Check an email preheader for length, subject repetition, filler and a sound hiding method
Email preheader checkFreeFor writers and designers
25/day
free runs a day, no sign-in.
Before: what you give it
The example input, as typed in. You paste text, CSS, markup or notes.
html:
<!DOCTYPE html>
<html lang="en"><head><meta charset="utf-8"><title>Your October receipt</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="color-scheme" content="light dark">
<style>.row{display:flex}.btn{background:#0a0a0a url(btn.png) no-repeat}@media (prefers-color-scheme: dark){.bg{background:#111!important}}</style></head>
<body style="margin:0;background:#ffffff">
<div style="display:none;max-height:0;overflow:hidden;opacity:0">Receipt for order 1042, total $48.00‌ ‌ ‌ </div>
...
After: what comes back
The first lines of a real run on that input. Nothing here is edited by hand.
Preheader: "Receipt for order 1042, total $48.00" (36 characters, 6 padding characters) - [medium] 36 characters. Under about 40, most clients append the start of the body text right after it, which can read as nonsense. Fix: Write 40 to 100 characters, then pad (see below). - [low] Only 6 padding character(s). After your text, clients pull in the next visible text from the body. If that is a logo alt or a nav link, it looks untidy. Fix: Follow the preheader with a run of ‌ pairs (about 100 characters' worth). Email client support notes are from our own testing notes dated 2026-10 and may have drifted. Confirm in real clients (Litmus, Email on Acid, or seed accounts) before sending. Limits: display lengths are estimates and differ by client, device width and font size. Only the first hidden block near the top of the body is read.
What you get
The preheader found, its length and padding, and findings with fixes
What it cannot do
Length cutoffs are estimates and vary by client; support notes are dated 2026-10 and may have drifted.
Who it is for
- Writers
- Designers
Free and Pro
| Free | 25 runs a day per IP address, up to 200,000 characters a run. |
|---|---|
| Pro | No daily limit, 300 requests a minute, up to 200,000 characters a run. |
Use it from your assistant
The tool is called check_email_preheader. Connect the server once, then ask for it by name or just describe the job. Install steps for each app.
claude mcp add --transport http designers https://designers.allthepossibles.com/mcpcurl -X POST https://designers.allthepossibles.com/api/email-preheader-check -H 'content-type: application/json' -d '{"html":"<!DOCTYPE html>\n<html lang=\"en\"><head><meta charset=\"utf-8\"><title>Your October receipt</title>\n<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">\n<meta name=\"color-scheme\" content=\"light dark\">\n<style>.row{display:flex}.btn{background:#0a0a0a url(btn.png) no-repeat}@media (prefers-color-scheme: dark){.bg{background:#111!important}}</style></head>\n<body style=\"margin:0;background:#ffffff\">\n<div style=\"display:none;max-height:0;overflow:hidden;opacity:0\">Receipt for order 1042, total $48.00‌ ‌ ‌ </div>\n<table role=\"presentation\" width=\"640\" align=\"center\" style=\"width:640px\"><tr><td class=\"bg\" style=\"background-color:#ffffff;color:#777777;font-size:12px\">\n<img src=\"https://example.com/logo.png\" width=\"120\">\n<h1 style=\"font-size:22px\">Thanks for your order</h1>\n<p>Your total was $48.00. <a href=\"https://example.com/receipt\" style=\"color:#0a64d8;text-decoration:none\">click here</a></p>\n<a href=\"https://example.com/track\" style=\"display:inline-block;padding:6px 10px;background:#0a0a0a;color:#ffffff;font-size:13px\">Track it</a>\n<p style=\"font-size:11px\">Sent by Example Co. <a href=\"https://example.com/unsubscribe\">Unsubscribe</a></p>\n</td></tr></table></body></html>","preheader":"","subject":"Your October receipt"}'Try it here
The form starts with the same example input, so you can run it first and then swap in your own.
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