Iris Warning: Unknown variable: BIOME_PALE_GARDEN

What the log looks like

java\.lang\.RuntimeException: Unknown variable: BIOME_PALE_GARDEN
Failed to resolve uniform inPaleGarden, reason: Unknown variable: BIOME_PALE_GARDEN

How to fix it

1. If your game is still running, safely ignore this message—Iris automatically skips the unsupported Pale Garden uniform and continues rendering normally.
2. If your game crashed, look further down in latest.log or open your crash-reports folder to find the actual FATAL error, because this warning is not the cause of a crash.
3. To silence the log spam on Minecraft 1.21.1, switch to an older version of your shaderpack released before Minecraft 1.21.4, or edit the shaderpack's shaders.properties file to remove the custom.uniform.inPaleGarden line.

Check your own log