If you received an offer for a new job and asked your current boss for a raise to match the new offer, would you tell him/her the real amount or would you inflate it by ~5-10% (or any other arbitrary amount)?

Would that be considered negotiating in bad faith?

I'm not in this position right now (unfortunately), I'm just genuinely curious about what you all would do in this situation.

I feel like it wouldn't technically be considered a bad faith negotiation. But would you be willing to show your current boss your other offer letter?

Usually proof is required, so yea... probably not.

But your gamble like any counter offer, they'll either match it or bid you farewell.

I don't think one should show the offer letter of another company to your current one.
I know they ask for proof, but offer letters are generally considered confidential, between the offering company and the candidate.
I also know that no one is going to pursue this legally, but, in the off chance that someone does, they will have a case.

About the negotiation, quote the amount, but don't show any letters. Cite confidentiality, because it's true. Tell them that they wouldn't want their offer letters strewn around too. That should shut them up.

It is definitely not bad faith to quote more. You should know your true worth, and also what the companies values you at.
In short, learn to assess yourself. Your knowledge of self worth is a very important tool in your arsenal.

You must have the will to go to other company if things go sooth.

Cards on the table

"Been offered x"

"We'll match it"

"I need x + 10% to stay"

One really should only pull this move out of desperation or spite.

You’re turning down the new job and poisoning the well at the old one.

Only time I did this was because I hated my boss. Made that bitch give me a 16k raise then still took the other job.

Never never never take a counter offer. Always go on to the new job. In most cases they company will just find someone to do your job at your old rate, and then replace you.

Note I’ve had a few situations where taking the counter offer seemed acceptable, but usually those were talking about engineers who were entrenched in the company in some way, or where the offer vs counter offer had more than salary and benefits in question (long distances from family, visa things, immigration). This doesn’t sound like your case.

To get higher salary in tech, its norm to move to other companies with a bump, and better for learning, building network

Listen to @jeeper. Most of the time taking the counter offer is the wrong answer.

This happened to me last year. I took a counter, a month later the company re-shuffled and I
took the new job.