Agent Skills: FastAPI Sweetener Skill

FastAPI Sweetener Skill

Add FastAPI server capabilities to an existing Python project, integrating as a CLI subcommand alongside existing functionality.

Prerequisites

This skill assumes you already have a Python project (typically created with /plinth:python-project-init) with:

• Existing pyproject.toml
• Package directory with __init__.py
• Existing CLI (typically cli.py)

What This Skill Adds

Adds FastAPI server capabilities to your existing project:

• FastAPI server with OpenAPI docs (/docs, /redoc)
• CLI subcommand - {package} server to start the API
• Configuration management (JSON/YAML config files)
• CORS middleware for web clients
• Lifespan management for startup/shutdown
• Dependencies - adds fastapi, uvicorn, click to pyproject.toml

Files Added/Modified

New files:

• {package}/server.py - FastAPI application
• {package}/config.py - Configuration management
• config.example.json - Example configuration
• .env.example - Environment variable template

Modified files:

• {package}/cli.py - Adds server subcommand (converts to Click if needed)
• pyproject.toml - Adds dependencies (fastapi, uvicorn, click, pyyaml)

Implementation Steps

Step 1: Detect Project Structure

Use Glob and Read to discover:

1. Find pyproject.toml in current directory
2. Extract project.name and package name from pyproject.toml
3. Locate package directory (usually {package}/ or src/{package}/)
4. Find existing CLI file (usually cli.py)
5. Read __init__.py or __version__.py to get description

If unable to detect structure, ask user for:

• PACKAGE_NAME
• PROJECT_NAME

Step 2: Check for Existing Server

Check if server already exists:

ls {package}/server.py

If it exists, ask user if they want to overwrite it.

Step 3: Gather Configuration

Ask user for (use AskUserQuestion if needed):

Optional parameters (with defaults):

1. SERVER_HOST (string) - Default: "0.0.0.0"

   • Host to bind server to

2. SERVER_PORT (integer) - Default: 8000

   • Port to bind server to

Step 4: Add Server Files

Create the following files from templates in skills/fastapi-sweetener/assets/:

{package}/server.py - FastAPI application:

• Read assets/server.py.template
• Replace template variables
• Write to {package}/server.py

{package}/config.py - Configuration management:

• Read assets/config.py.template
• Replace template variables
• Write to {package}/config.py

config.example.json - Example config:

• Read assets/config.example.json.template
• Replace SERVER_HOST and SERVER_PORT
• Write to project root config.example.json

.env.example - Environment template:

• Read assets/.env.example.template
• Write to project root .env.example

Step 5: Update CLI to Add Server Subcommand

This is the most critical step. The CLI needs to support subcommands.

If cli.py uses Click:

Add a new @main.command() for the server:

@main.command()
@click.option('--host', default=None, help='Server host (default: from config)')
@click.option('--port', default=None, type=int, help='Server port (default: from config)')
@click.option('--reload', is_flag=True, help='Enable auto-reload for development')
@click.option('--log-level', default='info',
              type=click.Choice(['critical', 'error', 'warning', 'info', 'debug']),
              help='Logging level')
def server(host, port, reload, log_level):
    """Start the FastAPI server.

    This starts the HTTP server that provides the API.
    The server will be accessible via http://HOST:PORT
    """
    import uvicorn
    from .config import Config

    config = Config()

    # Override config with CLI options if provided
    server_host = host or config.server_host
    server_port = port or config.server_port

    print(f"Starting server at http://{server_host}:{server_port}")
    print(f"OpenAPI docs: http://{server_host}:{server_port}/docs")

    uvicorn.run(
        "{{PACKAGE_NAME}}.server:app",
        host=server_host,
        port=server_port,
        reload=reload,
        log_level=log_level
    )

If cli.py uses argparse:

Convert to Click. This is a significant change but necessary for subcommands:

1. Read existing cli.py
2. Identify the main() function
3. Rewrite to use Click groups:
   • Convert ArgumentParser() to @click.group()
   • Convert existing logic to @main.command() named appropriately
   • Add the server subcommand as shown above
4. Use Edit tool to update the file

Step 6: Update pyproject.toml Dependencies

Add required dependencies to pyproject.toml:

Use Edit tool to add to the dependencies array:

dependencies = [
    # ... existing deps ...
    "fastapi>=0.104.0",
    "uvicorn[standard]>=0.24.0",
    "click>=8.0",
    "pyyaml>=6.0",
]

Step 7: Install Dependencies

Run uv sync to install new dependencies:

uv sync

Step 8: Create Config File

Guide user to create config from example:

cp config.example.json config.json

Step 9: Verification & Next Steps

Provide verification commands:

# Verify CLI still works
uv run {package} --version

# Start development server
uv run {package} server --reload

# Open in browser
open http://localhost:{port}/docs

Tell user:

1. Server added as {package} server subcommand
2. Configure via config.json (copy from config.example.json)
3. Add API endpoints to {package}/server.py
4. OpenAPI docs available at /docs and /redoc
5. Use /plinth:macos-launchd-service to set up auto-start

Template Variable Reference

| Variable | Example | Source | |----------|---------|--------| | {{PROJECT_NAME}} | "Temoa" | From pyproject.toml or user input | | {{PACKAGE_NAME}} | "temoa" | Detected from directory structure | | {{PACKAGE_NAME_UPPER}} | "TEMOA" | Derived from PACKAGE_NAME | | {{DESCRIPTION}} | "Semantic search" | From init.py or user input | | {{SERVER_HOST}} | "0.0.0.0" | User input or default | | {{SERVER_PORT}} | "8000" | User input or default |

Example Execution

User request: "Add FastAPI to my CLI project"

Detected:

• PROJECT_NAME: "MyProject" (from pyproject.toml)
• PACKAGE_NAME: "myproject" (from directory structure)
• Existing cli.py with Click

User provides:

• SERVER_PORT: 8080 (default: 8000)

Result:

Files added:

• myproject/server.py
• myproject/config.py
• config.example.json
• .env.example

Files modified:

• myproject/cli.py (added server command)
• pyproject.toml (added fastapi, uvicorn, etc.)

Usage:

myproject --help          # Shows all subcommands
myproject server          # Starts FastAPI on port 8080
myproject server --reload # Development mode with auto-reload

Idempotency

The skill should be safe to run multiple times:

• Check if server.py exists before creating
• Ask user before overwriting existing files
• Only add dependencies if not already present
• Don't duplicate CLI commands

Common Issues

CLI Not Using Click

If the existing CLI uses argparse, the skill will convert it to Click. This is necessary for subcommands. The conversion:

• Preserves existing CLI functionality
• Moves it to a default command or appropriately named subcommand
• Adds the server subcommand

Warn the user about this change.

Port Already in Use

If the default port is occupied, user can override:

myproject server --port 8001

Missing Config File

Server requires config.json. If missing:

cp config.example.json config.json

Edit values as needed.

What NOT to Do

• Don't create this from scratch - requires existing project
• Don't skip the CLI conversion if needed - subcommands require Click
• Don't overwrite server.py without asking
• Don't add emojis to any files
• Don't run python directly - always use uv run
• Don't modify README.md or documentation files (user does this)

Post-Addition Customization

After adding FastAPI, users typically:

1. Edit config.json with their specific configuration
2. Add API endpoints to server.py
3. Update tests to include API endpoint tests
4. Set up launchd service using /plinth:macos-launchd-service
5. Document API endpoints in README.md

Integration with Other Skills

Works well with:

• /plinth:python-project-init - Creates the base project first
• /plinth:macos-launchd-service - Auto-start the server
• /plinth:session-wrapup - Document the addition