API design guidelines are a set of rules that development teams follow when building and maintaining RESTful APIs to ensure consistency, reliability, and ease of integration.

Authentication and Authorization:
All API endpoints must require authentication using Bearer tokens in the Authorization header. Access tokens must expire after 15 minutes and refresh tokens after 7 days. Service-to-service communication must use OAuth 2.0 client credentials flow. Public endpoints that do not require authentication must be explicitly listed in the API security manifest. Rate limiting must be applied per-token with a default of 100 requests per minute for standard tier and 1000 for premium tier.

Request Validation:
All request payloads must be validated against JSON Schema definitions before processing. Path parameters must use UUID v4 format for resource identifiers. Query parameters for filtering must follow the format filter[field]=value. Pagination must use cursor-based navigation with a maximum page size of 100 items. Date-time values must be in ISO 8601 format with UTC timezone (suffix Z).

Error Handling:
All error responses must follow the Problem Details format (RFC 7807). Internal server errors must not expose stack traces or internal implementation details. Field validation errors must return HTTP 422 with a list of specific field errors. Rate limit exceeded responses must include Retry-After header with seconds until reset. Circuit breaker states must be logged but not exposed in API responses.

Caching:
GET endpoints must include appropriate Cache-Control headers. Mutable resource responses must include ETag headers for conditional requests. Cache keys must incorporate the Accept-Language header for localized responses. Stale-while-revalidate must be set to 60 seconds for collection endpoints. Private data endpoints must set Cache-Control to no-store.

Logging and Observability:
All requests must be logged with a correlation ID (X-Request-ID header). Response times exceeding 500ms must trigger a warning-level log entry. All 5xx errors must generate alerts in the monitoring system within 30 seconds. Distributed tracing must use W3C Trace Context format. Personally identifiable information must be redacted from all log entries.

Deprecation Policy:
Deprecated endpoints must continue functioning for a minimum of 6 months after announcement. API versions follow semantic versioning with breaking changes only in major versions. Migration guides must be published at least 90 days before deprecation takes effect. Deprecated endpoints must return the Deprecation header with the date of announcement. Usage metrics for deprecated endpoints must be tracked and reported monthly.
