This advice is only for a very specific situation. If you can certainly find a better position quickly, but your boss is a decent person who's interested in yo

This advice is only for a very specific situation. If you can certainly find a better position quickly, but your boss is a decent person who's interested in your success, then absolutely tell them that you're thinking about leaving. But that's a rare situation. Basically, it's people who were hired as junior engineers and have leveled up. Years 2-5 of your career are when your position can become mismatched with the broader market. If you're just testing the market, and you might find that your current job is your best available option, don't tell your boss. Similarly, if you're fleeing a bad situation but you need the paychecks for a little while, don't tell your boss.