Anthropic's subscription is the one that has historically been tough to use with other harnesses, where they have (had?) technical safeguards to ensure your subscription usage was limited to Claude Code CLI, with third party harnesses often implementing workarounds or faking it to use your Claude.ai subscription. Nowadays Jetbrains Air does officially allow using your Claude.ai subscription.

But OpenAI's subscription has seemingly always been available to use in other harnesses without trouble.