small Miles credit. At 50% slippage on the same trade, your floor is 0.5 of quote — half the trade value flows through as Miles instead of tokens. That's mechanically what the contract does; it's not an estimate or a probability.

For ordinary swaps, stick with auto at its base tier — the goal of normal trading is to receive tokens, not maximize Miles. Wider slippage is for cases where you specifically prefer Miles to tokens, or where market conditions require it.

Deadline

The swap deadline (default 30 minutes) controls how long the signed order remains valid. If the transaction isn't included within that window, it expires instead of executing at a stale price. Because fast swaps confirm in ~200ms, the deadline is effectively just a safety net for network-level failures.