I think I need to start prompt injecting all my correspondence.

While there are laws about minimum time in some states; the state is probably happy to enforce whatever extra time you grant. So if you tell your employees 20 they're probably going to be happy to hold you to 20.

Many states and many companies are “at will” employment.

As long as the termination isn’t for protected reasons you may not have much in the way of a civil law suit.

Ask me about the time I was “fired” for taking a vacation that my boss approved.

The day I returned, I was called into a meeting with HR and my boss, and berated for going on vacation without informing my boss (who again, approved my PTO), nor informing my team (a categorical lie). Later, my position was eliminated.

What you're describing isn't a plausible situation. In the real world, conversations with real people have to happen when you take so much time off or have such a life changing event (especially with benefits implications). Obvious mistakes aren't generally legally binding, either. You can argue what isn't or isn't an obvious mistake, sure, but if you signed a contract that says 8 weeks, your boss tells you 8 weeks, there's posted documentation that says 8 weeks, etc., then saying you legitimately thought you had 20 weeks doesn't hold water.

If a company intends an HR bot to be relied on by employees, then its words should be legally binding. If the company doesn't intend for employees to rely on the bot, then it shouldn't exist, because nobody can make any plans based on its responses.

In most U.S. states your employment agreement is on an at-will basis and outside dismissal or discrimination for a set of protected reasons, you’re free to leave employment at any time, and your employer is free to let you go at any time.

Very few verbal interactions create a “binding commitment” for a company. All might be admissible if you brought suit, but “he said; she said; they said” is often more difficult with the passage of time (people are forgetful) and worse than written interactions, and just because you may be allowed to present them as evidence in your claim, that does not make anything a slam dunk legally.

A lot of what you will formally get upon receiving i.e. an offer of employment will be on company headed paper, signed by a person with authority (this is often “Delegated Authority”, that Role can do that Action is written down in Company Policies or a Company Handbook, which itself is reviewed and approved at very senior levels). That’s more “binding” and if you signed and returned, rejected other offers, then it fell through, you might see success in making a claim for losses and injury because you reasonably believed there would be a job and compensation. Even there it is going to say something to the effect of “this is not an employment contract, your status is at-will”, which significantly limits the liabilities of the company in most U.S. states.

So, short of the person you are talking to being a C-level executive or Company Officer, no, most of what you hear verbally isn’t “legally binding”.

Well, it was worse than useless as described in the premise by someone else as spitting out verifiably wrong information.

If a company intends an HR bot to be relied on by employees, then its words should be legally binding.

No company would would intend this, and it would be fun to see this "legally binding" status tested, because as my previous post and sibling comment lays out, it would be pretty hard to construct a plausible situation where a reasonable person believes it to be.

Like you pointed out, by preponderance of evidence, you could show that the employee should've reasonably understood they had "X" amount of days since its documented in so many different places and if one place said it was "XY" and employee decided to follow XY instead of X, they're fine since it was "legal binding". However, that would most likely not hold up in court because you can't use ignorance as a defense and say you the only vacation amount you knew of was what the BOT told you.

I think any employment attorney would inform their client this is what would happen and probably wouldn't take the case since the likelihood of winning would be so low.