Threads character counter
500 characters, the shortest of the Meta family
A caption written for Instagram will not fit here, which is exactly why cross posting the same text to both rarely works. Threads wants the short version.
Long posts get split, and the split is not yours
Going over does not fail, it becomes a thread. Where it breaks is decided for you, and it rarely breaks where you would have.
Echoia does the rest: publishing to ten networks, every comment and message in one inbox. Free for three accounts.