Not really sure if all but decided means there is still a chance he stays in the agreement.
There is a "sources" report that Pres. Trump has "all but decided" to end the nuclear agreement with Iran.
They add, it is "unclear how he will withdraw from the agreement".
There seems to be some wiggle room there, but let's face it, his support of the agreement is not exactly enthusiastic (i.e. "This is the worst agreement ever")