People searching for an xdelta patcher usually hold a translation or a disc-sized diff, not a tiny IPS. VCDIFF is the IETF document RFC 3284 from June 2002, written by Korn, MacDonald, Mogul, and Vo. xdelta3 is the tool that writes the streams most readmes mean. Its repository describes itself as open-source binary diff and VCDIFF software. The release I recorded is v3.2.2 dated 9 Oct 2026. The command-line switch -S accepts lzma, fgk, djw, or none. In the source I read, secondary compression is compiled on when LZMA is present.
This website applies those streams with the VCDIFF module from Rom Patcher JS at commit 4f17297, dated 21 Oct 2026, whose message is that LZMA secondary decoding was added. The decoder recognizes secondary id 1 as DJW static Huffman and id 2 as LZMA. Any other id raises the exact text not implemented: secondary decompressor. That string is the whole point of the separate error guide. Older hosted copies of the same engine still show the message because they predate that commit. I pinned the commit instead of the v3.2.1 tag from 4 Nov 2025, and I do not treat the v3.5 label on marcrobledo.com as a GitHub release.
Size is the other half of the decision. Browser capacity depends on the device and on how much input, patch, and output data the decoder holds at once. Nintendo DS translation patches can target large cartridge images. GameCube images are about 1.4 GB. PS2 disc-image sizes depend on the game and dump layout. I will not print a browser maximum, because no specification states one. If the tab stalls, the honest next step is Delta Patcher or xdelta3 itself, which can stream. Rebuilding the patch with -S none removes the secondary compressor so an older decoder can read it. That is a new patch from the author, not a setting inside the browser.
xdelta-wasm is a second engine, Apache-2.0, aimed at the same online job and chunked work. I have not swapped it in as the default. The module on this site does not implement build-from-roms, so you cannot create an xdelta here. You can apply one. Related reading is the DS guide, the PS2 guide, and the GameCube guide, plus the secondary-decompressor fix.
