click2mcp

click2mcp turns a Click CLI into a typed MCP server. At startup it walks the command tree and creates one MCP tool per leaf command, using the command's parameter names, types, defaults, and allowed values.

The JSON Schema comes from a function signature. MCPServer.add_tool and Tool.from_function build it with pydantic and do not accept a hand-written inputSchema, so click2mcp gives each command a function whose __signature__ and __annotations__ are the Click parameters themselves.

Install

pip install click2mcp

Quickstart

Add an adapter module to your CLI package:

# src/mycli/mcp/server.py
from click2mcp import ClickMCP
from mycli.main import main as cli

bridge = ClickMCP(app="mycli", cli=cli, module="mycli.main")
mcp = bridge.server


def main() -> None:
    bridge.run()

Expose it as a console script and an optional dependency:

[project.scripts]
mycli-mcp = "mycli.mcp.server:main"

[project.optional-dependencies]
mcp = ["click2mcp>=0.1.0"]

Install the extra, register the server with an MCP client, and run it over stdio. For Claude Code:

claude mcp add mycli -- mycli-mcp

cli can also be a zero-argument callable that returns the root group or None. It is resolved once. If it raises, the server still starts with only the escape hatches. module="mycli.main" is the fallback launcher used when no installed mycli executable can be found.

Configuration

The launcher is resolved once, in this order: launcher verbatim, the console script next to the running interpreter, shutil.which(app), then [sys.executable, "-m", module]. If none of them exist, construction raises RuntimeError. The child process gets NO_COLOR=1, TERM=dumb, and PYTHONIOENCODING=utf-8, merged over os.environ and then over env.

Behavior notes

Tool generation. Every leaf command becomes one tool. Spaces and hyphens in the command path turn into _, so mycli browse following users is mycli_browse_following_users, and the description ends with the exact CLI spelling. --groups restricts the typed tools to the named subtrees, written with dots or spaces. An empty entry is rejected.

Escape hatches and inherited options. {app}_help(path) runs --help, and {app}_run(args) runs an arbitrary command line. Set escape_hatches=False to omit both. Only ToolError text reaches the client. Typed tools also expose the options of their ancestor groups, and the generated argv puts each option before the subcommand it belongs to: {app} --verbose sub --profile=x run -- n. A leaf parameter shadows a group option with the same name, the innermost group wins over outer ones, and eager options such as --help and --version are skipped. Set inherit_group_options=False to disable the inheritance.

Prompts and failures. An option declared with prompt= is exposed as required, because the child's stdin is /dev/null and a prompt can never be answered over MCP, even when Click does not mark the option required. A leaf whose parameters cannot be turned into a signature, or whose tool name collides with another command's, is logged and skipped. The rest of the server still starts.

Runtime and exclusions. A call returns when the child exits. There is no server-side timeout, and the child's stdin is /dev/null, so it cannot read the JSON-RPC stream; deciding when to give up is the MCP client's job. Options are passed as one token, --long=value, never --long value, and repeated options are repeated. Positionals follow an end-of-options -- in declaration order, so values that begin with - survive. Full-screen TUIs and endless streams cannot work over MCP. List them in exclude, because nothing here kills a subprocess and {app}_run refuses excluded paths before spawning.

License

MIT