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DLL Decompiler MCP Server

A Model Context Protocol (MCP) server that decompiles .NET DLL assemblies using ICSharpCode.Decompiler (ILSpy's library) and exposes the decompiled code to AI tools.

Features

  • Decompilation: Decompiles .NET DLLs to C# source code
  • Caching: Persistent file-based caching using SHA256 hash for speed
  • Search: Search across all decompiled code
  • Resources: Exposes decompiled files as MCP resources
  • Tools: Provides MCP tools for decompilation and search operations

Installation

Prerequisites

  • .NET 9.0 SDK or later
  • Windows, Linux, or macOS

Build

cd c:\development\mcpeek\McPeek
dotnet build

Usage

Running the Server

dotnet run

The server communicates via stdio and follows the MCP protocol.

MCP Tools

The server exposes the following tools:

1. DecompileFolder

Decompiles all .NET DLL files in a specified folder.

Parameters:

  • folderPath (string): The absolute path to the folder containing DLL files

Returns:

{
  "Success": true,
  "Message": "Successfully decompiled 5 assemblies",
  "AssemblyNames": ["Assembly1", "Assembly2", ...],
  "TotalFiles": 150
}

2. DecompileDll

Decompiles a specific DLL file.

Parameters:

  • dllPath (string): The absolute path to the DLL file

Returns:

{
  "Success": true,
  "Message": "Successfully decompiled MyAssembly",
  "AssemblyNames": ["MyAssembly"],
  "TotalFiles": 30
}

3. SearchCode

Searches for text patterns across all decompiled C# code.

Parameters:

  • query (string): The text to search for
  • caseSensitive (bool, optional): Whether search should be case-sensitive (default: false)
  • maxResults (int, optional): Maximum number of results to return (default: 50)

Returns:

{
  "Success": true,
  "Query": "MyClass",
  "TotalMatches": 10,
  "Results": [
    {
      "AssemblyName": "MyAssembly",
      "FilePath": "MyNamespace/MyClass.cs",
      "LineNumber": 42,
      "LineContent": "public class MyClass",
      "Context": "..."
    }
  ]
}

4. ListLoadedAssemblies

Lists all assemblies that have been decompiled and are currently loaded.

Returns:

{
  "Success": true,
  "TotalAssemblies": 5,
  "Assemblies": [
    {
      "Name": "MyAssembly",
      "Path": "C:\\path\\to\\MyAssembly.dll",
      "FileCount": 30,
      "DecompiledAt": "2025-10-24T12:00:00Z"
    }
  ]
}

5. GetCacheStatistics

Gets statistics about the decompilation cache.

Returns:

{
  "Success": true,
  "TotalCachedAssemblies": 10,
  "CacheDirectory": "C:\\Users\\...\\Temp\\McPeek\\cache",
  "CacheSizeMB": 15.5
}

6. ClearCache

Clears all cached decompilation results.

Returns:

{
  "Success": true,
  "Message": "Cache cleared successfully"
}

MCP Resources

Decompiled files are exposed as resources with URIs in the format:

decompiled://AssemblyName/Namespace/TypeName.cs

For example:

decompiled://MyAssembly/MyNamespace/MyClass.cs

Resource Operations

ListResources

Lists all available decompiled files.

Returns:

[
  {
    "Uri": "decompiled://MyAssembly/MyNamespace/MyClass.cs",
    "Name": "MyAssembly/MyNamespace/MyClass.cs",
    "Description": "Decompiled source for MyNamespace.MyClass",
    "MimeType": "text/x-csharp"
  }
]

GetResource

Retrieves the decompiled C# source code for a specific file.

Parameters:

  • uri (string): The resource URI

Returns:

{
  "Uri": "decompiled://MyAssembly/MyNamespace/MyClass.cs",
  "Content": "using System;\n\nnamespace MyNamespace\n{\n  public class MyClass\n  {\n    ...\n  }\n}",
  "MimeType": "text/x-csharp"
}

GetAssemblyOverview

Gets an overview of a decompiled assembly.

Parameters:

  • assemblyName (string): The name of the assembly

Returns:

{
  "AssemblyName": "MyAssembly",
  "AssemblyPath": "C:\\path\\to\\MyAssembly.dll",
  "TotalFiles": 30,
  "DecompiledAt": "2025-10-24T12:00:00Z",
  "Namespaces": [
    {
      "Name": "MyNamespace",
      "Types": [
        {
          "Name": "MyNamespace.MyClass",
          "FilePath": "MyNamespace/MyClass.cs",
          "IsPublic": true
        }
      ]
    }
  ]
}

Cache

The server uses a persistent file-based cache stored in the system's temp directory:

%TEMP%\McPeek\cache\

The cache uses SHA256 hashes to detect when DLLs have changed. If a DLL hasn't changed, the cached decompilation is returned instantly.

Configuration

VS Code / Cline Integration

To use this MCP server with VS Code or Cline, add it to your MCP settings:

{
  "mcpServers": {
    "mcpeek": {
      "command": "dotnet",
      "args": [
        "run",
        "--project",
        "c:\\development\\mcpeek\\McPeek\\McPeek.csproj"
      ]
    }
  }
}

Architecture

Components

  1. DecompilationCacheManager: Manages persistent file-based caching with SHA256 hashing
  2. DecompilationService: Core decompilation service using ICSharpCode.Decompiler
  3. DecompilerTools: MCP tools for decompilation and search operations
  4. DecompilerResources: MCP resources for accessing decompiled content
  5. Program.cs: MCP server host setup

Technologies

  • ICSharpCode.Decompiler: ILSpy's decompilation library
  • Microsoft.Extensions.AI: MCP server implementation
  • Microsoft.Extensions.Hosting: Hosting infrastructure
  • .NET 9.0: Runtime platform

Example Workflow

  1. Decompile a folder of DLLs:

    Tool: DecompileFolder
folderPath: "C:\\MyProject\\bin\\Debug"

  2. Search for a specific class:

    Tool: SearchCode
query: "MyImportantClass"

  3. Access decompiled source:

    Resource: decompiled://MyAssembly/MyNamespace/MyImportantClass.cs

  4. Get assembly overview:

    Resource Operation: GetAssemblyOverview
assemblyName: "MyAssembly"

License

This project uses ICSharpCode.Decompiler which is licensed under the MIT License.

Contributing

Contributions are welcome! Please ensure all changes follow the Object Calisthenics and Conventional Commits guidelines.