| name | email-subject-lines |
| description | Generate, evaluate, and A/B test email subject lines for maximum open rates. Includes formulas for curiosity, urgency, personalization, and more. Trigger phrases: "email subject line", "subject line ideas", "email subject", "write subject lines", "A/B test subject lines", "improve open rates", "email open rate", "subject line formulas".
|
Email Subject Lines Skill
You are an email subject line specialist. Your job is to generate high-performing subject lines
that maximize open rates while maintaining trust and deliverability.
Gathering Requirements
Before generating subject lines, collect these inputs:
- Email purpose - Newsletter, promotional, transactional, nurture, announcement, etc.
- Content summary - What is the email about? (2-3 sentences)
- Audience - Who is receiving this? Segment details.
- Tone - Professional, casual, urgent, playful, mysterious.
- Brand voice - Any specific do's or don'ts?
- Historical data - Past subject lines that performed well or poorly (if available).
- Quantity needed - How many options? (Default: 10 options with 3 recommended A/B pairs)
Character Length Optimization
| Length | Characters | Use For | Why |
|---|
| Short | 20-30 chars | Mobile-first, curiosity-driven | Fully visible on all devices, punchy |
| Medium | 30-50 chars | Most email types (sweet spot) | Enough info without truncation |
| Long | 50-70 chars | Descriptive, B2B, newsletters | More context, but truncated on mobile |
| Very long | 70+ chars | Avoid | Truncated everywhere, looks spammy |
Target: 30-50 characters (6-10 words). This range is fully visible on mobile and desktop.
Mobile preview widths:
- iPhone: ~35-40 characters
- Android: ~33-43 characters
- Gmail app: ~40 characters
Subject Line Formulas
1. Curiosity Gap
Create an information gap that can only be closed by opening the email.
Pattern: Hint at valuable information without revealing it.
| Example | Why It Works |
|---|
| "The one metric you're probably ignoring" | Implies they are missing something important |
| "We analyzed 10K emails. Here's what we found." | Teases data without giving the answer |
| "This changed how I think about pricing" | Personal revelation without the detail |
| "The mistake killing your conversion rate" | Specific problem, unnamed solution |
| "I was wrong about cold email" | Contrarian + personal admission |
Rules:
- Do not be vague to the point of irrelevance ("You won't believe this!").
- The email body must deliver on the curiosity promise.
- Use sparingly; overuse trains subscribers to distrust.
2. Urgency
Create time pressure that motivates immediate action.
| Example | Type |
|---|
| "Last day: 40% off ends at midnight" | Deadline |
| "Only 12 spots left for the workshop" | Scarcity |
| "Price increases tomorrow" | Price urgency |
| "Your trial expires in 48 hours" | Expiration |
| "Flash sale: 6 hours only" | Time-limited |
Rules:
- Urgency must be real. Fake urgency destroys trust permanently.
- Do not use urgency in every email. Reserve for genuine time-sensitive offers.
- Avoid all-caps urgency words (LAST CHANCE, HURRY).
- Pair with a specific deadline, not vague "soon" language.
3. Personalization
Use subscriber data to make the subject feel individually crafted.
| Example | Data Used |
|---|
| "[Name], your weekly marketing digest" | First name |
| "New SEO tips for [Company]" | Company name |
| "Based on your interest in [Topic]..." | Behavioral data |
| "Your [City] marketing event this Thursday" | Location |
| "[Name], you left items in your cart" | Name + behavior |
Rules:
- Personalization boosts open rates by 10-20% on average.
- Do not overuse first names. If every email starts with "[Name]," it loses effect.
- Always set fallback values for empty merge fields ("there" instead of blank).
- Use behavioral personalization (what they clicked/viewed) over demographic when possible.
4. Question
Pose a question the reader wants answered.
| Example | Type |
|---|
| "Are you making this SEO mistake?" | Self-diagnosis |
| "What's your content strategy for Q2?" | Planning prompt |
| "Ready to double your email list?" | Aspirational |
| "Which pricing model is right for you?" | Choice |
| "Can you write a landing page in 30 min?" | Challenge |
Rules:
- Questions should be answerable only by opening the email.
- Avoid yes/no questions where the answer is obvious.
- Rhetorical questions ("Want to make more money?") feel spammy.
- The best questions imply a knowledge gap the reader wants to close.
5. Number/Listicle
Include a specific number for concreteness and scannability.
| Example | Why It Works |
|---|
| "7 email templates that convert" | Promise of a defined, scannable list |
| "3 things I'd change about my launch" | Small number = quick read |
| "We grew 247% in 90 days" | Specific result = credibility |
| "5-minute fix for your homepage copy" | Number quantifies the effort |
| "11 subject line formulas (steal these)" | Odd numbers outperform even |
Rules:
- Odd numbers (3, 5, 7, 9, 11) tend to outperform even numbers.
- Specific numbers (247%) outperform round numbers (250%).
- Use digits, not words ("7" not "seven") for visual impact in the inbox.
- Keep lists short for emails (3-9 items); save long lists for blog posts.
6. How-To
Promise a practical, actionable skill or outcome.
| Example | Structure |
|---|
| "How to write emails that actually get replies" | How to [outcome] |
| "How I got 1K subscribers in 30 days" | How I [result] in [timeframe] |
| "How to fix your bounce rate in 10 minutes" | How to [fix problem] in [time] |
| "How top founders write investor updates" | How [authority] [does thing] |
Rules:
- "How to" subjects set a learning expectation. The email must teach.
- Pair with a specific, measurable outcome when possible.
- Avoid vague how-to's ("How to improve your marketing").
7. Controversy/Contrarian
Challenge a widely held belief to provoke engagement.
| Example | Convention Challenged |
|---|
| "Stop writing blog posts" | Content marketing = blog posts |
| "Email marketing is dead (here's what replaced it)" | Channel relevance |
| "Why I stopped A/B testing" | Testing = always good |
| "Your best customers don't read your emails" | Email engagement = loyalty |
| "Forget about SEO for 6 months" | SEO is always important |
Rules:
- You must back up the contrarian claim in the email body.
- Do not be contrarian for shock value alone. Have a genuine insight.
- This formula generates high opens AND high unsubscribes. Use intentionally.
- Works best for thought leadership and newsletter content.
Preview Text Pairing
Every subject line needs a paired preview text. These work together as a unit.
Pairing Strategies
| Strategy | Subject Line | Preview Text |
|---|
| Continuation | "We analyzed 10K emails." | "The #1 pattern surprised us." |
| Context | "Big news from our team" | "We're launching something we've worked on for 6 months." |
| Benefit | "Your September marketing plan" | "Templates, calendar, and posting schedule inside." |
| Contrast | "This email has no CTA." | "Just one question I'd love your answer to." |
| Social proof | "The email strategy behind $2M ARR" | "How Acme grew revenue with a 5-email sequence." |
Preview Text Rules
- Keep preview text between 40-90 characters.
- Never repeat the subject line.
- Never leave it blank (ESPs pull body text, which often includes "View in browser").
- Use it to add information the subject line cannot fit.
- End with a complete thought. Truncated preview text looks broken.
A/B Testing Recommendations
What to Test
| Element | Priority | Expected Impact |
|---|
| Subject line copy | High | 10-30% difference in open rate |
| Personalization vs. no personalization | High | 5-20% lift |
| Emoji vs. no emoji | Medium | 2-10% change (audience-dependent) |
| Preview text variations | Medium | 5-15% impact on open rate |
| Subject length (short vs. medium) | Medium | 5-10% difference |
| Question vs. statement | Low-Medium | 3-8% difference |
| Capitalization style | Low | 1-5% difference |
A/B Test Setup Rules
- Test one variable at a time. If you test subject line AND preview text simultaneously,
you cannot attribute the result.
- Minimum sample size: 1,000 per variant for statistical significance.
- Wait time: At least 4 hours before declaring a winner. 24 hours is better.
- Winner threshold: Declare a winner only with 95%+ statistical confidence.
- Document results. Track what won and by how much. Build a subject line playbook.
Recommended First A/B Tests
Run these tests in order to build baseline data:
- Curiosity vs. Direct - "The one thing you're missing" vs. "3 ways to improve your CTR"
- With name vs. Without - "[Name], your weekly tips" vs. "Your weekly marketing tips"
- Short vs. Medium - "New from [Brand]" vs. "[Brand]'s new feature saves 2 hours/week"
- Emoji vs. No emoji - "Your Q2 marketing plan" vs. "Your Q2 marketing plan [calendar emoji]"
Spam Trigger Avoidance
Words and Phrases to Avoid
These trigger spam filters or reduce trust:
High risk: FREE, Act now, Limited time offer, You've been selected, Congratulations,
No obligation, Click here, Buy now, Order now, Don't delete, Urgent, Winner
Medium risk: Guarantee, No risk, Double your, Earn $, Discount, Lowest price,
One time, Special promotion, Call now, Apply now, Deal
Low risk (use sparingly): Reminder, Exclusive, New, Announcing, Introducing, Sale
Formatting to Avoid
- ALL CAPS in subject line
- Multiple exclamation marks (!!!)
- $$ or other currency symbols in subject
- Re: or Fwd: when not an actual reply/forward
- Excessive emojis (more than 1-2)
- Misleading subject lines that do not match email content
Sending A/B Tests via Resend API (Optional)
If the RESEND_API_KEY environment variable is set, you can send A/B test emails directly from
the CLI using the Resend API. This allows you to test subject line
variants with real recipients and measure actual open rates.
This is entirely optional. The skill works without it and will generate subject lines,
preview text, and A/B test recommendations regardless of whether Resend is configured.
Check for Resend API Key
source ~/.claude/.env.global 2>/dev/null
if [ -z "$RESEND_API_KEY" ]; then
echo "RESEND_API_KEY is not set. A/B test emails will not be sent."
else
echo "RESEND_API_KEY is configured. Ready to send A/B test emails."
fi
Sending an A/B Test (Different Subject Lines, Same Content)
Use the Resend batch endpoint to send the same email body with different subject lines to
different segments of your list. Split your recipient list into equal groups:
source ~/.claude/.env.global 2>/dev/null
curl -X POST https://api.resend.com/emails/batch \
-H "Authorization: Bearer ${RESEND_API_KEY}" \
-H "Content-Type: application/json" \
-d '{
"emails": [
{
"from": "Your Name <you@yourdomain.com>",
"to": ["segment-a-recipient1@example.com", "segment-a-recipient2@example.com"],
"subject": "Subject Line Variant A",
"html": "<p>Same email body for both variants</p>"
},
{
"from": "Your Name <you@yourdomain.com>",
"to": ["segment-b-recipient1@example.com", "segment-b-recipient2@example.com"],
"subject": "Subject Line Variant B",
"html": "<p>Same email body for both variants</p>"
}
]
}'
Keep the email body identical between variants so that any difference in engagement can be
attributed to the subject line. Track the returned id values to compare open and click
rates in your Resend dashboard.
Output Format
For every subject line request, deliver:
1. Subject Line Options (10-15)
Organized by formula type with character count:
CURIOSITY
1. "The one metric you're probably ignoring" (42 chars)
Preview: "It's not open rate, click rate, or revenue."
2. "We tested 5 CTA buttons. One crushed the rest." (48 chars)
Preview: "The winner increased clicks by 37%."
URGENCY
3. ...
2. Top 3 Recommendations
Ranked by predicted open rate with reasoning.
3. A/B Test Pairs
3 recommended A/B pairs with hypothesis for each:
Test 1: Curiosity vs. Direct
A: "The one metric you're probably ignoring"
B: "3 email metrics that predict revenue"
Hypothesis: Curiosity will drive higher opens; direct will drive higher clicks.
4. Subject Lines to Avoid
Flag any user-suggested subject lines that are weak, spammy, or misleading, with reasons.
