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Roslyn Script Validation

By default, MCP for Unity uses a fast structural validator for scripts the LLM generates. For strict validation that catches undefined namespaces, types, and methods at write time — without a full Unity compile — install the optional Roslyn DLLs.

Most users don't need this. Enable it when:

  • You're letting the LLM write a lot of unsupervised C# and want stricter feedback loops
  • You're seeing recurring compile errors that survive the structural validator
  • You're building custom tools that depend on accurate symbol resolution
  1. Open Window → MCP for Unity.
  2. Scroll to the Runtime Code Execution (Roslyn) section in the Scripts / Validation tab.
  3. Click Install Roslyn DLLs.

The installer downloads the required NuGet packages, places the DLLs in Assets/Plugins/Roslyn/, and adds USE_ROSLYN to Scripting Define Symbols.

You can also trigger it from the menu: Window → MCP For Unity → Install Roslyn DLLs.

Manual install (if the installer isn't available)​

  1. Install NuGetForUnity.
  2. Open Window → NuGet Package Manager.
  3. Install:
    • Microsoft.CodeAnalysis v5.0
    • SQLitePCLRaw.core v3.0.2
    • SQLitePCLRaw.bundle_e_sqlite3 v3.0.2
  4. Add USE_ROSLYN to Player Settings → Scripting Define Symbols.
  5. Restart Unity.

Manual DLL install (no NuGetForUnity)​

  1. Download Microsoft.CodeAnalysis.CSharp.dll and its dependencies from NuGet.org.
  2. Place DLLs in Assets/Plugins/Roslyn/.
  3. Ensure .NET compatibility settings are correct for your Unity version.
  4. Add USE_ROSLYN to Scripting Define Symbols.
  5. Restart Unity.

Verifying it's active​

After restart, the MCP for Unity status panel shows Roslyn: enabled under the Scripts section. The validate_script tool now performs full semantic analysis rather than the structural pass.

Disabling​

Remove USE_ROSLYN from Scripting Define Symbols. The plugin falls back to structural validation; the DLLs can stay in Assets/Plugins/Roslyn/ or be removed.