From argparse

argparse is parser-first: you create an ArgumentParser, attach arguments, parsesys.argv, then call your own function. Milo is handler-first: write the function contract once, then let Milo derive argv parsing, MCP schema, and llms.txt.

Official reference: argparse documentation.

Before

import argparse


def deploy(environment: str, service: str, version: str) -> dict[str, str]:
    return {"environment": environment, "service": service, "version": version}


parser = argparse.ArgumentParser(prog="deployer")
parser.add_argument("environment")
parser.add_argument("service")
parser.add_argument("--version", default="latest")
args = parser.parse_args()
print(deploy(args.environment, args.service, args.version))

After

from typing import Annotated

from milo import CLI, Positional

cli = CLI(name="deployer", description="Deploy services")


@cli.command("deploy", description="Deploy a service")
def deploy(
    environment: Annotated[str, Positional("ENVIRONMENT")],
    service: Annotated[str, Positional("SERVICE")],
    version: str = "latest",
) -> dict[str, str]:
    """Deploy a service.

    Args:
        environment: Target environment.
        service: Service name.
        version: Version or image tag.
    """
    return {"environment": environment, "service": service, "version": version}


if __name__ == "__main__":
    cli.run()

Run it:

uv run python app.py deploy staging api
uv run python app.py deploy staging api --format json
uv run python app.py --llms-txt
uv run milo verify app.py

Mapping

argparse concept	Milo equivalent
`ArgumentParser(prog=..., description=...)`	`CLI(name=..., description=...)`
`add_argument("name")`	`name: Annotated[str, Positional("NAME")]`
`add_argument("--name", default="x")`	Defaulted parameter:`name: str = "x"`
`type=int`	Annotation:`count: int`
`choices=[...]`	`Literal[...]` or `Enum`
`parser.parse_args()`	`cli.run()` for processes, `cli.invoke([...])`for tests
`Namespace`	Function parameters

What To Watch

Parameters become named CLI options by default. UsePositional(...)to preserve established argv; MCP and programmatic callers still use the Python parameter name.
UseOption(aliases=("-x",))to preserve existing option spellings and surfaces=("cli",)for long-running commands that should not be MCP tools.
print()output is process output. For agent-facing commands, prefer structured returns andContextoutput helpers.
Parser errors become structured Milo/MCP diagnostics when the command is called throughtools/call.