What causes a WebGL shader link error from too many varyings?

The vertex shader declares more varying vectors than the device's MAX_VARYING_VECTORS limit allows, so the program fails to link on that hardware.

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 Shader Link Error From Too Many Varyings

Quick answer: Pack multiple values into fewer vec4 varyings, move constants out of varyings, and query MAX_VARYING_VECTORS to stay within the device limit.

WebGL passes interpolated data from vertex to fragment shader as varyings, and each device caps how many vec4-equivalents you can use. Exceeding it links fine on your desktop but fails on a phone. Here is how to fix it.

How to fix it

1. Pack values into vec4s

Combine separate floats and vec2s into shared vec4 varyings (for example pack UV and a fog factor together) to cut the varying vector count.

2. Move non-interpolated data to uniforms

Values that do not change across the triangle belong in uniforms, not varyings; this frees interpolator slots for data that genuinely varies.

3. Query the device limit

Read gl.getParameter(gl.MAX_VARYING_VECTORS) at startup and pick a shader variant that fits; low-end mobile GPUs often allow only 8.

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.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.