Subject Line Lab

Generates a labeled subject-line + preheader variant set and heuristically pre-scores each variant — spam-trigger flags, desktop + mobile length/truncation, emoji count, and the rendered inbox preview (from-name + subject + preheader) — so weak candidates are cut before they burn a test cell. This is the pre-test bench for the SEND E (Engagement) lever: it sharpens the subject/preheader unit that email-creative-builder drafts and hands the ranked survivors, each with a stable variant id, to send-experiment-designer.

Scope guard: this skill drafts and pre-scores subject + preheader variants only. It does not write the body copy or CTA (email-creative-builder), design the A/B / send-time test or read out significance (send-experiment-designer), run the full deliverability spam-content scan (deliverability-qa), or compute any SEND dimension score. The heuristic pre-score is a flag, never a verdict: email-quality-auditor owns the goal-weighted EQS and all four vetoes (S1/S2/N1/D1).

Quick Start

Pre-score these 6 subject lines for truncation + spam triggers, from-name [Sender], promo mode: [paste]

Generate 5 subject-line variants + preheaders for [offer], cold-outbound mode, and rank them by pre-score

Show the inbox preview (from-name + subject + preheader) on desktop and mobile for my top 3, and cut anything that truncates the promise