What Happens When a Solver's Swap Fails Onchain?

When a solver's swap fails onchain, the transaction is reverted and your order stays open. No tokens move, and you are charged no fee. A failed settlement is a rejected attempt, not a lost trade.

What is a solver?

A solver is a bot that competes to fill your order on CoW Protocol. You sign an order that describes the trade you want. The solver finds a way to execute it, often by matching it with other orders in the same batch. The solver that offers the best price wins the right to settle the batch.

Your tokens stay in your wallet until a settlement actually executes. The solver can try to fill the order, fail, and try again later. For a step-by-step look at how a pending order gets fixed after a failed attempt, read this cowswap explainer.

What exactly fails onchain?

What fails is a settlement transaction. The solver submits one transaction that contains a batch of swaps, including yours. If that transaction fails, the blockchain rolls back everything inside it. Your order is not part of the chain's state, because it was never executed. A direct cost is the solver's wasted gas.

A failed settlement does not burn your signature or cancel your order. The signed order can be picked up by another solver in a later batch.

Why do solver settlements revert?

Most reverts happen because market conditions changed between the solver's plan and the transaction reaching the chain. The prices the solver relied on are no longer good enough to satisfy your limit price. A solver's transaction can also fail if:

• A liquidity pool it depends on moves against the trade, so the batch no longer breaks even.
• Another order in the same batch becomes impossible to fill, taking the whole settlement down.
• The settlement is mined after its deadline, and the order refuses to execute.
• The solver contract hits a bug or runs out of gas during execution.

When any of those happen, the user is not the loser. The solver wastes time and gas. Your order remains where it was, waiting for a better attempt.

Could a failed swap touch your funds?

No. Funds move only when a settlement succeeds. Because your order is an offchain signature, no one can move your tokens just by trying to fill it. A failed attempt leaves both your wallet and your pending order unchanged.

The practical takeaway: if your order is still within its chosen time window, a failed solver is not a reason to panic. Leave it open and let another solver try. If it expires unfilled, you can simply sign a new one.