I took 5g this morning and totally didn’t consider that it might break my fast.
Depends on how strict you are. Amino acids do have a small number of calories. My general rule, if you’re fasting for certain physiological states, perhaps for a specific medical or religious reason, stick to just water - though some folks will still allow coffee and tea. If you’re fasting for wellness or as a caloric reduction, the number that gets thrown around a lot is 50 calories or less will usually not spike any meaningful response from the body. So any substance, especially if it helps in your journey or gets you to your next meal without eating, is worth it.
Though do watch out if your supplement has any added sugar and such.