How do you know a swap is atomic and not just a promise from the site

05 September 2026

You know a swap is atomic because the phishing-sites/smart-contract-front-end-cloning/">smart contract or protocol enforces the exchange in a single irreversible step, and neither party can steal funds by backing out. If the swap completes, both sides settle; if it fails, both sides get their original assets back. The site cannot keep your Bitcoin and refuse to send the other asset, because the mechanism itself prevents that outcome.

Atomic swaps rely on hash time-locked contracts (HTLCs). The process works like this: you and the counterparty both deposit your assets into a phishing-sites/smart-contract-front-end-cloning/">smart contract that holds them in escrow. You generate a secret number, hash it, and share only the hash. The contract is programmed so that if you reveal the original secret within a set time, the trade executes. If you do not, both sides can reclaim their deposits after a timeout. This structure means the swap either happens entirely or not at all. There is no intermediate state where one side holds both assets.

To verify that a swap is truly atomic rather than a mere promise, look at what you must do before sending Bitcoin. In a genuine atomic swap, you never send Bitcoin to an address controlled solely by the site. Instead, you send to a contract address that holds your coins until the counterparty fulfills their end. The contract is visible on-chain. You can inspect it. If the site asks you to send Bitcoin to a wallet they control and then "trust them" to send the other asset later, that is not atomic. That is a custodial exchange, and you are relying on their word.

Another test: examine the refund mechanism. In an atomic swap, if the counterparty never claims the other side of the trade, you can reclaim your Bitcoin after a timeout. This refund path must be coded into the contract from the start. If the site offers no clear, on-chain way to get your funds back after a delay, the swap is not atomic.

The technical details vary by chain. On Ethereum and other chains with smart contracts, the HTLC logic lives in solidity code. On Bitcoin, which has limited scripting, atomic swaps use a different approach: the Bitcoin script locks the coins to a hash, and the counterparty must reveal the secret to claim them. The timeout clause is also embedded. You can read the script before sending. If the script contains no hash-lock and no time-lock, it is not a swap contract.

Beware of sites that describe themselves as "instant" or "guaranteed" without showing the contract. They may be running a simple order book where the site holds your Bitcoin until they find a match, then manually forwards it. That is not atomic. That is a promise backed by the site's reputation, which can fail.

The most reliable check is to use a swap that publishes its contract addresses or allows you to verify the HTLC parameters on a block explorer. If the site provides no such transparency, assume the swap is not atomic. The page Swapping Bitcoin for other assets explains the broader context of moving between chains and why this distinction matters for security.

Finally, note that atomic swaps do not require you to trust the site. They require you to trust the code. If the code is open-source, audited, and deployed on a chain you can verify, you have a strong basis for confidence. If the code is hidden or the site refuses to show the contract, you are betting on their honesty. That is not an atomic swap.

Back to phishing