afd 0.1.0

Agent-First Development toolkit for Python

AFD - Agent-First Development for Python

A Python toolkit for building applications with the Agent-First Development methodology.

What is AFD?

Agent-First Development is a software development methodology where AI agents are treated as first-class users from day one. Instead of building UI first and adding API/agent access later, AFD inverts this:

Traditional:  UI → API → Agent Access (afterthought)
Agent-First:  Commands → Validation → UI (surface)

Installation

# Core types only
pip install afd

# With MCP server support
pip install afd[server]

# With CLI
pip install afd[cli]

# With testing utilities
pip install afd[testing]

# Everything
pip install afd[all]

Quick Start

Define a Command

from afd import CommandResult, success, error
from afd.server import define_command
from pydantic import BaseModel

class Todo(BaseModel):
    id: str
    title: str
    done: bool = False

@define_command(
    name="todo.create",
    description="Create a new todo item",
)
async def create_todo(title: str) -> CommandResult[Todo]:
    todo = Todo(id="todo-1", title=title)
    return success(
        data=todo,
        reasoning="Created new todo with default status",
    )

Create an MCP Server

from afd.server import create_server

server = create_server(
    name="todo-app",
    version="1.0.0",
)

@server.command(
    name="todo.create",
    description="Create a todo",
)
async def create_todo(input):
    todo = Todo(id="todo-1", title=input["title"])
    return success(data=todo)

# Run the server (stdio for VS Code/Cursor)
server.run()

Test Your Commands

import pytest
from afd.testing import assert_success

# Use the mock_server fixture
async def test_create_todo(mock_server):
    @mock_server.command("todo.create")
    async def handler(input):
        from afd import success
        return success({"id": "1", "title": input["title"]})

    result = await mock_server.execute("todo.create", {"title": "Test"})

    data = assert_success(result)
    assert data["title"] == "Test"

Core Types

CommandResult

The standard return type for all commands:

from afd import CommandResult, success, error

# Successful result
result = success(
    data={"id": "123"},
    reasoning="Created successfully",
    confidence=0.95,
)

# Error result
result = error(
    code="NOT_FOUND",
    message="Resource not found",
    suggestion="Check the ID and try again",
)

UX-Enabling Fields

AFD results include optional fields that enable rich agent experiences:

Field	Purpose
confidence	0-1 score for UI confidence indicators
reasoning	Explains "why" for transparency
sources	Attribution for verification
plan	Multi-step operation visibility
alternatives	Other options considered
warnings	Non-fatal issues to surface

Packages

Extra	Contents
(core)	CommandResult, success(), error(), error types, metadata types
[server]	MCP server factory, @define_command, create_server()
[cli]	Click-based CLI for connecting to MCP servers
[testing]	mock_server fixture, assertions, MockTransport

Related

• AFD TypeScript - Original TypeScript implementation
• AFD Philosophy - Why AFD?
• Command Schema Guide - Designing commands

License

MIT