What exactly changed in the new version of my prompt?
You have two versions of an instruction for a bot (an old one and a new one). That instruction is called a prompt. You want to see in 5 seconds what changed and whether the new version costs more.
Paste the old one on the left, the new one on the right. The tool shows the difference line by line. This is called a diff, just a comparison of two texts. Red background means something disappeared. Green means something was added. Everything else stays uncolored, unchanged.
Above the table you see how much longer or shorter the new version is. We measure in tokens, chunks of text the bot uses to count length. The more tokens, the more you pay each time the prompt is sent.
Everything runs locally in your browser. Your prompts never leave your computer. Safe even for internal company instructions.
How to use it
- Paste the old prompt in the left field, the new one on the right.
- Pick a model to count length for (GPT-4o, Claude, or Gemini). Each one counts a little differently, so pick the one you actually use.
- Pick a view: side by side (old on the left, new on the right) or one under the other (lines in a single column with pluses and minuses).
- Red background = a line that disappeared. Green = a line that was added. No color = unchanged.
- At the top you see the length difference. +50 = the new version is 50 tokens longer. -30 = 30 tokens shorter.
- The stats bar shows how many lines were added, how many removed, and how many stayed the same.
When this is useful
Six typical situations where the prompt diff gives you a concrete answer instead of a guess:
- Tweaking a prompt that already runs in an app. The first version worked OK, but you want to add 3 new rules. You paste old and new, the tool says: the new version is 50 tokens longer. With 10,000 questions a day that means an extra dollar a day. An informed decision, not a blind one.
- Reviewing what a teammate changed. Someone on the team edited a shared prompt. Instead of reading the whole thing from scratch, you paste the version before and the version after their edits, and immediately see the 3 lines that actually changed. A 30-second review.
- Shortening a prompt to make it cheaper. The old one had 3,000 tokens, the new one has 1,500. The tool confirms: half as long, so half the running cost. Plus you can see in red and green that no important rule disappeared along the way.
- Testing two versions side by side. You build version A and version B of the same prompt. The tool shows exactly how they differ, not just *"B is a bit longer"*. Easier to judge later which version gives better answers.
- Documenting changes for the team. Copy the comparison into a change description, a Notion page, or a chat message. Specifics instead of *"tweaked the prompt a bit"*.
- Onboarding someone new. Show them: *"this is what the prompt looked like a year ago, this is now"*. They can see the history of decisions and why certain rules are in there.