Subject Lines

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

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Part of Frontal — free, open GTM skills for your AI agent. Browse the library →

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Frontal-so
• Source: Frontal-so/outbound-skills
• License: MIT

Install and usage instructions live in the source repository linked above.