Exact-input and exact-output swaps: which amount is fixed?

Choose which side matters

A person spending a fixed 100 units of token A generally cares about the token B amount received. A person who needs exactly 50 B generally cares about how much A must be supplied. Those are different transaction instructions even when an interface initially quotes the same exchange rate.

Read the actual bounds

Suppose spending 100 A is quoted to receive 50 B. A 1% reduction from that quote gives a minimum of 49.5 B. A different instruction might request exactly 50 B while allowing at most 101 A. Neither instruction requires execution at the original quote.

The arithmetic is hypothetical and states the percentage convention explicitly. Interfaces can calculate tolerances differently. Inspect the minimum-output or maximum-input amount that will actually be enforced, along with the recipient and deadline.

Account for the integration

In Uniswap’s example, the exact-output helper temporarily takes the maximum input and refunds the unused portion. A custom integration needs to implement that refund correctly. A nonzero price limit can also cause an exact-input flow to use less than the requested amount, requiring careful refund handling.

These are v3 router examples. Fee-on-transfer tokens, custom hooks and other routers can have different supported behavior; a familiar swap label does not establish compatibility.