Copy.ai is purpose-built for short-form marketing copy and includes dedicated Instagram caption templates, making it the most direct fit for this task. You describe your post, pick a tone, and it generates multiple caption options with hashtag suggestions in seconds. ChatGPT is a strong alternative when you want more back-and-forth refinement or a conversational approach to dialing in voice and style. Claude Sonnet edges ahead for nuanced brand tone or longer creative briefs, and Jasper is worth considering if you need a full content marketing workflow beyond just captions.