Claude is quite bad at following instructions compared to other SOTA models.
As in, you tell it "only answer with a number", then it proceeds to tell you "13, I chose that number because..."
They all are. And once the context has rotted or been poisoned enough, it is unsalvageable.
Claude is now actually one of the better ones at instruction following I daresay.
I think its why its so good; it works on half ass assumptions, poorly written prompts and assumes everything missing.