The Subject Line Lab is a prompt you paste into Claude Code. Paste what your email is about and it writes a batch of subject lines — a fresh angle each, an open score on every one, spam-checked and length-checked so you send the winner.
No spam — just the tool. Unsubscribe anytime.
NEWSLETTER: The Solo Stack — a weekly newsletter for solo founders THIS ISSUE IS ABOUT: how I got my first 1,000 subscribers with $0 in ad spend — the 3 channels that worked and the 1 that wasted a whole month AUDIENCE: solo founders and indie hackers building in public CURRENT SUBJECT (optional): My growth update …
Score: 38/100 Why it's weak: it's a label, not a hook. Nothing new, no reason to open now. Reads like every other 'update' sitting in the inbox. Length: 16 chars — fine. Spam words: none.
That's the pre-loaded example — the tool re-runs on your own input, as many times as you want.