Why generate subject lines with two different models?

Because hook style is a model fingerprint: given the same brief, GPT-5-class output leans punchy-newsy while Claude leans specific-and-warm. Two models = two genuinely different candidate pools for the same A/B slot — variance you'd otherwise pay a copywriter for.

What subject-line length should I ask the AI for?

Cap at 50 characters in the prompt: mobile clients truncate around there, and the cap forces the model to front-load the hook. If your audience skews desktop, 65 is safe — but write the cap into the prompt either way, or the model will ramble.

Can AI subject lines hurt my deliverability?

Only if you let spam-trigger phrasing through — so ban it in the prompt ("no spam-trigger words") and keep the veto step human. The prompt template on this page includes that rule by default.

Write Email Subject Lines with AI

To write email subject lines with AI, give @vustbot your email's core message, audience and goal, and ask for 10 subject lines under 50 characters. Then regenerate the same brief with a second model — GPT-5 and Claude write noticeably different hooks — and A/B the two strongest. All in one Telegram chat, paid per action.

The tools ranking for this query optimize their own templates. The underused move is multi-model A/B: two frontier models given the same brief produce genuinely different hook styles — and no P.1 tool page does that today.

Open @vustbot in TelegramPay per action · no signup beyond Telegram

What the AI does in this scenario

10 options per run, length-capped for mobile preview
Multi-model A/B: same brief to GPT-5 and Claude, different hook styles
Goal-aware: open-rate tease vs announcement clarity are different asks
Spam-trigger words avoided on request
Per-action pricing — test batches without a tool subscription

Worked example: write email subject lines with ai

Input

Email: our project-management app now has offline mode. Audience: existing free-tier users. Goal: feature-announcement opens, no clickbait.

Output

1) Offline mode is here — plan anywhere. 2) Your boards now work without wi-fi. 3) New: full offline editing, synced later. 4) Trains, planes, basements: boards work now. 5) The most-requested feature just shipped. …(+5 more, each ≤50 chars)

How to write email subject lines with ai — step by step

1
Brief the message, audience and goal
One message to @vustbot: what the email says, who receives it, and whether you're chasing opens, clicks or clarity. The goal changes the hook style more than the topic does.
2
Ask for 10 under a hard length cap
"10 subject lines, each under 50 characters, no clickbait, no spam-trigger words" — the cap keeps them mobile-safe, the batch gives you real choice.
3
A/B across models, then across readers
Regenerate with a second model in the same chat and shortlist the strongest from each. Ship the top two as an actual A/B test — the models gave you the variance for free.

AI vs doing it manually

A human who knows the list writes ONE great subject line; AI writes ten plausible ones in seconds. The manual craft still matters at the shortlist step — you know that your audience hates emoji or that "free" tanks your deliverability. Use AI for the batch and the variance, yourself for the veto. Where AI clearly wins: the tenth send of the month, when your own hooks have converged to a formula.

The prompt to copy

Write 10 email subject lines for this email: [CORE MESSAGE]. Audience: [WHO]. Goal: [OPENS/CLICKS/CLARITY]. Rules: each under 50 characters; no clickbait; no spam-trigger words (free!!!, act now); vary the mechanism across the 10 (curiosity, benefit, news, question, specificity).

Frequently asked questions

Related in Marketing

Try it on your real task

The welcome bonus covers a first run — send the prompt above with your own facts and judge the output yourself.

Open @vustbot