Subject lines and preheaders

Blank means the model writes them

A subject line is the headline on a newspaper rack, and the model has read the whole story before it writes one. That is why its line usually beats the one you would guess before the email even exists.

Leave Subject line and Preheader empty and the model writes both straight from the body, so the two agree with the email itself. The subject comes back at 42 characters or under, which is what survives truncation on Gmail and Apple Mail on a phone. The preheader comes back under 100 characters and extends the subject instead of repeating it, so the preview line actually earns its space.

On this written path, a few patterns are banned by design: ALL CAPS, repeated exclamation marks, and the words Newsletter and Update. Those are the tells of bulk mail, so the generator will not produce them.

Typed means locked

Anything you type into Subject line or Preheader reaches the model as a hard constraint marked do not rewrite, and it comes back word for word. Use this when legal, a founder, or a brand guide has already settled the line, or when you are matching a subject across a set of sends. The model then builds the rest of the email around your exact words rather than trying to improve on them.

A good default

For most emails, leave both blank on the first pass and read what the model writes. It has the entire body in front of it, so its subject tends to be more specific than anything written before the email took shape. If you do not like it, type your own and run again. Locking a weak subject early is the most common way to end up with a worse email.