If you already use Claude, you've probably pasted a message into it at some point and asked, "how do I answer this?" It does a good job. The friction is everything around it: copying the thread over, explaining who the person is, then copying the draft back.
That's the gap tools like Grais are built for. Grais is a browser extension that sits on WhatsApp Web, Telegram Web, Instagram, LinkedIn and Tinder, reads the thread that's already open, and drafts the reply there. It also keeps notes on each person, so the next draft knows what you promised last time.
A rough way to split it:
Plenty of people will use both. Grais is invitation-only right now, and its iOS app is still in TestFlight, so Claude is the easier thing to start with today.
The side-by-side, with sources, is here: how Grais compares with Claude for replying to messages.