Hi @HeisenbergTwit -
Unsure if the ! in the Tweet is causing this…
Check relationship of the two users involved (original Tweet author, and user that is replying) if both are publically available or following one another.
The request for GET show/:id can be completed using app-only auth, however the POST statuses/update requires app-user auth.
Also, looking at the requirements for POST statuses/update, JSON format is required and to create a reply and the parameters: in_reply_to_status_id: and the @user must be included for a successful reply.