[LaundroMat]

Seconded. T&M leads to unproductive discussions, like when you had to try out different libraries to get the job done. The client can easily ask why you didn't know library X was the best candidate before you spent time they pay for to investigate.

[chiefalchemist]

Well, if it were fixed price you'd eat that discovery, correct? T&M doesn't stop you from not billing for something.

Again, the question was: There are too many unknowns. The answer is T&M. Btw, they can be combined. Fixed price on the knowns, and T&M on the unknowns.

Put it this way, I've seen 5x more fixed price projects go sideways than T&M. That's how bespoke work works best.