Text Diff
Compare two texts line by line and highlight added and removed lines.
Compare two text versions quickly
Paste the original text on the left and the modified text on the right, then click Compare. Added lines are shown in green, removed lines are shown in red, and unchanged lines stay neutral so you can scan the real change without rereading both versions.
This is useful for documentation edits, changelog drafts, support messages, prompt versions, config snippets, logs, and generated text where a small change can alter meaning.
Common text diff workflows
For Markdown or docs, compare the draft before and after an edit to catch accidental deletions. For logs or config files, compare the working version against a failing version to spot changed flags, missing environment variables, or reordered lines.
For API responses or JSON-like text, format both sides first so keys and arrays appear on stable lines. A clean line structure makes the diff more useful and reduces noisy false changes.
How to read the result
A green line means the line exists only in the modified text. A red line means the line existed only in the original text. Neutral lines exist in both versions and provide context around the change.
If one long paragraph appears as a single changed line, split it into shorter lines before comparing. Line-based diffs work best when related ideas or fields are already separated.
Private browser-side comparison
The comparison runs locally in your browser. FastDevTool does not upload the text you paste, and the result is computed with client-side JavaScript. You can use it for private drafts, internal notes, and sensitive snippets without creating an account.
Related Tools
Frequently Asked Questions
What is a text diff tool used for?
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A text diff tool compares two versions of plain text and highlights lines that were added, removed, or left unchanged. It is useful for reviewing notes, documentation, configuration snippets, API responses, prompts, support replies, and any text where small edits are easy to miss.
Does this compare words or lines?
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This tool compares text line by line. Line diff is usually the clearest view for config files, Markdown, logs, and multi-line notes. If one word changes inside a line, the whole line is marked as changed so you can inspect it in context.
Can I compare code or JSON here?
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Yes, you can compare code snippets, JSON, YAML, environment variables, logs, or any other plain text. For structured JSON, use JSON Formatter first if you want both sides consistently indented before comparing.
Is my data private?
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Yes. The diff runs entirely in your browser with JavaScript. Your text is not uploaded to FastDevTool, stored in a database, or sent to a server, which makes it suitable for sensitive drafts and internal snippets.
Why does the diff show more changes than I expected?
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Whitespace, line breaks, indentation, and reordered lines can all affect a line-by-line diff. If the output looks noisy, normalize formatting first, remove trailing spaces, or split long paragraphs into shorter lines before comparing.