subject-line-lab

subject-line-lab is a skill for Claude Code from aaron-he-zhu/aaron-marketing-skills. It costs 170 tokens per session (3,380 once invoked), scanned A, original, Apache-2.0.

A tool for creating subject-line and preheader options for email, then checking each for likely spam wording, length problems, mobile and desktop truncation, and inbox appearance. A preheader is the short preview text shown beside or below an email subject.

In plain words
What is it for?
Use it to generate and compare subject lines, flag possible spam triggers, check display length on different screens, and rank candidates for an email test.
Why use it?
It helps remove weak email variants before testing or sending them. The checks are warnings based on rules, not guarantees about performance or deliverability.

Skill for Claude Code

Written for Claude Code: argument-hint in frontmatter. Also seen: mentions Claude Code; built for openclaw.

Needs its repository: it reads a path above its own folder, which exists only inside the repository. The line is Emit the standard shape from [skill-contract.md §Handoff Summary Format](../../../references/skill-contract.md): Status / Objective / Key Findings / Evidence (l.

Part of the aaron-marketing plugin — 120 skills shipped together , and of aaron-marketing

Good fit Use it to generate and compare subject lines, flag possible spam triggers, check display length on different screens, and rank candidates for an email test.

Compare 6 skills from other repositories ↓
About the project

aaron-marketing-skills is a collection of 120 AI-agent skills covering marketing work such as brand narrative, search optimization, social media, email, advertising, influencer campaigns, and launches. Marketers and agent users can install it as a plugin, use its portable skills, or run its described bot team. The catalogue entries are components of this marketing workflow.

aaron-he-zhu/aaron-marketing-skills · 2,767 stars · on GitHub · aaronmarketing.ai

Install

Getting it into your agent

It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.

Clone the repo
git clone --depth 1 https://github.com/aaron-he-zhu/aaron-marketing-skills
agentmods
npx agentmods add skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab

Made for: Claude Code.

Or install aaron-marketing, the plugin that ships this one along with the rest of its 120 skills.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for subject-line-lab

README.md
[![agentmods](https://agentmods.dev/badge/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab/github.svg)](https://agentmods.dev/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab)
Your own site
<a href="https://agentmods.dev/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab"><img src="https://agentmods.dev/badge/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for subject-line-lab

Your own site · 80×15
<a href="https://agentmods.dev/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab"><img src="https://agentmods.dev/badge/skills/aaron-he-zhu/aaron-marketing-skills/subject-line-lab.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 170 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,380 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • Socket pass 2 Sept 2026
  • Snyk pass 2 Sept 2026
  • NVIDIA SkillSpector pass 7 Sept 2026
How audits are shown
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00170 $0.03380
Opus 5 $0.00085 $0.01690
Sonnet 5 $0.00034 $0.00676
Haiku 4.5 $0.00017 $0.00338

Measured 10d ago against content hash 6b7431ad52a5, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

subject-line-lab scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

email/engage/subject-line-lab/SKILL.md · 97 lines

How it starts

The opening of the file, as written. The whole thing — 97 lines — stays where its author put it; the contents beside it link to each section on GitHub.

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 profile-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

Output: a variant table (labeled SUBJ-A, SUBJ-B, …), a per-variant pre-score card (spam flags, desktop/mobile truncation, emoji count, preview render), and a ranked shortlist of survivors to carry into the test.

Skill Contract

Expected output: a subject-line + preheader variant set (3-8 variants, each with a stable variant id and an angle label) and a per-variant heuristic pre-score card covering spam-trigger flags, desktop + mobile length/truncation, emoji count, and the rendered inbox preview — plus a ranked shortlist of survivors and the standard handoff summary for memory/email/subject-line-lab/.

Read the full file on GitHub · 97 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 10d ago Changed 6b7431ad52a5
  2. 13d ago First seen · 97 lines · 170 tokens per session scan A 441793a7ecd7

Subscribe to this mod's changes

subject-line-lab is a skill published in the GitHub repository aaron-he-zhu/aaron-marketing-skills (2,767 stars, last pushed today), licensed Apache-2.0. It adds 170 tokens to every session and 3,380 once invoked, about $0.0009 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.