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fastapi-rest-api-design

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Designs and reviews REST APIs for FastAPI services using consistent resource naming, HTTP semantics, validation, security, and error handling patterns. Use for backend API tasks, endpoint design/refactors, or API review requests in FastAPI/Python projects.

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Source SKILL.md: https://github.com/open-edge-platform/anomalib/blob/HEAD/.agents/skills/fastapi-rest-api-design/SKILL.md

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FastAPI REST API Design

Use this skill when designing, implementing, or reviewing FastAPI REST endpoints.

Purpose and scope

This skill enforces practical REST design and FastAPI implementation quality: resource naming, HTTP semantics, schema quality, dependency boundaries, security checks, and concise review output.

Core design rules

Resource and path conventions

  • Use nouns in paths, not action verbs.
  • Use plural collection names such as /projects, /users.
  • Use stable item identifiers such as /projects/{project_id}.
  • Keep names lowercase and consistent.
  • Keep nesting shallow (typically max 2 levels), for example /projects/{project_id}/models.
  • Do not expose internal storage structure in public routes.

HTTP methods and status semantics

  • Use GET read, POST create, PUT full replace, PATCH partial update, DELETE remove.
  • Return 201 on create, 200 on standard success, and 204 when no response body is needed.
  • Choose precise error codes (400, 401, 403, 404, 409, 422, 500) and avoid vague fallbacks.
  • Keep semantics consistent across related endpoints.

Request/response contracts

  • Use JSON payloads for standard API requests/responses.
  • Use Pydantic models for request and response contracts.
  • Enforce explicit field constraints (enums, lengths, ranges, formats).
  • Prefer explicit response models for stable contracts.
  • Keep error response bodies consistent across endpoints.

FastAPI architecture patterns

  • Organize routes by resource/domain with APIRouter.
  • Inject dependencies with Depends(...); avoid hidden globals.
  • Keep handlers thin and move business logic into services/use-cases.
  • Raise domain exceptions in service layer and map to HTTP errors at API boundary.
  • Add explicit responses={...} metadata when custom errors must appear in OpenAPI docs.

Security and operability

  • Enforce HTTPS in deployed environments.
  • Enforce authentication and authorization per route/use case.
  • Apply least-privilege checks for each resource operation.
  • Avoid exposing sensitive internals in error details.
  • Add filtering, sorting, and pagination for large collection endpoints.
  • Introduce versioning (for example /v1/...) before shipping breaking API changes.

Review workflow

  1. Classify each endpoint as collection, item, or nested resource.
  2. Verify verb-to-action mapping.
  3. Verify status code and error semantics.
  4. Verify Pydantic schema quality and response model clarity.
  5. Verify DI and layer boundaries.
  6. Verify authn/authz and least-privilege behavior.
  7. Apply checklist and report material issues first.

Output style

When asked to design or review APIs, respond concisely with:

  1. Endpoint proposal (route + method list).
  2. Contract notes (request/response + validation).
  3. Security checks (authn/authz + sensitive data handling).
  4. Checklist verdict (pass/fail highlights).
  5. Top fixes in priority order.

Additional resources