Email Subject Line Tester

Email Subject Line Tester

AI-powered email subject line optimization agent — generates 10 variants, scores each on 7 open-rate predictor signals, and selects the top 3 for A/B testing.

Describe your email's topic, audience, and goal. The agent produces scored variants across multiple psychological angles (urgency, curiosity, personalization, social proof) and gives you a ready-to-launch A/B test plan with sample size guidance.

Commands

subject test                # generate and score 10 subject line variants for a topic
subject generate                   # generate variants with more context (audience, goal, tone)
subject score                # score a specific subject line you already have
subject ab test                    # structure a full A/B test plan with sample size formula
subject analyze competitors        # analyze subject lines from competitor emails you paste
subject by industry                # get industry-specific benchmarks and top-performing patterns
subject history                    # show previously tested subject lines and their scores
subject save                       # save current session results to ~/email-subjects/

What Data to Provide

The agent works with:

No integrations required. Works entirely from your descriptions.

Workspace

Creates ~/email-subjects/ containing:

Analysis Framework

1. The 7 Open-Rate Predictor Signals

Each subject line is scored 0-10 on each signal; total score is out of 70:

Signal 1 — Urgency Words

Signal 2 — Personalization Tokens

Signal 3 — Question Format

Signal 4 — Number Inclusion

Signal 5 — Emoji Presence

Signal 6 — Character Length

Signal 7 — Power Words

2. Industry Benchmark Reference

| Industry | Average Open Rate | Top Quartile | |----------|------------------|--------------| | Ecommerce | 15-20% | >25% | | SaaS / Software | 20-25% | >32% | | Newsletter / Media | 25-35% | >45% | | B2B Services | 20-28% | >35% | | Nonprofit | 26-30% | >40% | | Healthcare | 22-27% | >35% |

3. Spam Trigger Detection

4. Mobile Preview Check

5. A/B Test Setup Guidance

Output Format

Every subject test run outputs: 1. 10 Scored Variants — each with total score /70, per-signal breakdown, and character count 2. Top 3 Picks — recommended for A/B testing, with rationale for each selection 3. Spam Flag Report — any variants with trigger words highlighted 4. Mobile Preview Simulation — truncated rendering at 40 and 58 characters 5. A/B Test Plan — test setup instructions with sample size recommendation 6. Preheader Suggestions — paired preheader for each top-3 variant

Rules

1. Always generate exactly 10 variants before scoring — never fewer 2. Never recommend a variant containing known spam trigger words without flagging the risk 3. Score every variant on all 7 signals — no signal may be skipped 4. Flag when the audience or industry context makes certain signals inappropriate (e.g., emoji in B2B financial services) 5. Always include character count and mobile truncation preview for every variant 6. When scoring a user-provided subject line, explain each signal score individually — not just the total 7. Save session results to ~/email-subjects/history/ when the user requests subject save

1. Always generate exactly 10 variants before scoring — never fewer 2. Never recommend a variant containing known spam trigger words without flagging the risk 3. Score every variant on all 7 signals — no signal may be skipped 4. Flag when the audience or industry context makes certain signals inappropriate (e.g., emoji in B2B financial services) 5. Always include character count and mobile truncation preview for every variant 6. When scoring a user-provided subject line, explain each signal score individually — not just the total 7. Save session results to ~/email-subjects/history/ when the user requests subject save