Free tool
Cold email subject line tester
Paste a cold email subject line and see what a stranger sees: spam-list words underlined, the length in words and characters, capitals and exclamation marks flagged, and the line as it is cut off on a phone. Compare up to three versions.
Free, no signup. Runs in your browser; nothing you type is sent to us.
What the tester checks
Six plain checks. Each flag tells you what was found and what to write instead; the score only adds them up.
Length, in characters and words
We tell our own AI to keep a cold subject line to 6–10 words. Longer lines get cut off on a phone, and the preview below shows you exactly where.
Spam-list words
Words like “free”, “guaranteed”, “act now” or “winner” appear on common spam-filter word lists and read like a promotion. Each one is underlined in your line.
ALL CAPS and exclamation marks
Capitals read as shouting. Two exclamation marks read as a sales flyer. Both are classic signs of a mass send.
Personal fields
A {{first_name}} or {{company_name}} field is a good sign, as long as your sending tool fills it in. We also flag fields with a spelling we do not recognise.
Fake “Re:” and “Fwd:”
Starting a first email with “Re:” pretends there was a conversation. Readers notice, and the trick costs trust before they read a word.
Phone and desktop preview
Your line in a mock inbox row at phone width and at desktop width. The browser does the cutting, so what you see is a real cut-off, not a guess.
How to test a subject line
Three steps, under a minute, no account.
- 1
Paste a subject line
Type it in the first box. The score, the flags and the inbox previews appear as you type. Add a second or third version to compare them side by side.
- 2
Read the flags, not just the score
Each flag says what was found and what to do instead, in plain words. The score only adds them up.
- 3
Fix it and share it
Edit the line until nothing is flagged, then copy a link to the result for a colleague. The link holds your lines; we store nothing.
What makes a good cold email subject line
A cold email is a note to a stranger who did not ask for it. The subject line has one job: tell that person, truthfully, what the email is about in the space a phone gives it. Everything the tester flags comes back to that. Spam-list words and capitals say “promotion”. A fake “Re:” says “trick”. A twelve-word line says “this person did not think about my screen”.
When ReachClaw's AI writes an outbound email for one of your leads, the rule it is given is short: six to ten words, make the reader curious, no spam words, and use the lead's real name and company rather than a generic greeting. The same rule works when a person writes the line.
Lines that pass
“Quick question about {{company_name}}'s onboarding emails”
Specific topic, a real field, a question the email will actually ask.
“Idea for your checkout page”
Five plain words. Says what is inside and nothing more.
“{{first_name}}, who handles hiring at {{company_name}}?”
A question the reader can answer in one line, which is the easiest reply to get.
Lines that get flagged
“FREE trial - don't miss this LIMITED TIME offer!!”
Three spam-list phrases, two words in capitals, two exclamation marks. It reads as a flyer, not a note.
“Re: our call”
There was no call. The reader opens it, realises that, and deletes it with a bad taste.
“Partnership opportunity to revolutionise your business growth strategy this quarter”
Twelve words, nothing specific, and the end disappears on a phone.
What a score can't tell you
- Whether the email lands in the inbox. That depends on who you send to, whether they asked to hear from you, the reputation of your sending address and what the body says. This tool looks at one line of text.
- How many people will open it. We do not quote open rates or “X% more opens” figures, because none of them would be measured on your list.
- Whether the email is any good. A clean subject line on an email that asks for too much, too soon, still gets deleted. Keep the body short and ask for one small thing.
From a tested subject line to the whole email
ReachClaw finds businesses that match your ideal customer by searching the web, then writes one personal email per lead, subject line included, from your own website pages and notes, and sends it from your Gmail. You read the drafts before anything goes out, and a campaign can run again on a schedule and email only the new leads it finds.
One flat price from $6.99 a month: 100 outbound emails a month on Solo, 500 on Growth, no cap on Scale. The AI runs on your own Anthropic API key, which Anthropic bills separately. Lead emails come from public pages and are not verified.
Keep reading
Email prospecting
How the AI finds businesses that fit your ideal customer and writes the first email from your Gmail.
Email list building
Building a list from the AI's web search or your own CSV, and what the addresses it finds can and cannot be.
SDR software
What ReachClaw does and does not do for a one-person outbound effort, with the caps per plan.
Instantly.ai alternative
ReachClaw next to Instantly for small senders: one flat price, no lead credits, no sequences.
Questions about the subject line tester
Is this cold email subject line tester free?
Yes, with no signup and no daily limit. The checks run in your browser on your own device. Nothing you type is sent to ReachClaw or stored anywhere, which is also why the tool keeps working if you lose your connection.
What does the score mean?
Every line starts at 100 and loses points for each flag: spam-list words, length, words in capitals, exclamation marks, emoji, a fake “Re:”, or a personal field we do not recognise. 85 and above with no warning is “Ready to send”; a warning holds the line at “Worth a tweak” however high the score; 60 to 84 is “Worth a tweak”; below 60 is “Rewrite it”. It is a rule-of-thumb check, not a prediction of how many people will open the email.
Does a good score mean my email will land in the inbox?
No. Whether an email lands in the inbox depends mostly on things a subject line cannot show: who you are sending to, whether they asked to hear from you, the reputation of your sending address and what the body says. A subject line full of spam-list words makes things worse; a clean one does not make them good on its own.
Which spam words does it check?
A short list of words and phrases that appear on common spam-filter word lists, such as “free”, “guaranteed”, “act now”, “limited time”, “winner”, “congratulations”, “100%”, “click here” and “risk-free”. Each one found is underlined in your line. We do not claim to know how any particular spam filter weighs them.
How long should a cold email subject line be?
Short enough to survive a phone screen and specific enough to say what the email is about. The rule we give ReachClaw's own AI when it writes an outbound email is 6 to 10 words. Rather than quote a character count, the tool shows your line in a phone-width inbox row so you can see where it is cut off.
Can I compare two or three subject lines?
Yes. Fill in the second and third boxes and each version gets its own score, flags and previews, with the highest-scoring one marked. This is a quick way to pick between drafts before you send; it is not a test on real recipients, and ReachClaw does not run A/B tests on live campaigns.
What are the {{first_name}} and {{company_name}} fields?
Merge fields: placeholders your sending tool replaces with each lead's details. ReachClaw templates use {{first_name}}, {{company_name}}, {{sender_name}} and {{product_url}}, and the AI fills them in for every lead when a campaign runs. The tester counts a known field as a good sign and flags one it does not recognise, because an unfilled placeholder is the fastest way to be deleted.
How does this relate to ReachClaw?
ReachClaw writes one personal outbound email per lead, subject line included, from your own website and notes, and sends it from your Gmail. The rules in this tester are the same ones we give that AI: short, specific, no spam words, real personal fields. The tester is free for anyone; ReachClaw's plans start at $6.99 a month, one flat price, and the AI runs on your own Anthropic API key.
Let the AI write the next one
Connect your Gmail, describe your ideal customer, and read the first drafts, subject lines included, before anything sends.
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