tiangolo.fastapi/docs/PRD.md

PRD: FastAPI Framework

FastAPI is a modern, high-performance Python web framework for building APIs with Python type hints. This document outlines the product requirements for the FastAPI framework itself.

Implementation Status

The core framework is mature and well-established. This PRD tracks major feature initiatives and improvements beyond the stable release.

User Stories

P0: Core Framework (Must Have) — ✅ Complete

As a Python developer, I want to build APIs quickly using type hints, So that I get automatic validation, documentation, and IDE support.

• [P0-US1] ✅ Route definition via decorators with path parameters
• [P0-US2] ✅ Query, header, and cookie parameter extraction with type hints
• [P0-US3] ✅ Request body validation using Pydantic models
• [P0-US4] ✅ Automatic OpenAPI schema generation
• [P0-US5] ✅ Interactive API documentation (Swagger UI + ReDoc)
• [P0-US6] ✅ Dependency injection system
• [P0-US7] ✅ Error handling with custom exception handlers
• [P0-US8] ✅ Response model typing with automatic serialization

Acceptance Criteria:

• All core HTTP methods supported (GET, POST, PUT, DELETE, PATCH, OPTIONS)
• Type hints enable validation and documentation without boilerplate
• OpenAPI 3.0+ compliant schema generation

P1: Async & Performance (Must Have) — ✅ Complete

As a developer building high-throughput APIs, I want first-class async support, So that I can handle many concurrent connections efficiently.

• [P1-US1] ✅ Async route handlers with async def
• [P1-US2] ✅ Background tasks for deferred processing
• [P1-US3] ✅ WebSocket support for real-time communication
• [P1-US4] ✅ Middleware stack (request/response processing)
• [P1-US5] ✅ Sub-application mounting for modular apps

Acceptance Criteria:

• Async handlers perform comparably to Node.js/Go
• WebSocket connections maintained efficiently
• Middleware applies correctly to all routes

P1: Security & Authentication (Should Have) — ✅ Complete

As a developer building secure APIs, I want built-in authentication and authorization, So that I don't have to implement common patterns from scratch.

• [P1-US6] ✅ OAuth2 password flow implementation
• [P1-US7] ✅ JWT token support
• [P1-US8] ✅ CORS middleware
• [P1-US9] ✅ HTTP Basic Auth
• [P1-US10] ✅ Dependency-based authorization scopes

Acceptance Criteria:

• OAuth2 with JWT tokens works out of the box
• Security schemes documented in OpenAPI spec

P2: Developer Experience (Should Have) — ✅ Complete

As a FastAPI user, I want excellent tooling and error messages, So that I can develop quickly and debug easily.

• [P2-US1] ✅ FastAPI CLI for running and generating projects
• [P2-US2] ✅ Clear validation error messages with field locations
• [P2-US3] ✅ Static type checking support with mypy
• [P2-US4] ✅ Response model documentation in OpenAPI

Acceptance Criteria:

• CLI provides fastapi dev and fastapi run
• Error responses include details for each validation failure
• Type hints work with pyright/mypy

P2: Extensibility (Should Have) — ✅ Complete

As a framework contributor, I want to extend FastAPI without modifying core code, So that I can add custom behaviors.

• [P2-US5] ✅ Custom response classes
• [P2-US6] ✅ Custom parameter types
• [P2-US7] ✅ Custom middleware interfaces
• [P2-US8] ✅ OpenAPI extension hooks

Acceptance Criteria:

• Extensions integrate seamlessly with FastAPI
• Custom components documented in OpenAPI

P3: Advanced Features (Could Have) — ✅ Complete

As an advanced user, I want sophisticated deployment options, So that I can optimize for production.

• [P3-US1] ✅ Testing utilities with TestClient
• [P3-US2] ✅ Dependency overrides for testing
• [P3-US3] ✅ ASGI server integration (Uvicorn, Hypercorn)
• [P3-US4] ✅ Gunicorn worker support
• [P3-US5] ✅ Docker integration and multi-stage builds

Acceptance Criteria:

• Testing patterns documented and working
• Production deployment guides available

Architecture: Starlette + Pydantic

FastAPI builds on two key libraries:

Component	Role
Starlette	Routing, middleware, ASGI handler
Pydantic	Data validation, serialization, OpenAPI generation

Request/Response Flow

Request → ASGI Handler → Middleware → Route Handler → Dependency Injection
                                                        ↓
Response ← Serialization ← Pydantic Model ← Return Value

Key Components

Component	File	Role
FastAPI app	fastapi/applications.py	Main application class
Routing	fastapi/routing.py	Route resolution, OpenAPI generation
Dependencies	fastapi/dependencies.py	Dependency injection system
Params	fastapi/params.py	Parameter definitions
Response	fastapi/response.py	Response handling
OpenAPI	fastapi/openapi/	Schema generation

Non-Functional Requirements

Performance

• Request/response latency comparable to Node.js and Go
• Minimal overhead for sync handlers
• Efficient async task scheduling

Compatibility

• Python 3.8+ (subject to PEP 594)
• OpenAPI 3.0+ and JSON Schema
• ASGI specification compliance

Security

• Input validation via Pydantic
• Secure defaults for authentication
• No sensitive data in error messages

Reliability

• Comprehensive test coverage
• Deprecation warnings for breaking changes
• Stable API surface within major versions

Test Scenarios

1. Basic API: Create endpoint, verify request/response
2. Validation: Send invalid data, verify 422 error with details
3. Async: Test async handler with concurrent requests
4. Dependencies: Verify dependency injection ordering
5. Auth: Test OAuth2 flow end-to-end
6. Documentation: Verify OpenAPI schema accuracy
7. Middleware: Test request/response modification
8. Sub-app: Mount app, verify routing isolation