The big hoo-har about it earlier this year was because something (opencode?) was pretending to be the claude CLI and using the API that way. Anthropic made a fuss, then GPT said "we don't mind people using their own harnesses" and everyone relaxed about it, as long as the harness goes via the claude CLI and doesn't try to cheat.

Yeah there's even an "Agent SDK" from Anthropic that manages Claude CLI sessions for you, and a separate token budget for apps that use it, on top of the subscription tokens.