What happens inside a swap when a stablecoin starts depegging and you are only half filled

05 September 2026

You get a swap that is neither the asset you wanted nor the stable value you expected. Your order executes partially at one price, and the remaining portion settles at a different, often worse, rate while the system continues trying to complete the trade.

The mechanism is straightforward. When you submit a swap from a volatile asset into a stablecoin, the system does not lock the stablecoin's price. It sends your transaction to a decentralized exchange or aggregator, which quotes a rate based on the current market state. That quote has a short lifetime, typically measured in blocks. If the stablecoin begins to depeg mid-swap, the price feed shifts. The first half of your order might fill before the depeg accelerates. The second half sees a pool with altered reserves. The stablecoin you receive for the remaining portion is worth less in dollar terms than the first half's output.

You do not receive a warning. The swap proceeds until the liquidity it can access at the quoted rates is exhausted, or until slippage limits you set are breached. If you set no slippage tolerance, the system may chase the falling price across multiple pools, buying the depegging stablecoin all the way down. If you set a tight tolerance, the second half of the order may revert entirely, leaving you with a partial fill and the question of what to do next.

The risk is not symmetrical. Swapping into a stablecoin that depegs means you acquire an asset whose reference value is collapsing. Swapping out of a stablecoin that depegs means you escape at whatever discount the market offers. A partial fill in the first direction leaves you holding half your intended position in a sinking asset. A partial fill in the second direction leaves you with half your stablecoins still exposed.

What happens to the unfilled portion depends on the swap's design. Some implementations return the unused input tokens to your wallet. Others hold them in a pending state. The on-chain record shows two transactions: one successful swap at the initial rate, and one failed or partially executed swap at the degraded rate. No central party rebalances the loss. You bear the full spread between what you expected and what you received.

The real problem is not the partial fill itself. It is the time gap. A depeg can happen in minutes. A swap that routes through multiple liquidity pools takes seconds per hop. Each hop re-evaluates the price. If the second pool sees a 2% depeg that the first pool did not, your average fill price drifts downward. You end up with a blended rate that reflects the deterioration, but you never see the deterioration happen.

There is no protection built into standard swap contracts for this scenario. Stablecoin depegs are rare events, and most swap logic treats the quoted stablecoin ratio as a fixed reference. The system does not monitor whether the stablecoin's market price still matches the feed. It only checks whether the pool has enough tokens to complete the trade.

If you want to move between volatile assets and stablecoins, or between stablecoins on different chains, the hub page "Swapping into and out of stablecoins" covers the broader strategy. That page explains how to choose your routing and set your tolerances. This page answers only what happens during the event itself: you get a partial fill at a blended rate, and the system stops caring whether the stablecoin you received is still worth a dollar.

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