API Design Skill | Agent Skills

API Design

Comprehensive API design patterns covering REST/GraphQL framework design, versioning strategies, and RFC 9457 error handling. Each category has individual rule files in rules/ loaded on-demand.

Quick Reference

| Category | Rules | Impact | When to Use | |----------|-------|--------|-------------| | API Framework | 3 | HIGH | REST conventions, resource modeling, OpenAPI specifications | | Versioning | 3 | HIGH | URL path versioning, header versioning, deprecation/sunset policies | | Error Handling | 4 | HIGH | RFC 9457 Problem Details, agent-facing errors, validation errors, error type registries | | GraphQL | 2 | HIGH | Strawberry code-first, DataLoader, permissions, subscriptions | | gRPC | 2 | HIGH | Protobuf services, streaming, interceptors, retry | | Streaming | 2 | HIGH | SSE endpoints, WebSocket bidirectional, async generators |

Total: 18 rules across 7 categories

API Framework

REST and GraphQL API design conventions for consistent, developer-friendly APIs.

| Rule | File | Key Pattern | |------|------|-------------| | REST Conventions | rules/framework-rest-conventions.md | Plural nouns, HTTP methods, status codes, pagination | | Resource Modeling | rules/framework-resource-modeling.md | Hierarchical URLs, filtering, sorting, field selection | | OpenAPI | rules/framework-openapi.md | OpenAPI 3.1 specs, documentation, schema definitions |

Versioning

Strategies for API evolution without breaking clients.

| Rule | File | Key Pattern | |------|------|-------------| | URL Path | rules/versioning-url-path.md | /api/v1/ prefix routing, version-specific schemas | | Header | rules/versioning-header.md | X-API-Version header, content negotiation | | Deprecation | rules/versioning-deprecation.md | Sunset headers, lifecycle management, breaking change policy |

Error Handling

RFC 9457 Problem Details for machine-readable, standardized error responses.

| Rule | File | Key Pattern | |------|------|-------------| | Problem Details | rules/errors-problem-details.md | RFC 9457 schema, application/problem+json, exception classes | | Agent-Facing Errors | rules/errors-agent-facing.md | Agent extensions: retryable, error_category, content negotiation, token efficiency | | Validation | rules/errors-validation.md | Field-level errors, Pydantic integration, 422 responses | | Error Catalog | rules/errors-error-catalog.md | Problem type registry, error type URIs, client handling |

GraphQL

Strawberry GraphQL code-first schema with type-safe resolvers and FastAPI integration.

| Rule | File | Key Pattern | |------|------|-------------| | Schema Design | rules/graphql-strawberry.md | Type-safe schema, DataLoader, union errors, Private fields | | Patterns & Auth | rules/graphql-schema.md | Permission classes, FastAPI integration, subscriptions |

gRPC

High-performance gRPC for internal microservice communication.

| Rule | File | Key Pattern | |------|------|-------------| | Service Definition | rules/grpc-service.md | Protobuf, async server, client timeout, code generation | | Streaming & Interceptors | rules/grpc-streaming.md | Server/bidirectional streaming, auth, retry backoff |

Streaming

Real-time data streaming with SSE, WebSockets, and proper cleanup.

| Rule | File | Key Pattern | |------|------|-------------| | SSE | rules/streaming-sse.md | SSE endpoints, LLM streaming, reconnection, keepalive | | WebSocket | rules/streaming-websocket.md | Bidirectional, heartbeat, aclosing(), backpressure |

Integrations

Messaging platform integrations and headless CMS patterns.

| Rule | File | Key Pattern | |------|------|-------------| | Messaging Platforms | rules/messaging-integrations.md | WhatsApp WAHA, Telegram Bot API, webhook security | | Payload CMS | rules/payload-cms.md | Payload 3.0 collections, access control, CMS selection |

Quick Start Example

# REST endpoint with versioning and RFC 9457 errors
from fastapi import APIRouter, Depends, Request
from fastapi.responses import JSONResponse

router = APIRouter()

@router.get("/api/v1/users/{user_id}")
async def get_user(user_id: str, service: UserService = Depends()):
    user = await service.get_user(user_id)
    if not user:
        raise NotFoundProblem(
            resource="User",
            resource_id=user_id,
        )
    return UserResponseV1(id=user.id, name=user.full_name)

Key Decisions

| Decision | Recommendation | |----------|----------------| | Versioning strategy | URL path (/api/v1/) for public APIs | | Resource naming | Plural nouns, kebab-case | | Pagination | Cursor-based for large datasets | | Error format | RFC 9457 Problem Details with application/problem+json | | Error type URI | Your API domain + /problems/ prefix | | Support window | Current + 1 previous version | | Deprecation notice | 3 months minimum before sunset | | Sunset period | 6 months after deprecation | | GraphQL schema | Code-first with Strawberry types | | N+1 prevention | DataLoader for all nested resolvers | | GraphQL auth | Permission classes (context-based) | | gRPC proto | One service per file, shared common.proto | | gRPC streaming | Server stream for lists, bidirectional for real-time | | SSE keepalive | Every 30 seconds | | WebSocket heartbeat | ping-pong every 30 seconds | | Async generator cleanup | aclosing() for all external resources |

Common Mistakes

Verbs in URLs (POST /createUser instead of POST /users)
Inconsistent error formats across endpoints
Breaking contracts without version bump
Plain text error responses instead of Problem Details
Sunsetting versions without deprecation headers
Exposing internal details (stack traces, DB errors) in errors
Missing Content-Type: application/problem+json on error responses
Supporting too many concurrent API versions (max 2-3)
Caching without considering version isolation

Evaluations

See test-cases.json for 9 test cases across all categories.

Related Skills

fastapi-advanced - FastAPI-specific implementation patterns
rate-limiting - Advanced rate limiting implementations and algorithms
observability-monitoring - Version usage metrics and error tracking
input-validation - Validation patterns beyond API error handling
streaming-api-patterns - SSE and WebSocket patterns for real-time APIs

Capability Details

rest-design

Keywords: rest, restful, http, endpoint, route, path, resource, CRUD Solves:

How do I design RESTful APIs?
REST endpoint patterns and conventions
HTTP methods and status codes

graphql-design

Keywords: graphql, schema, query, mutation, connection, relay Solves:

How do I design GraphQL APIs?
Schema design best practices
Connection pattern for pagination

endpoint-design

Keywords: endpoint, route, path, resource, CRUD, openapi Solves:

How do I structure API endpoints?
What's the best URL pattern for this resource?
RESTful endpoint naming conventions

url-versioning

Keywords: url version, path version, /v1/, /v2/ Solves:

How to version REST APIs?
URL-based API versioning

header-versioning

Keywords: header version, X-API-Version, content negotiation Solves:

Clean URL versioning
Header-based API version

deprecation

Keywords: deprecation, sunset, version lifecycle, backward compatible Solves:

How to deprecate API versions?
Version sunset policy
Breaking vs non-breaking changes

problem-details

Keywords: problem details, RFC 9457, RFC 7807, structured error, application/problem+json Solves:

How to standardize API error responses?
What format for API errors?

agent-facing-errors

Keywords: agent error, AI agent, retryable, retry_after, error_category, content negotiation, accept header, token efficient, machine readable Solves:

How to design error responses for AI agent consumers?
How to reduce token cost of error responses?
How to enable deterministic agent error handling?
Content negotiation for agents vs browsers vs LLMs

validation-errors

Keywords: validation, field error, 422, unprocessable, pydantic Solves:

How to handle validation errors in APIs?
Field-level error responses

error-registry

Keywords: error registry, problem types, error catalog, error codes Solves:

How to document all API errors?
Error type management

Agent Skills: API Design

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