CompletionException: RuntimeException: could not reload shaders

What the log looks like

java\.util\.concurrent\.CompletionException:\s*java\.lang\.RuntimeException:\s*could not reload shaders
java\.lang\.RuntimeException:\s*could not reload shaders

How to fix it

1. Open logs/latest.log and scroll below 'could not reload shaders' to find the 'Caused by:' line (typically ShaderParseException or GlException), which identifies the exact shader JSON, .vsh, or .fsh file that failed.
2. Disable resource packs that include custom core shaders (often custom UI, font/text, or visual effect packs) and reload resources with F3+T.
3. If the game crashes on startup before you can open the Resource Packs menu, open options.txt in your Minecraft instance folder, change the resourcePacks line to resourcePacks:[], and save the file.
4. If no resource packs are active and you use Sodium or Iris, check for incompatible versions of shader-injecting mods such as Immersive Portals or Physics Mod and disable or update them to versions matching your Sodium release.

Check your own log