What happens to a swap when you send Bitcoin with a low transaction fee

05 September 2026

Your swap will stall. The exchange will not see your Bitcoin arrive in time, if at all. The transaction sits unconfirmed in the mempool while the swap timer runs down.

Here is what happens in detail, and why.

The swap timer and confirmation requirement

Every swap on starname.me that involves Bitcoin uses a time lock. You send your Bitcoin to a deposit address generated for that swap. The exchange waits for a set number of on-chain confirmations before it releases the other asset. That number depends on network conditions and the swap amount, but the principle is fixed: the exchange must see your transaction confirmed in a block.

If you set a low transaction fee, miners prioritise transactions that pay more. Your transaction may wait hours or days. The swap timer, typically a few hours, can expire before your transaction confirms.

What the exchange sees

The exchange monitors the Bitcoin network for an unconfirmed transaction to your deposit address. It can detect that you broadcast something. But it will not act on unconfirmed Bitcoin. Too risky. A double-spend could replace that transaction, and the exchange would have no coin. So it waits for confirmations.

If the timer expires and your transaction remains unconfirmed, the swap fails. The timer is not a suggestion; it is a safety mechanism built into the contract. The exchange returns your Bitcoin, but not instantly. It must first wait for your transaction to confirm so it can spend that unspent output. Until then, your Bitcoin is effectively locked.

The low-fee trap

A low fee does not mean your transaction fails immediately. It means it competes poorly. During quiet periods on the Bitcoin network, a very low fee might still confirm within an hour. During congestion, it might never confirm at all. The uncertainty is the problem. Swaps require predictable timing. Low-fee transactions introduce unpredictability that the protocol cannot tolerate.

Can you accelerate it?

If your transaction is stuck in the mempool and the swap timer still runs, you can try Replace-by-Fee (RBF) if your wallet supports it. This allows you to rebroadcast the same transaction with a higher fee. Some wallets also support Child-Pays-for-Parent (CPFP), where you spend the unconfirmed output in a new transaction that pays enough to incentivise miners to include both. Neither is guaranteed. And they require technical steps most users will not have time for during a live swap.

If the timer expires, acceleration does nothing. The swap is dead.

How to avoid this

Use a fee estimator that targets confirmation within the next 2-3 blocks for the swap amount. Overestimating costs you a few dollars. Underestimating can cost you the swap entirely and leave your Bitcoin stranded while you wait for the timer to expire and the exchange to process a refund.

Where this fits

This is one specific way Bitcoin's design affects swaps. The hub page - "Swapping Bitcoin for other assets" - covers the broader picture: why Bitcoin swaps work differently from swaps between faster chains, how deposit addresses are generated fresh per swap, and what makes Bitcoin's proof-of-work consensus a constraint, not a bug. If you are new to this, that page explains the foundation this scenario sits on.

Final note on fees

Bitcoin's fee market is an auction. When you send Bitcoin into a swap, you are not paying for speed in the abstract. You are bidding against every other transaction in the mempool for a limited number of block spaces. Low bids lose. The exchange does not care about your bid; it cares about on-chain finality. If you do not pay the market rate, you are gambling that the mempool will clear before your timer expires. Sometimes it does. Often it does not. The swap outcome is not subjective. It is binary.

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