I am traveling to Paris for 12 days in late September. Do I need to apply for a short stay visa?
What is the nationality of your passport? If USA, you do not need a visa.
What country are you a citizen of?
U.S. citizens don’t need a visa for a 12 day stay for tourism purposes. It is possible that the electronic travel authorization requirement will be in effect in Sept, but the specific start date hasn’t been announced yet. You can read about it here:
https://travel-europe.europa.eu/etias_en
All good info above: if you have a US passport, you don't need a visa. You WILL need ETIAS if that system has been activated by the dates of your trip. Laura provided a link to the official website, and understand that it is NOT a visa. Tourists who currently require a visa to enter France will still need one.
Be sure to keep checking that official website for activity in the coming months, and don't fall for any scam sites which may require large payments.
Information about ETA and ETIAS on the RS website right here.