What counts as breaking a fast
Whether protein breaks a fast depends on the goal of the fast. If the aim is strictly zero insulin response and maximal autophagy, then only water keeps the fast technically intact, since any caloric intake provokes some metabolic response. Protein, even in small amounts such as a scoop of whey or a few grams of amino acids, raises insulin and signals through mTOR more than black coffee does, so it interrupts the deepest fasted state.
Relative to carbohydrate, however, a modest protein intake produces a smaller and slower insulin rise, so it sits between plain water and a carbohydrate-containing snack on the fast-breaking spectrum.
Matching the rule to your purpose
If the fast is mainly for calorie control or simplicity, a little protein may matter little. If the specific goal is to maximize autophagy or keep insulin at floor, protein is best saved for the eating window. The honest answer is that it depends on which benefit is being chased, and human autophagy timing data is too limited to set a precise protein cutoff.
This is general educational information, not medical advice. Anyone fasting for a medical reason, such as before a procedure or a fasting blood test, must follow the specific instructions of a qualified professional.