What causes a WebGL fragment shader missing a precision qualifier?

The fragment shader has no default float precision declared, which desktop drivers tolerate but mobile WebGL requires, so compilation fails on phones.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a WebGL Fragment Shader Failing on Mobile From a Missing Precision Qualifier

Quick answer: Add a precision statement such as precision mediump float; at the top of every fragment shader so it compiles across all WebGL implementations.

Desktop GLSL ES drivers often assume a default float precision, but the spec requires fragment shaders to declare one. Mobile drivers enforce this strictly, so a shader that works on your PC fails on devices. Here is how to fix it.

How to fix it

1. Declare a default precision

Add precision mediump float; (or highp where needed) as the first line of every fragment shader so all float variables have a defined precision.

2. Use highp for position-like math

For values needing range and accuracy such as world positions or large UV offsets, declare those specific variables highp while keeping color math at mediump for speed.

3. Test compile logs on device

Check gl.getShaderInfoLog on a real device; the desktop browser will not reproduce the strict mobile precision requirement.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every HTML5 error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Ship the fix, watch the signature disappear from the next build. That's how you know it's really gone.