How much of a jazz composition is defined by the composer, and how much by the musicians?

Question

Do jazz music composers need to write all notes for the every instrument, do the players write themselves a new suitable extravaganza or do they completely improvise the song?

Answer 1

Hi, Jazz musicians usually write the heads (this is a common term for the melody of the song in Jazz). This is often a lead sheet, like the music written in Real Books. Once the head played once or twice they start improvising on the chord changes.