Did you have a non-disclosure agreement with your former employer?

Did he literally say "I want you to build the same product", or did he say "I want you to build XYZ" which happens to be the same as your previous company's product? There is a vast difference between the two.

@Flater he asked me to build XYZ, and then how long did he take at my previous job and how long would it take me to build it again. The prospect did not ask for this product, my boss approached him, talked to him about this and said that he has someone in the team that did it before

If this knowledge is so valuable, why not go into business for yourself?

@alephzero I agree. If you can whip up something equivalent just because you're that good then that's just a skill you bring to the table. Go you!

@AndreiROM Because depending on where you live, setting up a business for yourself, finding the relevant customers, getting everything in order and handling the administration is simply very frustrating to do.

Are the machine learning models specifically the things that your new boss wants you to duplicate? It seems that they'd be the crux of the ethical problem right there. If you are building something that uses them, you are legally and ethically at risk. If the thing that you're building uses different models, then it is a different thing.

@PatriciaShanahan I only have what I wrote in my edit, plus a sentence about patents being protected, but I don’t think they filed one

Where are you located? In many jurisdictions there is no need to include a "non-disclosure" agreement directly in contracts because there are employment laws that already specify the duty of employees in not selling trade secrets.

@AndreiROM 1. I don’t want to build a company, 2. I am not capable of recreating everything from scratch, only the part I was working on 3. I think it is morally wrong to do that to my previous employer.

@BenBarden my boss does not understand what a machine learning model is, but he wants me to do it very quickly, ie not thinking and just use the same strategy as in my previous company

How does your new boss expect you to recreate the models without your old employer's data to train it on? If you don't need proprietary data to train it, I don't think there's a conflict necessarily.

Also just a matter of semantics, but due to the inherently random nature of model development the odds that you'd recreate the exact model as your old employer are astronomically small, even given identical training data, model design, and initialized values.

So... basically, your boss is looking at you and saying "I don't know what it involves, but I want you to steal all of your old employer's stuff." The opportunity to do this was, apparently, one of the first things he thought when you walked in the door. How badly do you need this job? Legal/ethical difficulties seem like they might be an ongoing hazard with this guy.

@FooTheBar no, we need a third party data provider

Although I understand your legal worries, I don't get the ethical concern (other than if it is illegal or dishonest). Using the ideas in the new business doesn't stop the old one from using them.

@BenBarden I don’t really need this job, I could easily find something else, except it’s my first job here and I would have preferred to stay, say one year at least, and leave with a good reference.

If the product your former company is so vital and important, they should not have let you go, or let you go with an NDA. You are currently employed for your knowledge and experience, if you weren't hired for that, what would you be there for?

@anonymous1 So you get to decide if you want to continue working for a company that is asking you to do something you believe is morally wrong. Only you can decide what your morals are worth.

@anonymous1 I think you used to work at the company that I currently work at lol