fs-mcp 1.26.0

A secure MCP filesystem server with Stdio and Web UI modes.

fs-mcp 📂

Universal, Provider-Agnostic Filesystem MCP Server

Works with Claude, Gemini, GPT — zero configuration required.

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https://github.com/user-attachments/assets/132acdd9-014c-4ba0-845a-7db74644e655

💡 Why This Exists

MCP (Model Context Protocol) is incredible, but connecting AI agents to filesystems hits real-world walls:

Problem	fs-mcp Solution
Container Gap — Stdio doesn't work across Docker boundaries	HTTP server by default — connect from anywhere
Token Waste — Agents dump entire files to find one function	Smart grep → read pattern with section hints
Schema Hell — Gemini silently corrupts nested object schemas	Auto-transforms schemas at runtime — just works
Blind Overwrites — One hallucination wipes your main.py	Human-in-the-loop review with VS Code diff

fs-mcp is a Python-based server built on fastmcp that treats efficiency, safety, and universal compatibility as first-class citizens.

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🚀 Core Value

1. Agent-First Efficiency

Tools are designed to minimize token usage and maximize context quality:

flowchart LR
    A["grep_content('def calculate')"] --> B["Returns: Line 142<br/>(hint: function ends L158)"]
    B --> C["read_files(start=142, end=158)"]
    C --> D["17 lines instead of 5000"]
    
    style D fill:#90EE90

• Section Hints: grep_content tells you where functions/classes end
• Pattern Reading: read_files with read_to_next_pattern extracts complete blocks
• Token-Efficient Errors: Fuzzy match suggestions instead of file dumps (90% savings)

2. Human-in-the-Loop Safety

The propose_and_review tool opens a VS Code diff for every edit:

sequenceDiagram
    participant Agent
    participant Server
    participant Human

    Agent->>Server: propose_and_review(edits)
    Server->>Human: Opens VS Code diff
    
    alt Approve
        Human->>Server: Add double newline + Save
        Server->>Agent: "APPROVE"
        Agent->>Server: commit_review()
    else Modify
        Human->>Server: Edit directly + Save
        Server->>Agent: "REVIEW" + your changes
        Agent->>Agent: Incorporate feedback
    end

Safety features:

• Full overwrites require explicit OVERWRITE_FILE sentinel
• Batch edits with edits=[] for multiple changes in one call
• Session-based workflow prevents race conditions
• Optional dangerous mode: create FS_MCP_FLAG in the workspace root to bypass path restrictions and auto-commit propose_and_review edits without human review until the file is deleted

3. Universal Provider Compatibility

The problem: Gemini silently corrupts JSON Schema $ref references — nested objects like FileReadRequest degrade to STRING, breaking tool calls.

The fix: fs-mcp automatically transforms all schemas to Gemini-compatible format at startup. No configuration needed.

Before (broken):     "items": {"$ref": "#/$defs/FileReadRequest"}
                              ↓ Gemini sees this as ↓
                     "items": {"type": "STRING"}  ❌

After (fs-mcp):      "items": {"type": "object", "properties": {...}}  ✅

This "lowest common denominator" approach means the same server works with Claude, Gemini, and GPT without any provider-specific code.

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⚡ Quick Start

Run Instantly

# One command — launches Web UI (8123) + HTTP Server (8124)
uvx fs-mcp .

Selective Launch

# HTTP only (headless / Docker / CI)
fs-mcp --no-ui .

# UI only (local testing)
fs-mcp --no-http .

Docker

# In your Dockerfile or entrypoint
uvx fs-mcp --no-ui --http-host 0.0.0.0 --http-port 8124 /app

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🔌 Configuration

Claude Desktop (Stdio)

{
  "mcpServers": {
    "fs-mcp": {
      "command": "uvx",
      "args": ["fs-mcp", "/path/to/your/project"]
    }
  }
}

OpenCode / Other HTTP Clients

Point your MCP client to http://localhost:8124/mcp/ (SSE transport).

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🧰 The Toolbox

Discovery & Reading

Tool	Purpose
grep_content	Regex search with section hints — knows where functions end
read_files	Multi-file read with head/tail, line ranges, read_to_next_pattern, or per-file reads arrays for multi-slice requests
directory_tree	Recursive JSON tree (auto-excludes .git, .venv, node_modules)
search_files	Glob pattern file discovery
get_file_info	Metadata + token estimate + chunking recommendations

Editing (Human-in-the-Loop)

Tool	Purpose
propose_and_review	Safe editing — VS Code diff, batch edits, fuzzy match suggestions
commit_review	Finalize approved changes

Structured Data

Tool	Purpose
query_json	JQ queries on large JSON files (bounded output)
query_yaml	YQ queries on YAML files

Utilities

Tool	Purpose
list_directory_with_sizes	Detailed listing with formatted sizes
list_allowed_directories	Show security-approved paths
create_directory	Create directories
read_media_file	Base64 encode images/audio for vision models

Analysis

Tool	Purpose
analyze_gsd_work_log	Semantic analysis of GSD-Lite project logs

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🏗️ Architecture

src/fs_mcp/
├── server.py          # Tool definitions + schema transforms
├── gemini_compat.py   # JSON Schema → Gemini-compatible
├── edit_tool.py       # propose_and_review logic
├── web_ui.py          # Streamlit dashboard
└── gsd_lite_analyzer.py

scripts/schema_compat/ # CLI for schema validation
tests/                 # pytest suite (including Gemini compat CI guard)

Key dependency: ripgrep (rg) must be installed for grep_content.

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📊 Why Token Efficiency Matters

Scenario	Without fs-mcp	With fs-mcp
Find a function	Read entire file (5000 tokens)	grep + targeted read (200 tokens)
Edit mismatch error	Dump file + error (6000 tokens)	Fuzzy suggestions (500 tokens)
Explore large JSON	Load entire file (10000 tokens)	JQ query (100 tokens)

Result: 10-50x reduction in context usage for common operations.

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🧪 Testing

# Run all tests
uv run pytest

# Run Gemini compatibility guard (fails if schemas break)
uv run pytest tests/test_gemini_schema_compat.py

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📜 License & Credits

Built with ❤️ for the MCP community by luutuankiet.

Powered by FastMCP, Pydantic, and Streamlit.

Now go build some agents. 🚀