Actually the Old Testament wasn’t “fixed” - there were differences between the Septuagint and the Babylonian in terms of the minor prophets. Everyone agreed with the Pentateuch, but which of the minor prophets and other books were to be added was left until the council fo Jamnia in 70 AD that defined Jewish canon
And yet before one jot or tittle of the so call New Testament was place on plant fibers or animal skins, or chiseled in stone, Christ declared in Mark 13:23 But take ye heed: behold, I have foretold you all things.
It was 'fixed' in the Words of Christ.