303-fastapi

Design FastAPI applications with Service Layer pattern and Pydantic v2 validation.

Updated May 21, 2026
One-click install
npx skills add https://github.com/ulf1/trading-regime --skill 303-fastapi
Or copy as Structured Prompt for Agent
Please help me install this Agent Skill.
Skill: 303-fastapi
Source: https://github.com/ulf1/trading-regime/tree/main/.agent/skills/303-fastapi
Command: npx skills add https://github.com/ulf1/trading-regime --skill 303-fastapi

SYSTEM DOCUMENTATION & REQUIREMENTS

💡 This Skill includes assets (resource) components.

What problem does it solve?

It solves the challenge of building production-grade FastAPI backends reliably by enforcing a clean Service Layer architecture, correct async/sync concurrency behavior, and consistent error handling that keeps business logic independent from HTTP concerns.

Core Features & Use Cases

  • Service Layer Pattern by design: Separates Transport (FastAPI routers/controllers) from Business Logic (services) so your rules remain testable and stable.
  • Pydantic v2 validation & schema rigor: Encourages explicit request/response models to reduce runtime contract drift.
  • Async/sync concurrency matrix: Provides clear rules for async I/O, sync I/O via FastAPI’s threadpool, and CPU-bound work via process offloading.
  • Testability-first approach: Promotes unit testing with TestClient, dependency_overrides, and fixture-based cleanup to prevent cross-test leakage.
  • Use Case: You’re developing a REST API for a trading data service and need endpoints that validate input with Pydantic, delegate logic to services, map service exceptions to correct HTTP responses, and stay reliable under high-concurrency traffic.

Quick Start

Ask the AI to scaffold a FastAPI app using the Service Layer pattern, add Pydantic v2 request/response models, implement global exception mapping for service errors, and generate unit tests using TestClient with dependency overrides.

Frequently Asked Questions about 303-fastapi

High-intent search queries and answers about installing and using this skill.

FAQPage Schema
How do I separate business logic from HTTP routing in FastAPI?

Use the Service Layer pattern in FastAPI to separate Transport (routers/controllers) from Business Logic (services). This keeps your rules stable and testable, independent from HTTP concerns. It enforces strict Transport-to-Service separation for reliable backends.

How do I test FastAPI services with pytest and dependency overrides?

Test FastAPI services using TestClient and pytest-asyncio with dependency_overrides. This approach ensures deterministic unit tests and uses fixture-based cleanup to prevent cross-test leakage, keeping your service layer testable and stable.

How should I handle async and sync I/O concurrency in FastAPI?

Handle async/sync concurrency in FastAPI by using async I/O for non-blocking operations, routing sync I/O through FastAPI's threadpool, and offloading CPU-bound work to processes. This matrix ensures reliable behavior under high-concurrency traffic.

What is the best way to map service exceptions to HTTP responses in FastAPI?

The best way to map service exceptions to HTTP responses in FastAPI is by implementing global exception handling at the perimeter. This keeps business logic independent from HTTP concerns while ensuring consistent error responses across your RESTful API.

Do I need Pydantic v2 for request and response validation in FastAPI?

Yes, using Pydantic v2 for request and response validation in FastAPI enforces explicit schema rigor. This reduces runtime contract drift and ensures your API endpoints validate input correctly before delegating logic to the service layer.

Can I build high-concurrency trading data APIs with FastAPI?

Yes, you can build high-concurrency trading data APIs with FastAPI using the Service Layer pattern. It supports Pydantic v2 validation, correct async/sync concurrency, and deterministic unit tests for production-grade behavior.