File Reader MCP Server

A professional, production-ready Model Context Protocol (MCP) server built in Python that allows LLM agents (like Claude Desktop) to securely explore, read, search, and analyze text-based files within a restricted sandbox environment.

Architecture Overview

graph TD
    Client[MCP Client e.g., Claude Desktop] <-->|JSON-RPC over stdio| Server[server.py / MCPServer]
    Server <--> Tools[tools/ folder]
    Tools --> Validator[utils/validators.py]
    Tools --> FileUtils[utils/file_utils.py]
    Validator -->|Security Check| Sandbox[(sample_files/ Sandbox)]
    FileUtils -->|Read/Scan| Sandbox
    Logger[utils/logger.py] -->|Log execution & time| stderr[sys.stderr]

Modular Structure

• server.py: The entry point. Initializes MCPServer and registers the tools.

• config.py: Loads environment variables and configures parameters like the sandbox directory, allowed extensions, and size thresholds.

• utils/validators.py: Path traversal checker (preventing ../ escapes), null byte detector, file size validator, and extension filter.

• utils/file_utils.py: Low-level filesystem operations (encoding fallback loop, file stats, recursive filename search, stream line reader).

• utils/logger.py: Standard error logger setup (ensures stdout remains unpolluted) and execution timer decorator.

• tools/: Domain-specific implementation wrappers for each API action.

Safety Features & Validation Rules

1. Strict Sandboxing: The server converts all user-supplied paths to absolute paths and verifies that they fall within the configured SANDBOX_DIR using Path.relative_to(). Any attempt to escape using parent directory constructs (e.g. ../) will trigger a traversal warning and block execution.

2. Nullable Byte Check: Paths containing null characters (\x00) are immediately rejected to prevent truncate injection attacks on OS-level calls.

3. Safe File Extensions: Standard configuration whitelists text-based files (.txt, .md, .json, .csv, .log, .xml, .yaml, .yml, .ini, .conf). Binary files or system executables are blocked.

4. Memory Exhaustion Safeguard: A file size limit of 5 MB is enforced by default to prevent LLM agents from loading massive log files or SQL dumps into system memory.

5. Early-Exit Line Streaming: The read_lines tool opens a file context stream and exits as soon as the requested line threshold is met, preventing high memory consumption.

Installation & Setup

This project requires Python 3.11 or newer.

1. Create a Virtual Environment

Navigate to the root directory and create a virtual environment to isolate the project packages:

On Windows:

c
.venv\Scripts\activate

On macOS/Linux:

python3 -m venv .venv
source .venv/bin/activate

2. Install Dependencies

Install the required Python SDK and dependencies listed in requirements.txt:

pip install -r requirements.txt

3. Configure the Environment

Create your .env file from the example template:

copy .env.example .env

Review or modify .env as needed:

# Verbosity options: DEBUG, INFO, WARNING, ERROR, CRITICAL
LOG_LEVEL=INFO

# Relative or absolute path to the sandbox folder
SANDBOX_DIR=sample_files

Running the Server

MCP servers run locally on your machine and communicate using Standard Input/Output (stdio).

Running in Development Mode

You can use the official MCP command-line tool mcp dev to run the server in developer mode with an interactive inspector interface. This is the recommended way to test during development:

# Install mcp-cli globally if you haven't already
npm install -g @modelcontextprotocol/inspector

# Run the dev server
mcp dev server.py

This command opens an inspector window in your browser (usually at http://localhost:5173) allowing you to manually trigger tools and view outputs.

Starting Directly via Python

You can also run the script directly:

python server.py

Note: The command-line will seem to hang and wait, because it is waiting for JSON-RPC messages via stdin.

MCP Integration Examples

To use this server with Claude Desktop, add its configuration in the Claude Desktop configuration file (typically located at %APPDATA%\Claude\claude_desktop_config.json on Windows or ~/Library/Application Support/Claude/claude_desktop_config.json on macOS).

Replace paths below with your actual project location:

{
  "mcpServers": {
    "file-reader-mcp": {
      "command": "python",
      "args": [
        "d:/Projects/file-reader-mcp/server.py"
      ],
      "env": {
        "SANDBOX_DIR": "d:/Projects/file-reader-mcp/sample_files",
        "LOG_LEVEL": "INFO"
      }
    }
  }
}

Tool API and Example Requests/Responses

1. list_files

List files inside a folder in the sandbox, with an optional extension filter.

• Request Parameters:

  • directory (string, optional): Folder relative to sandbox root. Defaults to ".".

  • extension_filter (string, optional): Filter by suffix (e.g. txt or .md).

• Response:

  [
    {
      "name": "notes.txt",
      "relative_path": "notes.txt",
      "size_bytes": 482
    },
    {
      "name": "todo.md",
      "relative_path": "todo.md",
      "size_bytes": 354
    }
  ]

2. read_file

Read a complete text file inside the sandbox.

• Request Parameters:

  • path (string, required): File path relative to sandbox.

• Response:

  Meeting Notes - Project Kickoff
  Date: 2026-07-29
  Participants: Alice, Bob, Charlie
  ...

3. read_lines

Read a slice of lines from a text file.

• Request Parameters:

  • path (string, required): File path relative to sandbox.

  • start_line (integer, required): Starting line (1-indexed).

  • end_line (integer, required): End line (inclusive).

• Response:

  Agenda:
  1. Define project scope and deliverables.
  2. Setup Model Context Protocol (MCP) server integration.

4. search_file

Search files inside the sandbox recursively by name matching a keyword.

• Request Parameters:

  • keyword (string, required): Case-insensitive keyword to locate.

• Response:

  [
    "notes.txt",
    "report.txt"
  ]

5. file_info

Get file size, type, creation, and modified times.

• Request Parameters:

  • path (string, required): File path relative to sandbox.

• Response:

  {
    "filename": "notes.txt",
    "extension": ".txt",
    "size_bytes": 482,
    "created_at": "2026-07-29T20:32:00.123456",
    "modified_at": "2026-07-29T20:34:10.789123"
  }

6. count_words

Calculate characters, words, and lines.

• Request Parameters:

  • path (string, required): File path relative to sandbox.

• Response:

  {
    "words": 62,
    "characters": 482,
    "lines": 17
  }