About The Diff/Patch Tool (Unified Diff Parser + Apply Preview)

This free online Diff/Patch Tool applies a unified diff to pasted original text and shows the patched result in your browser. It is useful for previewing .patch and .diff changes, checking hunk context, and confirming what a patch would produce before touching a real file.

The tool validates hunk headers, context lines, deleted lines, added lines, and expected line counts. If the original text does not match the patch context exactly, it stops with a mismatch message instead of producing a misleading patched output.

How to Preview a Patch

1. Paste the original file content into Original text.
2. Paste the unified diff into Unified diff.
3. Click Apply Preview to validate the hunks and generate the patched output.
4. Use Copy Output when you need to move the preview result into another editor or review note.

Choosing Input Correctly

Original text:
- Paste the exact version the diff was generated from.
- Keep indentation, blank lines, and trailing content unchanged when testing a failing patch.

Unified diff:
- Include the hunk header lines such as @@ -1,3 +1,4 @@.
- File header lines like --- a/file and +++ b/file are accepted, but the preview applies the hunks to the single pasted text block.

Common Use Cases

• Preview a patch before applying it to a config file, script, or code snippet.
• Debug context mismatches when a command-line patch fails.
• Check small documentation, template, or generated-code changes without opening Git tooling.

Quick FAQ

What is a unified diff?
It is a patch format that shows removed and added lines with surrounding context so changes can be reviewed or applied.

Why does the patch fail with a context or delete mismatch?
The base text may not match the version used to create the patch, or the patch context may have been edited.

Does this edit my files?
No. It previews the result in the browser and does not write to your local files.

Can I test it without my own patch?
Yes. Load sample fills both inputs and generates a working patch preview.