Writing email subject lines with AI is a Skill job you finish in one sitting, about fifteen to twenty minutes per email. You paste the finished email into ChatGPT or Claude, it returns ten subject lines with matching preview text, and you pick two to test in your send tool, or one if your list is small. The AI writes the options. Your send tool does the testing, and you make the call. Do it once for the email you are about to send today, then save the prompt so the next send takes five minutes.
- Shape
- Skill
- Sitting
- one sitting
The shape: Skill
A Skill job is one prompt, one answer, and your judgment on top. There is no system to build and nothing to connect to the AI. You bring it the email, it brings you options, and the decision stays yours.
Subject lines fit this shape because the hard part is range, not effort. Most people write one subject line, usually the first one that comes to mind, which is often a label for the email ("October update") instead of a reason to open it. Ten options side by side show you angles you would not have reached alone: a question, a plain statement of the news, a line that names the reader's problem, a line that names the one thing inside. Seeing them together also makes the weak ones obvious.
The other half of the shape is that the AI cannot tell you which line will win. It has never seen your list, and no prompt changes that. That is why the job splits in two: the AI writes, and the send tool tests. Many send tools, Mailchimp and Kit among them, can send two subject lines to two parts of a list and report which one did better. Whether your plan includes that feature is on your tool's own pricing page.
What this means for you: put your effort into the inputs and the final choice. A vague prompt returns ten vague lines, and a test between two vague lines only tells you which one is less vague.
What you give the AI
The AI writes a subject line as a promise about the email, so it needs the email first. Give it these, by name, in one message:
- The finished email body, pasted as text, with the final call to action in place. Write the subject line last, never first.
- Who is on the list, in one sentence: "past clients of a home cleaning business" beats "my subscribers".
- The one thing you want a reader to do after opening: book, reply, read, buy, show up.
- Your sender name as it appears in the inbox, since the subject line is read next to it.
- Up to five subject lines you sent before, with a note on which ones you felt worked, so the AI hears your voice and avoids repeating you. If this is your first send, say so.
- Words you never use, such as "exclusive", "act now" or anything your industry rules forbid.
- Any hard date or deadline in the email, typed exactly as the email states it.
If the email has a single fact that is the real news (a new service, a date, a change in hours), say so in one line. The AI will otherwise guess what matters, and its guess is often the greeting.
What comes back
You get ten subject lines, each with preview text that finishes its thought, and each labeled with the angle it takes. You also get two lines picked as finalists, with one sentence on why they differ enough to test.
Paste this brief into ChatGPT or Claude, with your inputs below it:
Write ten subject lines for the email below, each with its own preview text.
Rules:
- Each subject line takes a different angle. Label each one: plain news, question, the reader's problem, the one thing inside, a specific detail, or a direct ask.
- Put the most important words first. A phone may cut off the end.
- Every claim must be true of the email body below. Nothing the email does not say: no numbers, deadlines, discounts or results that are not in it.
- No capital letters for emphasis, no exclamation marks, no emoji, no fake "Re:" or "Fwd:".
- Never use the words on my "never use" list.
- Preview text is one short sentence that finishes the subject line's thought. It never repeats the subject line.
- Match the voice of my past subject lines. Do not repeat any of them.
- After the ten, pick two finalists that take truly different angles, and say in one sentence what a test between them would teach me.
Sender name:
[name as it shows in the inbox]
Who is on the list:
[one sentence]
What I want a reader to do:
[one action]
The real news in this email, if any:
[one line, or "none"]
Words I never use:
[list]
Up to five past subject lines and how they did, in my words:
[paste, or "none yet"]
The email:
[paste the finished body]If none of the ten works, ask for one more round with a single direction: "five more in the plain news angle, shorter" is enough. Asking for a third and fourth round tends to sand the lines down until they sound like everyone else's. Then copy your two finalists, and their preview text, into your send tool.
How to judge it
Read each line as one of your readers would, scrolling a full inbox on a phone. You can reject a bad line in seconds once you know the signs.
A bad result looks like this:
- It promises something the email does not deliver. If the line says "the one mistake" and the email lists four tips, it fails, however good it sounds. A broken promise costs you the next open too.
- It invents a fact. A percent, a deadline, a number of spots left, or a result that is not in your email has to go.
- It is a label, not a reason. "Monthly newsletter" or "Updates from our team" tells the reader what it is, not why to open it.
- It shouts. Capital words, stacked punctuation and urgency words read as an ad, and many readers skip them on sight.
- It sounds unlike you. If the line is far more playful or far more formal than the email, the reader feels the switch before they read a word.
- It loses its meaning when cut. Send yourself a test and look at it in your phone's inbox. If what shows reads as half a thought, rewrite it with the key words first.
- The preview text repeats the line. That wastes the second line of the inbox and makes the email look careless.
A good line passes one test: a reader who sees only your sender name, the subject line and the preview text knows what is inside and why it is for them.
When you test, judge the result with care. On a small list, the difference between two lines can be chance, so treat it as a hint and not a rule. Open counts are also less reliable than they used to be, because Apple Mail's privacy feature can load emails for readers who never looked. Where your tool allows, judge the test on clicks or replies, which only happen when a person actually read the email.
What stays with you
The AI can write ten lines. It cannot know your readers, and it cannot press send.
These parts stay with you:
- The final pick. Only you know whether your readers respond to a plain statement or a question, and whether a line sounds like the business they signed up to hear from.
- The facts. Check every date, price and claim in the line against the email, and the email against reality. A subject line with a wrong date does more harm than a dull one.
- The test setup. Choosing the two lines, the size of each test group, and how long to wait before the winner goes out is done in your send tool, by you.
- What you learned. Keep a short running note of which angles won and which lost. That note becomes the "past subject lines" input next time, and it is the part that makes this job better each month.
- Your list. The best subject line cannot rescue an email nobody asked for. Send to people who signed up to hear from you.
- The send button. Read the line once more next to your sender name, in your own inbox, before you send to everyone.
If the email itself is the harder job, start there instead. Turning one useful idea into a post, an email and a week of social shows where the body of the email can come from.
An example (invented business)
This is an editorial example with a made-up business, to show the shape of the work.
Say you run a small dog grooming shop, and your monthly email to past clients says you have added Saturday appointments starting in November. Your old subject line for this kind of email was "News from the shop", and you know it was easy to ignore.
You paste the email, describe the list as "past grooming clients who have booked at least once", and set the action as "book a Saturday slot". You add five past subject lines and a note that the one naming a specific service did better than the general ones.
Four of the ten might come back like this:
1. Plain news: Saturday grooming starts in November
Preview: Book the first weekend slots now from our booking page.
2. Question: Is a weekday grooming slot hard to fit in?
Preview: Starting in November, we are open Saturdays too.
3. The reader's problem: No more taking a weekday morning off for a bath and trim
Preview: Saturday appointments open in November.
4. Direct ask: Book your dog's first Saturday groom
Preview: Weekend slots start in November.You check them. Each one is true to the email, none invents a number of slots or a discount, and line three is long, so you check it on your phone and see it is cut before "trim". You keep line one and line two as finalists, because one states the news and the other asks a question, which is a real difference to test. If your list is a few hundred people, you skip the test and send line one, since it names the new Saturday service, the kind of line that did better for you before.
Test or just pick
There is one real choice here, and the size of your list mostly decides it.
| Your situation | What to do | Why |
|---|---|---|
| A small list, or a tool without testing | Pick one line with the checks above | A test on a small list mostly measures chance |
| A larger list and a tool that tests | Test your two finalists, then send the winner to the rest | Two different angles teach you something you can reuse |
| A time-sensitive send, like a same-week event | Pick one and send now | Waiting for a test result can cost more than a weaker line |
Whichever you choose, write the result in your running note. After a few sends, the note tells you which angles your readers open, and the AI gets better because you give it better inputs.
Where the same habit goes next
Ten options, then a judgment, works for other short lines too. The email signature under every message is the same job done once, and the hooks that open social posts follow the same rule: specific inputs in, specific lines out, as using AI for social media without the slop explains.
Of every job on this list, social media is the one that comes back every week, and Boomp is software that does that one for you. See ten posts made from your website, free
