[Bug]: execute_code tool fails with BOM character (U+FEFF) in code parameter

[stanLy490]

What happened?

Description

The execute_code tool consistently fails with CodeDom compilation error when the code parameter contains a UTF-8 BOM character (U+FEFF, ) at the beginning.

Error

{
  "success": false,
  "message": "Compilation failed",
  "data": {
    "errors": ["Line 1: "],
    "compiler": "codedom"
  }
}
Steps to Reproduce
Call execute_code with any valid C# code:
{
  "action": "execute",
  "code": "return \"hello\";"
}
Observe compilation failure with BOM error
Expected Behavior
Code should compile and execute successfully. The BOM character should be stripped before compilation.

Root Cause
The code parameter arrives with a leading U+FEFF (BOM) character. This likely occurs during JSON serialization/deserialization in the MCP protocol layer. The ExecuteCode.HandleExecute method does not strip BOM before passing the code to WrapUserCode → CodeDomCompile.

Suggested Fix
In ExecuteCode.cs, add BOM stripping at the entry point:

string code = @params["code"]?.ToString()?.TrimStart('\uFEFF');
Environment
MCP for Unity: main branch (78ee541841)
Unity Version: 2022.3.x (URP)
OS: Windows 10
MCP Client: opencode with stdio transport

### Reproduction steps

## Reproduction Steps

1. Open Window > MCP for Unity
2. Click Start Server
3. In MCP client, call execute_code tool with any valid C# code:
   ```json
   { "action": "execute", "code": "return \"hello\";" }

### Unity version

2022.3.8f1

### MCP for Unity package version

v9.7.1

### Python server version

_No response_

### MCP client

Other (specify in description)

### Transport

stdio

### OS

Windows

### Relevant logs / console output

```text

Checks

• I searched existing issues and did not find a duplicate
• I included logs / steps to reproduce

[lgarczyn]

Ran into this on Unity 2022.3.62f2 (Windows/Mono) and dug into ExecuteCode.cs. It's not a BOM in the code param, so TrimStart('\uFEFF') won't fix it.

The compile actually succeeds — mcs returns 0 and writes the /out: DLL. The problem is mcs prints a lone U+FEFF line on stdout, and Mono's CodeDom can't parse it as file(line,col): error CSxxxx, so it turns it into a phantom CompilerError (no ErrorNumber, no FileName, Line 0, ErrorText is just the BOM). That trips HasErrors, CodeDom leaves CompiledAssembly null, and execute_code reports a failure for a build that was fine.

Trace for return 1 + 1;:

exit=0  asmPath=  asmNull=True  errCount=1
error: file=  line=0  col=0  num=  warn=False  text=

The input code is clean here (first chars 72,65,74,75,72,6E = "retur…"), so there's nothing to strip on that side.

Two changes fixed it: ignore the phantom error (no ErrorNumber and ErrorText is only BOM/whitespace), and compile to a known path (GenerateInMemory=false + OutputAssembly) so you can load the DLL yourself when there are no real errors, rather than relying on results.CompiledAssembly:

foreach (CompilerError error in results.Errors)
{
    if (error.IsWarning) continue;
    string text = (error.ErrorText ?? "").Trim().Trim('\uFEFF').Trim();
    if (string.IsNullOrEmpty(error.ErrorNumber) && text.Length == 0) continue; // stray mcs BOM line
    int userLine = Math.Max(1, error.Line - WrapperLineOffset);
    errors.Add($"Line {userLine}: {error.ErrorText}");
}
if (errors.Count > 0) return null;
if (File.Exists(outputAssemblyPath)) return Assembly.Load(File.ReadAllBytes(outputAssemblyPath));
return results.CompiledAssembly;

Env: Unity 2022.3.62f2, Windows, stdio, codedom.

[lgarczyn]

fixed it

Index: Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs
--- a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs	(revision 6d7882f13a728b5fedf6f5c867ecdfb2273bfeb6)
+++ b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs	(date 1780530186224)
@@ -294,32 +294,58 @@
                     }
                 }
 
+                // Compile to a controlled DLL path instead of in-memory. mcs prints a stray BOM line on
+                // stdout that Mono's CodeDom can't parse, so it fabricates a bogus error (no error number,
+                // text is just the BOM) and refuses to surface the assembly — even though mcs exits 0 and
+                // wrote the DLL. We skip that bogus error and Assembly.Load the produced DLL ourselves.
+                string outputAssemblyPath = Path.Combine(Path.GetTempPath(), $"mcp-codedom-{Guid.NewGuid():N}.dll");
                 using (var provider = new CSharpCodeProvider())
                 {
                     var parameters = new CompilerParameters
                     {
-                        GenerateInMemory = true,
+                        GenerateInMemory = false,
+                        OutputAssembly = outputAssemblyPath,
                         GenerateExecutable = false,
                         TreatWarningsAsErrors = false,
                         CompilerOptions = "@\"" + responseFilePath + "\"",
                     };
 
-                    var results = provider.CompileAssemblyFromSource(parameters, source);
+                    try
+                    {
+                        var results = provider.CompileAssemblyFromSource(parameters, source);
 
-                    if (results.Errors.HasErrors)
-                    {
+                        bool hasRealErrors = false;
                         foreach (CompilerError error in results.Errors)
                         {
-                            if (!error.IsWarning)
-                            {
-                                int userLine = Math.Max(1, error.Line - WrapperLineOffset);
-                                errors.Add($"Line {userLine}: {error.ErrorText}");
-                            }
+                            if (error.IsWarning)
+                                continue;
+
+                            // The bogus BOM "error": no error number, text is just the BOM/whitespace.
+                            string text = error.ErrorText?.Trim('', ' ', '\t', '\r', '\n');
+                            if (string.IsNullOrEmpty(error.ErrorNumber) && string.IsNullOrEmpty(text))
+                                continue;
+
+                            hasRealErrors = true;
+                            int userLine = Math.Max(1, error.Line - WrapperLineOffset);
+                            errors.Add($"Line {userLine}: {error.ErrorText}");
                         }
-                        return null;
-                    }
+
+                        if (hasRealErrors)
+                            return null;
+
+                        if (!File.Exists(outputAssemblyPath))
+                        {
+                            errors.Add("CodeDom reported success but produced no assembly.");
+                            return null;
+                        }
 
-                    return results.CompiledAssembly;
+                        return Assembly.Load(File.ReadAllBytes(outputAssemblyPath));
+                    }
+                    finally
+                    {
+                        try { if (File.Exists(outputAssemblyPath)) File.Delete(outputAssemblyPath); }
+                        catch { /* best effort */ }
+                    }
                 }
             }
             finally

[stanLy490]

You are the BEST! Thank you for everything~ 漫画家 ***@***.*** 原始邮件 发件人：Lou Garczynski ***@***.***&gt; 发件时间：2026年6月4日 19:51 收件人：CoplayDev/unity-mcp ***@***.***&gt; 抄送：stanLy490 ***@***.***&gt;, Author ***@***.***&gt; 主题：Re: [CoplayDev/unity-mcp] [Bug]: execute_code tool fails with BOM character (U+FEFF) in code parameter (Issue #1186) lgarczyn left a comment (CoplayDev/unity-mcp#1186) fixed it Index: Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs IDEA additional info: Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP <+&gt;UTF-8 =================================================================== diff --git a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs --- a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs (revision 6d7882f13a728b5fedf6f5c867ecdfb2273bfeb6) +++ b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs (date 1780530186224) @@ -294,32 +294,58 @@ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// Compile to a controlled DLL path instead of in-memory. mcs prints a stray BOM line on + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// stdout that Mono's CodeDom can't parse, so it fabricates a bogus error (no error number, + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// text is just the BOM) and refuses to surface the assembly — even though mcs exits 0 and + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// wrote the DLL. We skip that bogus error and Assembly.Load the produced DLL ourselves. + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;string outputAssemblyPath = Path.Combine(Path.GetTempPath(), $"mcp-codedom-{Guid.NewGuid():N}.dll"); &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; using (var provider = new CSharpCodeProvider()) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; { &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; var parameters = new CompilerParameters &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; { - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;GenerateInMemory = true, + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;GenerateInMemory = false, + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;OutputAssembly = outputAssemblyPath, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; GenerateExecutable = false, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; TreatWarningsAsErrors = false, &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; CompilerOptions = "@\"" + responseFilePath + "\"", &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }; - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;var results = provider.CompileAssemblyFromSource(parameters, source); + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;try + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;var results = provider.CompileAssemblyFromSource(parameters, source); - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (results.Errors.HasErrors) - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;bool hasRealErrors = false; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; foreach (CompilerError error in results.Errors) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; { - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (!error.IsWarning) - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{ - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;int userLine = Math.Max(1, error.Line - WrapperLineOffset); - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;errors.Add($"Line {userLine}: {error.ErrorText}"); - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (error.IsWarning) + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;continue; + + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// The bogus BOM "error": no error number, text is just the BOM/whitespace. + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;string text = error.ErrorText?.Trim('', ' ', '\t', '\r', '\n'); + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (string.IsNullOrEmpty(error.ErrorNumber) &amp;&amp; string.IsNullOrEmpty(text)) + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;continue; + + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;hasRealErrors = true; + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;int userLine = Math.Max(1, error.Line - WrapperLineOffset); + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;errors.Add($"Line {userLine}: {error.ErrorText}"); &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return null; - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} + + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (hasRealErrors) + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return null; + + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if (!File.Exists(outputAssemblyPath)) + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;errors.Add("CodeDom reported success but produced no assembly."); + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return null; + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} - &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return results.CompiledAssembly; + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return Assembly.Load(File.ReadAllBytes(outputAssemblyPath)); + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;finally + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{ + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;try { if (File.Exists(outputAssemblyPath)) File.Delete(outputAssemblyPath); } + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;catch { /* best effort */ } + &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; finally — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.

[Huaian666]

the fact that this works offline is huge for our use case. most competitors need constant internet for no reason.

[Scriptwonder]

fixed it

Index: Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8

diff --git a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs
--- a/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs (revision 6d7882f13a728b5fedf6f5c867ecdfb2273bfeb6)
+++ b/Packages/com.coplaydev.unity-mcp/Editor/Tools/ExecuteCode.cs (date 1780530186224)
@@ -294,32 +294,58 @@
}
}

•            // Compile to a controlled DLL path instead of in-memory. mcs prints a stray BOM line on
  

•            // stdout that Mono's CodeDom can't parse, so it fabricates a bogus error (no error number,
  

•            // text is just the BOM) and refuses to surface the assembly — even though mcs exits 0 and
  

•            // wrote the DLL. We skip that bogus error and Assembly.Load the produced DLL ourselves.
  

•            string outputAssemblyPath = Path.Combine(Path.GetTempPath(), $"mcp-codedom-{Guid.NewGuid():N}.dll");
             using (var provider = new CSharpCodeProvider())
             {
                 var parameters = new CompilerParameters
                 {
  

•                    GenerateInMemory = true,
  

•                    GenerateInMemory = false,
  

•                    OutputAssembly = outputAssemblyPath,
                     GenerateExecutable = false,
                     TreatWarningsAsErrors = false,
                     CompilerOptions = "@\"" + responseFilePath + "\"",
                 };
  

•                var results = provider.CompileAssemblyFromSource(parameters, source);
  

•                try
  

•                {
  

•                    var results = provider.CompileAssemblyFromSource(parameters, source);
  

•                if (results.Errors.HasErrors)
  

•                {
  

•                    bool hasRealErrors = false;
                     foreach (CompilerError error in results.Errors)
                     {
  

•                        if (!error.IsWarning)
  

•                        {
  

•                            int userLine = Math.Max(1, error.Line - WrapperLineOffset);
  

•                            errors.Add($"Line {userLine}: {error.ErrorText}");
  

•                        }
  

•                        if (error.IsWarning)
  

•                            continue;
  

•                        // The bogus BOM "error": no error number, text is just the BOM/whitespace.
  

•                        string text = error.ErrorText?.Trim('', ' ', '\t', '\r', '\n');
  

•                        if (string.IsNullOrEmpty(error.ErrorNumber) && string.IsNullOrEmpty(text))
  

•                            continue;
  

•                        hasRealErrors = true;
  

•                        int userLine = Math.Max(1, error.Line - WrapperLineOffset);
  

•                        errors.Add($"Line {userLine}: {error.ErrorText}");
                     }
  

•                    return null;
  

•                }
  

•                    if (hasRealErrors)
  

•                        return null;
  

•                    if (!File.Exists(outputAssemblyPath))
  

•                    {
  

•                        errors.Add("CodeDom reported success but produced no assembly.");
  

•                        return null;
  

•                    }
  

•                return results.CompiledAssembly;
  

•                    return Assembly.Load(File.ReadAllBytes(outputAssemblyPath));
  

•                }
  

•                finally
  

•                {
  

•                    try { if (File.Exists(outputAssemblyPath)) File.Delete(outputAssemblyPath); }
  

•                    catch { /* best effort */ }
  

•                }
             }
         }
         finally
  

Hi, thanks for the error fixing, can you submit a PR for this? Would love to review and update it

[lgarczyn]

Sure!