Why does my additional vertex stream not affect the shader?

The auxiliary mesh assigned to additionalVertexStreams must have the same vertex count as the source. Mismatch silently disables the stream. Check both meshes' vertexCount and rebuild if they diverge.

What channel should I write to?

UV1 (TEXCOORD1) and UV2 (TEXCOORD2) are common; the shader reads via NORMAL or TEXCOORDn semantic. The aux mesh writes only the channels you set; others are zero.

Should I bake bake into the source mesh instead?

For static data, yes — baking into the source mesh's UV channel avoids the matching problem entirely. additionalVertexStreams is for runtime-varying per-instance vertex data.

Fix: Unity MeshRenderer Additional Vertex Streams Mismatch

Quick answer: The aux mesh on additionalVertexStreams must have identical vertex count to the source mesh. Set only the channels you intend to override. Shader reads them via the same TEXCOORDn semantic.

You bake per-instance wind weights into a UV2 stream and assign via additionalVertexStreams. Shader reads zero — aux mesh has 100 verts, source has 200.

The Symptom

additionalVertexStreams assignment silently fails. Shader reads default (zero) values. No error in console.

The Fix

var aux = new Mesh();
aux.SetVertices(sourceMesh.vertices);   // must match count + order
aux.uv2 = customWeights;                // length = sourceMesh.vertexCount

renderer.additionalVertexStreams = aux;

Vertex count and order must match. Set only the channels (uv2, colors, etc.) you want to override; others stay at the source values.

Shader Declaration

// Input semantic in vertex stage
struct Attributes
{
    float4 positionOS : POSITION;
    float2 uv2        : TEXCOORD1;   // reads aux.uv2
};

Verifying

Pick a known vertex. Read its uv2 in the shader and visualize. Should match what you wrote into the aux mesh. If zero, vertex count or channel mismatch.

“Match vertex count. Right channel. Shader reads.”

Related Issues

For mesh deformer job, see deformer. For CombineMeshes, see CombineMeshes.

Vertex count match. Channel set. Shader sees.