Email Subject Line Optimizer Skill for Claude

Email Subject Line Optimizer

You receive an email subject line (and optionally the email body for context) as input.

Your job: analyze the subject line against the best practices below, diagnose its weaknesses, and produce 5 optimized alternative versions — each using a different psychological angle.

Process

Step 1 — Silent analysis

Before writing anything, internally evaluate the subject line across these dimensions:

Length & mobile readability

• Is it under 50 characters? (ideal: 30–50 chars; under 30 for mobile-first)
• Will it get truncated on mobile screens?
• Does it front-load the most important word/concept?

Clarity vs. cleverness

• Is it specific or vague/generic?
• Does it make a clear promise or does it try to be cryptic?
• Would the reader know what the email is about after reading it?

Psychological trigger

• Which primary trigger does it use (or fail to use)? FOMO, curiosity, pain point, benefit, personalization, social proof, urgency, humor?
• Is the trigger genuine or does it feel forced/spammy?

Spam risk

• Does it use all caps, excessive punctuation, or known spam trigger words?
• Does it make unverifiable claims ("FREE MONEY", "GUARANTEED")?

Deliverability signals

• Does it contain words commonly flagged by spam filters?
• Is it formatted cleanly (no weird symbols, no misleading previews)?

Alignment with email content

• If the body was provided: does the subject line accurately reflect the email's main value?
• Is there a mismatch between promise (subject) and delivery (body)?

Step 2 — Output

Produce this structured output:

🔍 Subject Line Audit

Original subject: [the subject line as provided]

A concise diagnosis (3–5 lines) identifying:

• What works (if anything)
• The 2–3 main weaknesses reducing open rates
• The primary psychological lever that's missing or underused

✏️ 5 Optimized Alternatives

For each alternative, provide:

• The subject line (formatted as inline code)
• The character count
• The psychological angle it uses
• One sentence explaining why it works

Format:

1. [Label: e.g., Curiosity Gap]

[Subject line here] — *[X chars]*

Angle: [Psychological trigger]

Why it works: [One sentence explanation]

Repeat for all 5 alternatives.

💡 Recommendation

One short paragraph (3–4 lines) indicating:

• Which of the 5 alternatives is the strongest pick and why
• What to A/B test (e.g., version 1 vs. version 3)
• One optional tip on the preview text to pair with the winning subject

Reference: Subject Line Best Practices

Length

• Optimal: 30–50 characters for desktop; under 30 for mobile-first audiences
• Over 60 characters: truncated on most mobile clients — the key message disappears
• Short ≠ vague. Short + specific is the winning formula
• Front-load: the most compelling word or phrase goes first, not at the end

Psychological triggers (use one primary angle per subject line)

| Trigger | How to use it | Example |

|---|---|---|

| FOMO / Urgency | Real deadlines, limited availability | "Last 6 hours: your spot expires tonight" |

| Curiosity gap | Open a loop the reader must close by opening | "The mistake 90% of founders make on day one" |

| Pain point | Name a specific problem your audience has | "Still losing leads after the first call?" |

| Benefit / value | State a clear, concrete outcome | "3 templates that saved me 4h/week" |

| Personalization | Name, role, company, behavior, location | "Luca, your October report is ready" |

| Social proof | Numbers, peers, recognition | "How 1,200 solopreneurs grew their list this month" |

| Humor | Unexpected twist, self-aware joke | "We'd be better together (said the email to the inbox)" |

| Straightforward | No tricks — just clarity. Works when trust is established | "Your invoice for October" |

Words & formatting that work

• Numbers: signal concrete, scannable value — "5 subject line fixes", "doubled open rates in 3 weeks"
• [Brackets] and {Curly braces}: create visual contrast in the inbox — "[NEW]", "{Quick question}"
• Verbs that imply action or change: discover, stop, avoid, steal, fix, unlock
• Questions: engage the reader's brain before they open — but avoid yes/no questions with obvious answers
• "You" in triggered/transactional emails: works well (it's personal); avoid in broadcast campaigns (feels like a sales pitch)

Words & formatting to avoid

• ALL CAPS (except strategic 1-word emphasis like "FREE" — use sparingly)
• Multiple exclamation points!!!
• Classic spam trigger words: "guaranteed", "act now", "earn money", "click here", "no risk", "100% free"
• Vague openers: "Newsletter #14", "Check this out", "Important update", "Quick question" (overused in cold email)
• Misleading hooks that the email doesn't deliver on — kills trust and future open rates

Personalization

• Including the recipient's first name boosts open rates by ~10–14% across industries
• Beyond the name: city, company, recently viewed product, membership tier, behavior-based triggers
• Always set a fallback for missing data — "Hi [FNAME]" errors destroy credibility

Emoji usage

• A single relevant emoji increases open rates by ~45% (Experian data) in B2C contexts
• Use max 1–2 emojis per subject line
• Place at the start or end — not embedded mid-sentence
• Don't replace words with emojis (e.g., "I ❤️ this" is fine; "I ❤️ ur 🛒" is not)
• Test rendering: emojis display differently across Gmail, Outlook, Apple Mail
• Avoid in formal B2B, legal, financial, or sensitive contexts

A/B testing guidance

• Test one variable at a time: length, tone, trigger type, personalization, emoji presence
• Need minimum ~1,000 recipients per variant for statistically significant results
• Let the test run at least 4 hours before declaring a winner
• Build a testing log: what you tested, which won, why — it compounds over time
• High-value tests: question vs. statement, short vs. medium, benefit vs. curiosity, emoji vs. no emoji

The preview text pairing rule

The subject line and preview text function as a two-part pitch. Never write one without considering the other.

• Preview text: max 50 characters, front-loaded with the key message
• It must complement the subject, not repeat it
• Think of it as: subject = hook, preview = the reason to open
• If left empty, email clients auto-pull the first line of body copy (often "View in browser" — a wasted opportunity)

Deliverability & spam filters

• Subject lines are the primary spam filter trigger — 69% of recipients mark email as spam based on the subject alone (Zippia)
• Avoid deceptive RE: or FWD: prefixes on cold/broadcast email
• Don't use "$$$", "FREE FREE FREE", or excessive symbols
• Consistent sender name + relevant subject = improved sender reputation over time

Quick reference: 5 angles to always consider

When generating alternatives, cover at minimum these 5 psychological angles:

• Curiosity/open loop — what question does the reader need to answer?
• Pain point — what problem are they actively trying to solve?
• Concrete benefit — what specific, measurable outcome do they get?
• Urgency/scarcity — is there a real reason to open now vs. later?
• Social proof or authority — who else is doing this / what do the numbers say?