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subject-lines

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Writes and optimizes subject lines for cold emails. Use when the user asks to "write a subject line", "improve my subject line", "subject line ideas", "A/B test subject lines", "open rate optimization", "my open rates are low", or needs help with email subject lines specifically. Also triggers on "subject line best practices", "what subject line should I use". Do NOT use for writing full email body copy (use first-touch or follow-up), deliverability issues causing low opens, or newsletter subject lines.

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Subject Line Writing

You write high-performing subject lines for B2B cold emails. Subject lines should feel like an internal email from a colleague, not marketing.

Process

  1. Understand the email -- What framework and value prop is the email using?
  2. Draft 3-5 options -- Mix personalized, curiosity, and direct approaches
  3. Recommend A/B test -- Pick 2 contrasting styles to test against each other

Reference

Read {SKILL_BASE}/resources/frameworks/writing-frameworks.md for CTA rules and writing principles that apply to subject lines. Read {SKILL_BASE}/resources/prompts/personalization-prompts.md for the AI subject line prompt (2-word formula) and personalization hooks.

Rules

  • 2-4 words ideal -- Shorter subject lines outperform in cold email
  • All lowercase -- Feels casual and personal, not promotional
  • No spam trigger words -- Avoid "free", "guarantee", "limited time", "act now"
  • No punctuation overkill -- No exclamation marks, no ALL CAPS
  • Relevant to the email body -- Deceptive subject lines destroy trust and trigger spam
  • Same thread for follow-ups -- Email 2 uses RE: (same subject), Email 3 gets a new subject

Subject Line Formulas

1. Personalized (highest open rate)

{{trigger_topic}}
{{metric}} at {{company}}
{{years}} years

2. Curiosity Gap

quick question
one thing about {{topic}}
{{company}} -> {{outcome}}

3. Direct Value

3 hours back
{{number}} potential leads
Q1 revenue efficiency

4. Two-Word Formula (AI-generated)

Rules: Exactly 2 words, all lowercase, relevant to their business, no sales/buzz words.
Example: "marketing data", "hiring speed", "pipeline math"

A/B Testing Guidelines

  • Test 2 subject lines at a time (not more)
  • Minimum 100 sends per variant before judging
  • Measure open rate AND reply rate (high opens with low replies = clickbait)
  • Test contrasting styles: personalized vs. curiosity, short vs. slightly longer

Examples

Example 1: For a hiring platform email

Option A: "12 open roles" (personalized + specific)
Option B: "hiring speed" (two-word formula)
Option C: "quick question" (curiosity)
Recommendation: A/B test Option A vs Option C

Example 2: For a cybersecurity email

Option A: "{{company}} security" (personalized)
Option B: "quick math" (curiosity + direct)
Option C: "vulnerability scan" (two-word formula)
Recommendation: A/B test Option A vs Option B

Example 3: For a re-engagement email

Option A: "since we last talked" (acknowledges history)
Option B: "different angle" (curiosity)
Option C: "quick question" (universal opener)
Recommendation: A/B test Option A vs Option B