Do I have to tell someone I'm trans before we meet?
It is a safety decision and a personal one. It is not a moral obligation, whatever anyone tells you.
This question is usually asked as if there is a rule, and there is not. What there is is a set of trade-offs that only you can weigh, because you are the one carrying the outcome either way.
One thing first, because it gets used as a weapon: not volunteering something about your own body or your own history is not deception. You are allowed privacy about yourself. Anyone who frames this as you owing strangers a disclosure has smuggled in a premise you do not have to accept.
The case for saying it early
It filters. People who were never going to be alright with it remove themselves before you have spent an evening, or three months, finding out. That is a real saving.
It also moves the moment of risk to a place you control — a message, from your own home, with the block button in reach — rather than a bar at 10pm or someone's flat. For a lot of people that is the whole argument, and it is a strong one.
And it means you are not managing it during the date, which is a kind of work that spoils evenings.
The case for not
It is nobody's business, and putting it up front makes it the first thing about you, which it may not be to you at all.
If you are stealth, telling someone is a genuine risk with consequences that reach your job, your housing and your family. Weighing that carefully is not cowardice.
Some people would simply rather be met as themselves and decide later. That is a legitimate way to date.
What most people actually do
Something in between: on the profile for the people who want it visible; in messages before meeting for the people who want the filter without the billboard; in person for the people who want to be met first.
A common middle path is to say it before a first meeting but after a real conversation has started — late enough that they know something about you, early enough that you are not alone with them when it happens.
Safety comes before everything else here
Where the reaction could be violent, in person and alone is the worst place for it. If you have any doubt, do it in writing, in advance, from somewhere you are safe.
And the reaction tells you what you needed to know. Someone who is briefly surprised and then interested in you is fine. Someone who becomes angry, or whose interest suddenly narrows to your body, has answered a different question.
Why t4t changes the maths
In a t4t space the question mostly stops existing, because it is not news. That is not the only reason those spaces exist, but it is a large part of why people who find them stay.