Compare two versions
See exactly what changed between two versions of a prompt, and what the change does to length, token cost, structure and its score out of 10. Nothing you paste leaves this browser.
New to this? What it does, in plain words
It puts two versions of a prompt side by side and highlights every word that was added or removed.
Use it when someone edits a prompt your team depends on and you want to see what really changed before it goes live. A team lead can review a colleague's edit to the customer-reply prompt in seconds. An operations manager can check whether a longer version will cost noticeably more per thousand uses.
What you need
No API key and no sign-up. The comparison, the cost estimate and the score all run in this browser, and nothing you paste is sent anywhere. Only the optional Explain this change button asks an AI model for a summary, and that one needs a key.
How to use it
- Paste the earlier version in A and the changed one in B, or press My Prompts to load a saved version.
- Read the Changes panel: added words are highlighted, removed words are struck through. Switch Compare by to Lines for lists and templates.
- Check the summary beside it for words, tokens, cost per 1,000 calls, variables and the ten checks.
- To see how the change affects real answers, press Test both in the Prompt Tester.
Changes
Read a prompt change before you ship it
A one-line edit can change how long the replies are, what format they come back in, and what each call costs. The diff shows the edit and measures what it does.
- Free, in your browser. Paste two versionsThe old prompt and the new one, with their system prompts if they have them.
- Free, in your browser. See every wordEach added and removed word is marked, so the one sentence that matters cannot hide.Instant
- Free, in your browser. What the change doesLengthWords added, removed and kept.CostInput tokens and cost per call, before and after.ScoreThe score out of 10, before and after.
- With your key. Explain itOptionally, ask an AI to say in plain words what the change will do to the answers.Optional
Why diff prompts
Prompts change all the time: a teammate tightens the wording, a new rule is added after a bad output, an example is swapped. Read side by side, two long versions look almost the same, and the one sentence that matters is easy to miss. A word-level diff puts every added and removed word in front of you, so a review takes seconds and nothing slips through.
Version Compare is also the quickest way to understand someone else's change before you test it. When you know what changed, you know what to look for in the outputs, and Prompt Tester can then run both versions on the same test cases.
How to read the summary
- Words and similarity. Words added and removed across the system prompt and the prompt. Similarity is the share of words the two versions have in common, counted regardless of order.
- Tokens and cost. An offline estimate of the input tokens for each version, usually within 10 to 15 percent for English prose, and the input cost per call and per 1,000 calls at the chosen model's list price. Output tokens depend on the reply, so they are not included.
- Variables.
{{placeholders}}that appear in one version but not the other. A removed variable usually means some input is no longer used; a new one means every caller has to supply it. - Structure. XML-style tags such as
<context>and Markdown headings that were added or removed. Structure changes tend to affect how reliably a model follows the format. - Score out of 10. The free, rule-based count of the ten checks from the Grader, for each version, with the checks the change gained or lost. It checks whether the prompt says things, not whether it says them well, so treat it as a checklist rather than a verdict.
Questions
Is my prompt uploaded anywhere?
No. The diff, the token estimate and the score all run in this browser. Your draft is kept in this browser's storage so it survives a reload. The only time text leaves the page is when you ask a model to explain the change, and then it goes straight from your browser to the provider you chose, with your own key.
Do I need an API key?
Not for the diff or the summary. A key is only needed for "Explain this change", which asks a model to describe the likely effect of the edit. Google AI Studio, OpenRouter and Groq all offer free keys.
What is the difference between words and lines?
Compare by words marks each changed word, which suits prose prompts where a paragraph is one long line. Compare by lines marks whole lines, which suits lists, code and templates where each line is a unit. Side by side keeps the two versions aligned line by line; Inline shows one text with removals and additions marked in place.
How does it handle very long prompts?
Long unchanged stretches are folded so the changes stay in view; select a fold to expand it. For very large inputs the comparison runs line by line first and then word by word inside the changed lines, which keeps the page responsive.
How do I share a comparison?
Share creates a link that carries both versions inside the link itself, after the # sign, which browsers never send to a server. Anyone with the link can open the same comparison. Long prompts make long links, and some chat apps cut very long links short.