fix(roslyn): install missing System.Runtime.CompilerServices.Unsafe v6 + surface inner errors

MCPForUnity/Editor/Setup/RoslynInstaller.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.IO.Compression;
+using System.Reflection;
 using UnityEditor;
 using UnityEngine;
 using UnityEngine.Networking;
@@ -13,19 +14,44 @@ public static class RoslynInstaller
 
         private static readonly (string packageId, string version, string dllPath, string dllName)[] NuGetEntries =
         {
-            ("microsoft.codeanalysis.common",    "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.dll",       "Microsoft.CodeAnalysis.dll"),
-            ("microsoft.codeanalysis.csharp",    "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.dll","Microsoft.CodeAnalysis.CSharp.dll"),
-            ("system.collections.immutable",     "8.0.0",  "lib/netstandard2.0/System.Collections.Immutable.dll", "System.Collections.Immutable.dll"),
-            ("system.reflection.metadata",       "8.0.0",  "lib/netstandard2.0/System.Reflection.Metadata.dll",   "System.Reflection.Metadata.dll"),
+            ("microsoft.codeanalysis.common",         "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.dll",                   "Microsoft.CodeAnalysis.dll"),
+            ("microsoft.codeanalysis.csharp",         "4.12.0", "lib/netstandard2.0/Microsoft.CodeAnalysis.CSharp.dll",            "Microsoft.CodeAnalysis.CSharp.dll"),
+            ("system.collections.immutable",          "8.0.0",  "lib/netstandard2.0/System.Collections.Immutable.dll",             "System.Collections.Immutable.dll"),
+            ("system.reflection.metadata",            "8.0.0",  "lib/netstandard2.0/System.Reflection.Metadata.dll",               "System.Reflection.Metadata.dll"),
+            // Transitive dep of Microsoft.CodeAnalysis.* on netstandard2.0. Without it, Roslyn's StringTable
+            // static cctor throws FileNotFoundException for v6.0.0.0 and every Roslyn entry point fails to
+            // initialize. Unity ships a v4.x of this assembly which does NOT satisfy the v6 reference.
+            ("system.runtime.compilerservices.unsafe","6.0.0",  "lib/netstandard2.0/System.Runtime.CompilerServices.Unsafe.dll",   "System.Runtime.CompilerServices.Unsafe.dll"),
         };
 
         public static bool IsInstalled()
         {
             string folder = Path.Combine(Application.dataPath, PluginsRelPath);
             foreach (var entry in NuGetEntries)
             {
-                if (!File.Exists(Path.Combine(folder, entry.dllName)))
+                string path = Path.Combine(folder, entry.dllName);
+                if (!File.Exists(path))
                     return false;
+
+                // Defense-in-depth: a stale DLL whose assembly version is older than what
+                // Roslyn references (e.g. a v4.x System.Runtime.CompilerServices.Unsafe
+                // shadowing the v6 we actually need) would still satisfy file-existence but
+                // leave Roslyn unable to load. Compare the on-disk assembly version against
+                // each entry's declared NuGet version, treating "older or unreadable" as not
+                // installed so Install() can rewrite it.
+                if (Version.TryParse(entry.version, out var requiredVersion))
+                {
+                    try
+                    {
+                        var actual = AssemblyName.GetAssemblyName(path).Version;
+                        if (actual == null || actual < requiredVersion)
+                            return false;
+                    }
+                    catch
+                    {
+                        return false;
+                    }
+                }
             }
             return true;
         }

MCPForUnity/Editor/Tools/ExecuteCode.cs

@@ -672,7 +672,13 @@ public static Assembly Compile(string source, string[] assemblyPaths, out List<s
             }
             catch (Exception e)
             {
-                errors.Add($"Roslyn compilation error: {e.Message}");
+                // Walk to the deepest cause: TargetInvocationException (and friends) wrap the real
+                // failure inside .InnerException, and reporting only e.Message hides everything
+                // useful (e.g. a missing transitive dep manifests as the generic "Exception has been
+                // thrown by the target of an invocation.").
+                Exception root = e;
+                while (root.InnerException != null) root = root.InnerException;
+                errors.Add($"Roslyn compilation error: {root.GetType().Name}: {root.Message}");
                 return null;
             }
         }