GrammarNaziBot

Add the bot to a group and it checks messages as they arrive, replying with the corrected spelling. Half the point is social — the running joke of a chat that polices its own typos — and half is real: in a working group where posts get copied into documents, the typos stop leaving the chat.

Because it reacts to every message, it belongs in chats that opted into it. In a large or serious group the same behaviour reads as noise, so agree on it first and give an admin the ability to remove it quickly.