Write email subject lines from the body
Eight subject lines drawn from what the email actually says, each under 50 characters and labelled with the approach it uses.
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Your prompt
Model-agnostic — works in Claude, ChatGPT, Gemini or any AI chat.
Why this prompt works
- →Stating the character limit as a rule to enforce, not a suggestion, is what keeps every line usable in a real inbox instead of truncating mid-word.
- →Labelling each approach turns the output into a genuine test set instead of eight lines that all lean on the same one trick.
- →Tying every subject line back to what the body actually says guards against the classic failure: urgency or a hook the email doesn't deliver on.
Part by part
- 50 characters or fewer
- A hard limit tied to where subject lines truncate on mobile. Precision matters more than the exact number, since truncation is silent and destroys the ending.
- count the characters and discard any that don't fit
- The enforcement. Models estimate length poorly, so asking for short produces long, and asking them to count improves compliance noticeably.
- no promising content, urgency or a deadline the body doesn't contain
- The honesty constraint, aimed at the three specific overclaims subject lines attract.
- skip the deadline-based subject line rather than inventing one
- False urgency is the most damaging option here — it works once, and it teaches recipients to distrust every later message.
Common mistakes
Write subject lines that get opens.Opens are maximised by urgency and curiosity, both of which are easy to fake. A message opened under a false pretext costs more in unsubscribes than the open was worth.
Write 8 subject lines for this email.Without the length limit, several will be seventy or eighty characters and truncate on a phone — usually mid-word, and usually just before the part that carried the meaning.
Add urgency to make people act.Manufactured urgency is detectable and it degrades every subsequent send. Real deadlines work precisely because the recipient learned the previous ones were real.
Questions
Is 50 characters the right limit?
It is a safe one. Mobile clients truncate somewhere between roughly 30 and 60 depending on device and orientation, so anything important should be inside the first 40 regardless of the total.
Does the preview text matter as much?
Often more. It is the second line the recipient reads and it is frequently left as the first sentence of the email, which is usually a greeting. Setting it deliberately is a bigger win than another subject line variation.
Why does personalisation get its own category?
Because merge-tag personalisation performs differently from genuine relevance, and the two are easy to confuse. A first name in a subject line is a pattern recipients recognise; a subject line about something they actually did is not.
How should I choose between eight options?
Test if you can, and otherwise pick the most specific one. Specificity is the property that most reliably survives contact with a crowded inbox, where a clever line reads as marketing.