"CORS Error" During Modrinth Download — What It Means and How to Fix It

The error

You see a message like: "Access to fetch at ... has been blocked by CORS policy." The conversion stops or skips the affected mod.

What CORS is

CORS (Cross-Origin Resource Sharing) is a browser security rule that prevents websites from fetching arbitrary data from other domains. Modrinth's CDN explicitly permits cross-origin downloads, but some third-party mod hosts do not.

How the converter handles it

The converter first tries a direct fetch. If CORS blocks it, the request is routed through a public CORS-relaxing proxy as a fallback. This is slower but works in most cases.

If the fallback also fails

Option A — Manually download the offending mod

The converter shows which URL was blocked. Open it in a new tab, save the JAR, and add it manually to your converted ZIP's mods/ folder.

Option B — Use a desktop launcher with native .mrpack support

Prism Launcher, the Modrinth App, and ATLauncher do not have CORS restrictions because they are not browsers. If a particular mod refuses to convert in the browser, use a native launcher instead.