When someone comments a keyword on your Instagram post, you can have a direct message sent to them automatically. Meta supports this officially through the Instagram Graph API using a feature called private replies. It is not a browser bot and it does not need your password.
Here is the sequence. Your Instagram Professional account is connected to an approved app through Facebook Login. Meta sends your app a webhook every time a new comment arrives. Your automation checks the comment text against the keywords you configured. If it matches, the app calls the private reply endpoint and Instagram delivers a DM to that commenter from your account.
The important constraint is timing. A private reply can only be sent in response to a recent comment. Older comments fall outside the window and the send fails. This is why re-triggering an automation across an old viral post does not reach everyone who ever commented, and any tool promising otherwise is either ignoring the API or using an unofficial method that risks your account.
There is a second window that matters. Once a person messages you, or you private-reply to them, you have a 24 hour window for standard messaging. After that, follow-ups need to fit Meta's allowed message types. Good automation is designed around these windows rather than pretending they do not exist.
Rate limits are the third constraint. Meta caps how many private replies an account can send per hour. A well built tool paces sends below that ceiling and queues the rest. If a tool advertises unlimited instant sending, ask how it handles pacing, because hitting the ceiling repeatedly is what gets accounts restricted.
The practical takeaway is that comment-to-DM works extremely well for live and recent posts, and it works best when the offer in the caption matches the message the person receives. Treat it as a way to answer intent quickly, not as a way to blast everyone who ever touched your content.