It's not clear to me why the employer investing in training has somehow shouldered some additional obligation to the employee.

It's the employer modifying the at-will employment arrangement and attempting to restrict the employee's freedom to choose jobs at will, which should result in a symmetric obligation. The default situation in most U.S. states is at-will employment: either side may terminate the employment relationship at any time, for any or no reason, with any notice they choose to give or none.

If a more durable arrangement is desired, so the employee cannot leave tomorrow with no notice (thereby giving up the normal freedom to work only willingly for employers, and leave when it is no longer willing), then IMO it should be set out in advance and honored by both sides, e.g. by using a fixed-term or minimum-term contract, with penalties for early termination by either side. That way both sides can plan on the stability of the relationship (barring serious misconduct).

For example in my current job, either my employer or myself can terminate the employment with four months notice, after the first six months. This gives us both some guarantee that we will be able to plan for major changes— they will have time to find a replacement if I give notice, and I'll have time to find another job if they give notice.

The employer is not "modifying the at-will arrangement". The employer is doing the same thing employers do with signing bonuses: "here is a benefit you can have, but if you leave before 12 months is up, you have to give it back".

Obviously, training benefits can be abused. "Here is a 'benefit' that is not really a 'benefit' but instead a job requirement for which we have assigned a nearly arbitrary value; if you leave within 12 months, you must pay us this random number back." But as long as "training" has some value assigned by the market with a reference price not controlled by the employer, what's the problem?

Of course, it is all about consideration. I just can't think of an example where training would be enough consideration for a non-compete.

Employers always have to pay for experience. You either pay more for an experienced employee or pay for training.

With an experienced employee you can include actual benefits, like you mentioned, or you can pay to train someone who will take a lower salary.

I'm not suggesting that training should be adequate consideration for a noncompete.

> what's the problem?

It's uncomfortably close to indentured servitude to me to say: you can't work for anyone else until you pay off this up-front investment we made in you (working only for us until you do).

Do you feel the same way about signing bonuses? All of them work that way, I think.

Are you W2 employed in the US with an employment contract that says you can't quit without 4 months notice?

What does the contract specify as a remedy for the company if you quit early?

Regularly employed, but not in the US. I believe the remedy is monetary on both sides; they can't get a court order to literally drag you back into the building and make you work, and you can't get a court order to force them to let you into it, either. I don't know of anyone who's violated such a period and haven't looked in detail into what would happen, so the details are sketchy to me. But I believe there would be some kind of court or arbitration hearing, where the company alleges damages from the disruption due to breach of the employment contract, and the person may be ordered to pay any damages that stem from the too-short notice.