FieldCure Filesystem Server

NuGet License: MIT

A secure Model Context Protocol (MCP) server that exposes local filesystem operations to AI clients. Built with C# and the official MCP C# SDK.

Features

• 17 filesystem tools: read, write, append, delete, search, copy, move, convert-to-markdown, and more
• Sandboxed access: all operations restricted to explicitly allowed directories
• MCP roots protocol: clients can change allowed directories at runtime without restarting the server
• Security hardened: path traversal prevention, symlink resolution, NTFS ADS blocking, Windows reserved name rejection
• Atomic writes: temp-file-then-rename pattern prevents data corruption
• Binary detection: automatically returns base64 for binary files, UTF-8 for text
• Built-in document parsing: supported document formats can be read directly and converted to markdown on disk
• Stdio transport: standard MCP subprocess model via JSON-RPC over stdin/stdout

Installation

dotnet tool

dotnet tool install -g FieldCure.Mcp.Filesystem

From source

git clone https://github.com/fieldcure/fieldcure-mcp-filesystem.git
cd fieldcure-mcp-filesystem
dotnet build

Requirements

• .NET 8.0 Runtime or later

Configuration

Claude Desktop

{
  "mcpServers": {
    "filesystem": {
      "command": "fieldcure-mcp-filesystem",
      "args": [
        "C:\\Users\\me\\Documents",
        "C:\\Projects"
      ]
    }
  }
}

VS Code

{
  "servers": {
    "filesystem": {
      "command": "fieldcure-mcp-filesystem",
      "args": [
        "${workspaceFolder}"
      ]
    }
  }
}

Tools

File Operations

Tool	Description
read_file	Read file contents (text as UTF-8, binary as base64)
read_multiple_files	Read multiple files at once with per-file error handling
read_file_lines	Read a specific range of lines
convert_to_markdown	Convert one supported document file into a markdown file on disk
convert_directory_to_markdown	Batch-convert supported document files in a directory to markdown files
write_file	Create or overwrite a file with atomic write
append_file	Append content to end of file with auto-newline
modify_file	Find and replace text (plain text or regex)
copy_file	Copy a file or directory recursively
move_file	Move or rename a file or directory
delete_file	Delete a file or directory

Directory Operations

Tool	Description
list_directory	List contents with type markers, sizes, and dates
create_directory	Create a directory recursively
directory_tree	Hierarchical tree view with configurable depth

Search & Info

Tool	Description
search_files	Find files by glob pattern
search_within_files	Search text content across files with line numbers
get_file_info	File or directory metadata

MCP Roots Protocol

The server supports the MCP roots protocol for runtime directory changes. When the client changes roots:

1. The server requests the updated roots list.
2. Allowed directories are replaced entirely.
3. CLI args provide the initial sandbox.
4. An empty roots list means all file access is denied.

Security Model

All paths are validated through IPathValidator before any filesystem operation:

1. Allowed directories define the sandbox boundary.
2. . and .. are normalized via Path.GetFullPath().
3. Symlinks must resolve within allowed directories.
4. Prefix matching is directory-separator-aware.
5. NTFS alternate data streams are blocked.
6. Windows reserved names are rejected.
7. Allowed directory updates are thread-safe.

The markdown conversion tools validate both source and output paths through the same sandbox rules. They write converted markdown directly to disk instead of returning full document text through MCP, which is usually more token-efficient than read_file plus write_file.

Project Structure

src/FieldCure.Mcp.Filesystem/
|- Program.cs
|- Security/
|  |- IPathValidator.cs
|  \- PathValidator.cs
|- Tools/
|  |- FileOperationTools.cs
|  |- DirectoryTools.cs
|  \- SearchTools.cs
\- Utilities/
   |- EncodingDetector.cs
   \- FileSize.cs

Development

dotnet build
dotnet test
dotnet pack src/FieldCure.Mcp.Filesystem -c Release

Limitations

Supported	Not Yet Supported
Text file read/write (UTF-8)	Non-UTF-8 encoding auto-detection
Binary file detection + base64	Streaming for very large files (>100 MB)
Atomic writes	File watching / change notifications
Glob pattern search	Full-text indexing
Regex find-and-replace	Multi-file transactional writes
Symlink resolution	Cross-platform symlink creation
MCP roots protocol	HTTP transport
Document text extraction / markdown conversion for formats supported by DocumentParsers	OCR for scanned documents

See Also

Part of the AssistStudio ecosystem.

License

MIT