Assalaamu alaykum. If I fast a voluntary fast but ate after the Fajr - for example, I knew that the azan had been made but I still had tea till 6 minutes after it and I knew that - is my fast valid? Or should I break it because it might be invalid?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

Your fast is not valid; if a person eats after the Fajr, then his fast is not valid because Allah commanded us to fast from the beginning of the Fajr; Allah says (what means): {And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset.} [Quran 2:187]

Also, the Prophet said, ‘‘Bilaal calls the athaan when it is still night, so eat and drink until Ibn Umm Maktoom calls the athaan." [Al-Bukhaari and Muslim]

Ibn Umm Maktoom used to call the athaan at the break of dawn.

Since your fasting was a voluntary one, then you are not obliged to make up for it, but if you do so, it would be better because it is desirable to make up for a voluntary fast if it was invalidated.

The Fiqh Encyclopedia reads, “The Shaafi'i and Hanbali Schools are of the view that a person who invalidated a voluntary fast is not obliged to make up for it because the ruling of making up the fast depends on the ruling of the fast one wants to make up for, so if fasting on that day is not obligatory, then it is not obligatory to make up for it, but it is desirable to do so regardless of whether he invalidated the voluntary fast with or without an excuse as a way of avoiding the difference of opinion of those who hold that it is obligatory to make up for it.”

Allah knows best.