Prompt Diff / A-B Tester
Prompt A
Prompt B
A↔B
Enter text in both prompts to see differences
Additions shown in green · Deletions in red with strikethrough
About
The Prompt Diff / A-B Tester shows a word-level diff between two prompts. Enter Prompt A and Prompt B, and the tool highlights every word that was added (green background) or removed (red strikethrough). Use it to understand exactly what changed between prompt iterations, compare different phrasings, or audit edits made to a system prompt.
How to use
- 1 Type or paste your first prompt into the "Prompt A" textarea.
- 2 Type or paste your second prompt into the "Prompt B" textarea.
- 3 The diff panel below updates instantly — green highlights show additions, red shows removals.
- 4 The stats bar shows character count, word count, and approximate token count for both prompts with delta values.
- 5 Click "Clear both" to reset.
- What level does the diff operate at?
- The diff is word-level — it splits text by whitespace and computes the longest common subsequence (LCS) between the two word lists. This produces a readable diff that shows meaningful changes. Character-level diffing can be noisy for prose; word-level is better for prompt comparison.
- How is token count estimated?
- Token count is approximated as character count ÷ 4, which is accurate for typical English prose. For exact token counts, use the Token Counter tool.
- Can I use this to compare system prompt versions?
- Yes — paste the old version in Prompt A and the new version in Prompt B. The diff makes it immediately clear what rules, constraints, or instructions were added, removed, or reworded. This is especially useful when debugging changed model behavior.