What It Means & How to Fix It in UE5

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🧠 Why You’re Seeing This Error

This error means Unreal’s shader compiler is trying to compile a shader and looking for a function named MainVS, but it doesn’t exist in the shader file being compiled.

In UE5, this usually happens when:

• You didn’t define the MainVS() function in your .usf file
• You used a different name for the entry point
• There’s a typo in the IMPLEMENT_SHADER_TYPE macro
• The function is hidden behind a #define or #if block

In short: Unreal is told to compile MainVS, but it can’t find it anywhere in your shader code.

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💥 Example Error Message

error X3501: 'MainVS': entrypoint not found

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🛠️ Common Shader Scenarios That Cause This

❌ Missing the Function Altogether

// File has no MainVS() defined
// Unreal expected: void MainVS(...) { ... }

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❌ Typo or Wrong Name

hlslCopyEditvoid mainVS() { } // ← Wrong case: should be MainVS

// Or maybe:
void MyCustomVertexShader() { } // ← Doesn’t match what C++ expects

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❌ Hidden Behind a Preprocessor Directive

#ifdef USE_MY_SHADER
void MainVS(...) { ... }
#endif

// If USE_MY_SHADER is undefined → MainVS doesn't exist at compile time

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❌ Wrong Shader Stage or Entry Name in C++ Macro

IMPLEMENT_SHADER_TYPE(, FMyShader, TEXT("/Shaders/MyShader.usf"), TEXT("MainVS"), SF_Vertex);

But your .usf file only has:

void VertexMain(...) { ... }

Unreal is looking for MainVS, but only finds VertexMain.

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✅ How to Fix X3501 in UE5 – Step-by-Step

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✔️ 1. Define a Function Named Exactly as Expected

If your shader is registered with "MainVS" as the entry point, make sure your .usf file contains:

hlslCopyEditvoid MainVS(...) {
    // shader logic
}

Match the spelling and casing exactly.

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✔️ 2. Match Entry Name in C++ Shader Registration

In your shader registration macro:

IMPLEMENT_SHADER_TYPE(, FMyVertexShader, TEXT("/Shaders/MyShader.usf"), TEXT("MainVS"), SF_Vertex);

Make sure "MainVS" matches the actual function in the shader file.

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✔️ 3. Watch for Conditional Compilation Hiding the Entry Point

If your function is inside a macro block:

#if ENABLE_MY_SHADER
void MainVS(...) { ... }
#endif

Make sure ENABLE_MY_SHADER is defined when compiling. Otherwise, MainVS doesn’t exist during compile time.

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✔️ 4. Rebuild Shader Cache and Flush If Needed

Sometimes even after you fix the code, Unreal may cache the old version.

basDelete DerivedDataCache/
Restart Editor

Or use:

Console Command: ShaderCompileWorker -clean

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✅ Summary: How to Fix “error X3501: ‘MainVS’: entrypoint not found” in UE5

Cause	Fix Example
Function MainVS() missing	Define it in your shader file exactly as expected
Mismatch in macro vs function name	Sync IMPLEMENT_SHADER_TYPE with your shader function
Preprocessor hiding the function	Ensure relevant #defines are active during compile
Wrong casing	Use exact name: MainVS, not mainvs or vertexMain