Basic Fns

A streamable-HTTP/stdio MCP server exposing the core filesystem, shell, search, and listing tools that mirror the default agent toolset.

Tools

Tool	Hint	Purpose
read	readOnly	Read a text file with cat-style line numbers
write	destructive	Create or overwrite a file atomically
edit	destructive	Targeted string replacement in a text file
bash	destructive	Execute a shell command and capture output/exit code
grep	readOnly	Search file contents by regex (uses rg --no-ignore if available, with a Python fallback)
find	readOnly	Find files/directories by glob-style name match
ls	readOnly	List directory contents

All filesystem paths are resolved relative to the configured server root. The server refuses any path that resolves outside that root, including symlinks that escape it.

Quick start

Install and run over HTTP:

uv sync
uv run basic-fns-mcp --root . --port 8080

Add it to Claude Code:

claude mcp add --transport http basic-fns http://127.0.0.1:8080/mcp

stdio

The same server runs over stdio, where the client spawns it as a subprocess:

uv run basic-fns-mcp --stdio            # or: --transport stdio

Add it to Claude Code:

claude mcp add basic-fns -- uv run --directory /path/to/basic-fns-mcp basic-fns-mcp --stdio

Or in an MCP client config file:

{
  "mcpServers": {
    "basic-fns": {
      "command": "uv",
      "args": ["run", "--directory", "/path/to/basic-fns-mcp", "basic-fns-mcp", "--stdio"],
      "env": { "BASIC_FNS_ROOT": "/path/to/your/project" }
    }
  }
}

On stdio, stdout is the protocol channel — the startup banner is suppressed and all logging goes to stderr. The --host/--port flags are ignored.

The module can also be served directly, since tools are registered at import time:

uv run fastmcp run src/basic_fns_mcp/server.py

CLI flags / environment variables

Flag	Environment variable	Default	Meaning
--root	BASIC_FNS_ROOT	current working directory	Sandbox root; all paths resolve under it
--host	BASIC_FNS_HOST	127.0.0.1	HTTP bind address
--port	BASIC_FNS_PORT	8080	HTTP bind port
--max-read-bytes	BASIC_FNS_MAX_READ_BYTES	2_000_000	Refuse read for files larger than this
--max-output-chars	BASIC_FNS_MAX_OUTPUT	100_000	Cap any tool result to this length
--bash-timeout	BASIC_FNS_BASH_TIMEOUT	120	Default shell timeout in seconds
--read-only	BASIC_FNS_READ_ONLY	false	Disable write and edit
--no-bash	BASIC_FNS_ALLOW_BASH	true	Do not register the bash tool
--transport / --stdio	BASIC_FNS_TRANSPORT	http	http or stdio

Command-line arguments win over environment variables.

Security

• Path sandbox: read, write, edit, grep, find, and ls resolve every path inside --root. Symlinks are followed and checked; any path that resolves outside the root is rejected.
• bash is not sandboxed: bash runs with the full privileges of the server process. A shell command can cd anywhere, read any file the server user can read, and execute arbitrary code. Only bind to loopback (127.0.0.1) and expose this server to trusted clients.
• No command blocklist: Blocklists are easily bypassed and create a false sense of security, so none is implemented.
• Read-only mode: Use --read-only to prevent file modification via this server.

Development

Run the test suite:

uv run pytest -q

License

MIT