tiangolo.fastapi/.specify/memory/constitution.md

FastAPI Constitution

Core Principles

I. Standards-Based

FastAPI MUST be built on open standards for APIs. All features MUST support OpenAPI (formerly Swagger) and JSON Schema. The framework MUST generate compliant API documentation automatically. Rationale: Ensures interoperability and automatic tooling support (client generation, documentation).

II. Type Safety (NON-NEGOTIABLE)

FastAPI MUST leverage Python type hints for request validation, response serialization, and automatic documentation. Type checking MUST pass with mypy at strict level. Rationale: Catches bugs at development time, enables IDE autocomplete, powers automatic documentation.

III. Test-First Development

All new features MUST have tests written before implementation. The framework MUST maintain comprehensive test coverage. Tests MUST verify both functionality and API contract compliance. Rationale: FastAPI is a framework used in production by millions; regressions are costly.

IV. Performance

FastAPI MUST achieve performance comparable to NodeJS and Go frameworks. Async support MUST be first-class with no performance penalty for sync code paths. Rationale: Performance is a primary feature claim; benchmarks MUST remain competitive.

V. Developer Experience

The framework MUST provide intuitive APIs with minimal boilerplate. Error messages MUST be clear and actionable. Documentation MUST include runnable examples. Rationale: "Fast to code" is a core value proposition; reduces developer cognitive load.

Additional Constraints

Technology Stack

• Language: Python 3.8+ (subject to Python release support policy)
• Core Dependencies: Starlette for routing, Pydantic for data validation
• Async: First-class support for asyncio
• Standards: OpenAPI 3.0+, JSON Schema

Compatibility

• Python Versions: Support Python versions according to PEP 594
• Breaking Changes: Follow semantic versioning; MAJOR version bumps require deprecation warnings in prior minor releases
• Dependencies: Pin minimal required versions; allow compatible updates

Development Workflow

Code Quality Gates

• All PRs MUST pass type checking (mypy --strict)
• All PRs MUST maintain or increase test coverage
• All PRs MUST include documentation for new features
• All PRs MUST pass CI/CD pipeline (lint, test, coverage)

Contribution Process

1. Fork and create feature branch
2. Write tests first (must fail)
3. Implement feature
4. Ensure tests pass and coverage maintained
5. Update documentation
6. Submit PR with description

Release Process

• Follow semantic versioning strictly
• Maintain changelog
• Publish release notes
• Ensure backwards compatibility within MAJOR version

Governance

The FastAPI Constitution supersedes all other development practices. Amendments to this constitution MUST be documented with:

1. Rationale for the change
2. Impact assessment on existing workflows
3. Migration plan if applicable

All PRs and code reviews MUST verify compliance with these principles. Complexity MUST be justified against simpler alternatives.

Version: 1.0.0 | Ratified: 2026-02-23 | Last Amended: 2026-02-23