What is the XDELTA file format?
An .xdelta file is a binary patch created by xdelta, an open-source delta compression tool and C library started by Joshua P. MacDonald. Instead of holding a whole file, it stores only the differences between an original file and a modified version. Because two similar files usually differ in just a few places, the patch is far smaller than the files themselves, which makes .xdelta a compact way to ship changes.
Modern xdelta (the xdelta3 line) writes patches in the VCDIFF format defined by RFC 3284. A VCDIFF stream begins with the bytes V, C, D with their high bits set (hex D6 C3 C4) followed by a version byte. Inside, the patch is a series of copy, add, and run instructions that tell xdelta how to rebuild the target from the source. You will meet .xdelta files most often in game modding and ROM translations, where authors distribute a small patch rather than a copyrighted original.
Security & safety
RISK: MEDIUMA .xdelta file is passive data and cannot run on its own. The real risk is what it patches: applying a patch produces a new file (often a game ROM, ISO, or executable) whose safety depends on the patch author. Only apply patches to the correct, expected original file and from sources you trust, since a malicious patch could turn a clean file into a modified executable. Applying a patch to the wrong source usually just fails a checksum rather than corrupting anything.
Format details
in a nutshell- xdelta1 legacy patches - Files from the original 1997-era xdelta (version 1) also used the .xdelta name but a different, non-VCDIFF on-disk layout; they are not compatible with xdelta3.
Programs that open XDELTA files
Windows3 apps
xdelta3 (command line)
Open-source Run xdelta3 -d -s original.bin patch.xdelta output.bin in a terminal to apply the patch against the original file.
Delta Patcher
Open-source Open the app, pick the original file and the .xdelta patch, then click Apply Patch to produce the patched file. It is self-contained and does not need a separate xdelta binary.
xdelta UI
Free Load the original file and the .xdelta patch in the Apply tab to generate the patched output; commonly used for ROM patches. 
macOS2 apps
xdelta3 (command line)
Open-source Install with brew install xdelta, then run xdelta3 -d -s original patch.xdelta output to apply the patch.
Delta Patcher
Open-source Use the cross-platform GUI to select the source file and .xdelta patch and apply it without touching a terminal. 
Linux2 apps
xdelta3 (command line)
Open-source Install the xdelta3 package from your distro, then run xdelta3 -d -s original patch.xdelta output to rebuild the patched file.
Delta Patcher
Open-source Run the wxWidgets GUI to apply or create .xdelta patches with a graphical interface.
Xdelta patcher (WebAssembly)
Open-source Drag the original file and the .xdelta patch into the browser page to apply the patch locally without installing anything. Technical details
deep spec| Encoding | Binary VCDIFF (RFC 3284) delta stream with COPY, ADD and RUN instructions referencing a source file |
| Byte order | Endian-independent by design; integers are stored as variable-length, byte-order-neutral values |
| Container | VCDIFF window/section layout (header, then one or more windows each with instruction, data and address sections) |
| Compression | Secondary compression selectable at encode time: djw (default in many builds), lzma, fgk, or none |
| Typical size | Small relative to the target file; often a tiny fraction of the original when the two files are similar |
| Structure | A .xdelta file stores the difference between a source file and a target file. It is not a standalone document: applying it requires the exact original (source) file, from which xdelta reconstructs the target byte-for-byte. |
| Integrity | xdelta3 can embed and verify source and output checksums (Adler-32, and BLAKE3 in recent 3.2.x builds) to confirm the correct source file and a correct reconstruction |
| Platforms | Windows, macOS, Linux, BSD |
| Notes | Created by the xdelta command-line tool and C library. Widely used to distribute game translations and mods (ROM/ISO patches) because delta encoding accounts for shifted data and avoids embedding copyrighted source bytes. |
| Released | 1997 (xdelta1); xdelta3 series in the 2000s |
| Latest version | xdelta3 3.2.x |
| Open standard | Yes · royalty-free |
| Specification | datatracker.ietf.org |
XDELTA conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about XDELTA files.
Frequently asked questions
How do I open an .xdelta file?
xdelta3 command line (xdelta3 -d -s original patch.xdelta output) or a GUI such as Delta Patcher or xdelta UI, which ask for the source file and the patch.Why won't my .xdelta patch apply?
What program creates .xdelta files?
xdelta3 -e -s original modified patch.xdelta. GUI tools like Delta Patcher can also generate them by diffing two files you provide.Is an .xdelta file the same as a VCDIFF file?
.xdelta and a .vcdiff made by xdelta3 hold the same kind of data. The extension is just a naming choice.Are .xdelta files safe?
Can I see what is inside an .xdelta file?
xdelta3 printhdr patch.xdelta or xdelta3 -v, but the meaningful result only appears after you apply it to the source file.
