Subject Line Lab

What it does

Subject lines are judged on a phone, truncated, next to preview text pulled from the first line of the body. Most subject line advice ignores all three. This skill generates variants, then screens each against character truncation on the common clients, against filter triggers, and against the preview text it will actually appear beside.

When to use it

• write subject lines
• my open rates are low
• test subject variants
• improve this subject line
• subject line ideas

What it needs from you

• email: the body the subject sits on
• audience: who receives it
• history: past subject lines with their open rates, if any

How it works

1. Generate variants across four distinct shapes: the specific observation, the direct question, the two word fragment, and the plain statement of the thing. Not four wordings of one shape.
2. Screen for truncation. Show each variant cut at 30 characters and at 45, which is roughly where mobile and desktop clients stop, and reject any variant whose meaning dies at 30.
3. Screen for filter triggers: currency symbols, exclamation marks, all caps words, and the specific phrases that read as promotional.
4. Render the preview text each variant will produce, which is the first line of the body. If the subject and the preview say the same thing, one of them is wasted.
5. Reject anything that promises what the body does not deliver. An open earned by a mismatch costs a reply.
6. Rank by expected performance and say what each variant is testing, so a test result means something.

What you get back

A variant table with the 30 and 45 character truncations, the filter flags, the preview text pairing, and what each variant tests.

The quality bar

• Variants span four distinct shapes, not four wordings
• Every variant is shown truncated at 30 and 45 characters
• Preview text is rendered beside each subject
• Each variant states what it is testing

Where it runs

• Claude Code
• Codex CLI
• Cursor
• MCP-compatible agents