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Added during sampling by adding a bias to token probabilities in places where several tokens are equally likely.

Reading their technical report https://cdn.openai.com/pdf/e9508624-d767-41b6-a26d-e34ca798a... it's a bit more complicated than that, but yes, the watermark is added during sampling.

And that's part of inference engine or model?

It doesn't involve changing the model weights, if that's what you mean.

Yes, thank you