Recognize an unreadable file
Messages such as invalid mod file, failed to load mod, ZIP end header not found, or error reading a JAR strongly indicate a damaged or incorrect file. The evidence line usually includes a path or filename that should be checked first.
A valid JAR can still be the wrong kind of JAR. Server plugins, installers, source archives and API documentation packages do not become mods merely because they use the same extension.
Download a clean copy safely
Remove the named file from the active instance and download it again from the author's official CurseForge, Modrinth or project release page. Confirm the file size is plausible and that the selected entry targets your Minecraft version and loader.
Do not use generic re-upload sites. Besides licensing and security concerns, mirrors may rename files, provide obsolete builds or wrap downloads in installers that are not Minecraft mods.
Do not unzip normal mod JARs
Most loader mods belong in the mods folder as downloaded. Extracting their contents creates folders the loader may ignore. The main exceptions are packages whose authors explicitly instruct you to unpack and select files, so follow the project's installation notes.
If many unrelated JARs suddenly report corruption, check disk health, antivirus quarantine and incomplete cloud synchronization instead of redownloading the entire pack repeatedly.
Quick troubleshooting checklist
- ✓Identify the exact unreadable filename
- ✓Remove it from the active instance
- ✓Use the official project source
- ✓Match game version and loader
- ✓Leave a normal mod JAR unextracted
- ✓Investigate storage if many files fail
Every launch creates a new result. After making one change, use the newly generated latest.log rather than continuing from an older report.