0x Overbuying buyToken when setting slippagePercentage > 0

Question

Based on the documentation here: https://0x.org/docs/api#request-1, I expected the 0xApi to obtain the exact amount of buyToken specified in the request, while applying the "slippageTolerance" to the sellAmount.

However the api / router seems to buy overbuy the buyToken returning roughly (1+slippagePercentage)*buyAmount instead of the actual specified amount.

Did I misunderstand the documentation or is this an error in the api / router ?

To reproduce you can run this test against a local mainnet fork : https://github.com/ckoopmann/0xApiExperiments/blob/main/test/index.ts

Which resulted in the following output for me (note that the api was queried for a fixed buyAmount of 100 sushi tokens with 5% slippage and ended up buying slightly more than 105 tokens:

Answer 1

When using buyAmount in 0xAPI, the buyAmount is guaranteed as a minimum. Not all underlying liquidity sources provide buyExact functionality so buys are converted to sells.

The settlement will revert if boughtAmount < buyAmount. So it is often the case that when using buyAmount the trader buys more than expected.