API Design

Use this skill before implementing or reviewing any HTTP API contract. It defines the contract-first procedure for this repo. For backend implementation mechanics (DTO/Swagger/SDK patterns, tests, Nx verification) follow apps/chat-api/AGENTS.md — do not restate them here. For library isolation follow the "Library isolation" section in the root AGENTS.md: host/API/transport details stay in apps; hand-authored libs/* receive resolved data, values, or callbacks. Exception: libs/chat-api-client is the generated OpenAPI client.

Start Here

1. Read openspec/config.yaml for API architecture, project boundaries, and proposal/spec/task rules.
2. Inspect the closest existing endpoint and its matching frontend client before designing a new contract:
   • Backend controllers: apps/chat-api/src/**/**.controller.ts
   • Backend DTOs: apps/chat-api/src/**/dto/*.dto.ts
   • Frontend API helpers: apps/chat/src/server-api/
   • Shared interfaces: libs/chat-shared/

Contract Checklist

For every new or changed endpoint, define these before coding:

• Method and full path. Business endpoints are URI-versioned (/api/v1/<resource>); infrastructure endpoints stay unversioned (/api/<resource>).
• Request shape: path params, query params, body DTO, headers, cookies.
• Success response shape and status code.
• Error status codes and what each means: at least validation/auth/authorization/not found, plus 502 (upstream non-OK), 503 (upstream timeout/unavailable). Never 500 exposing internals.
• Authentication and authorization requirements, including whether the endpoint is explicitly public.
• Cache behavior when applicable: TTL, invalidation, and key naming (<domain>:<resource>[:<param>]).
• Pagination for list endpoints: prefer cursor for large/append-heavy datasets; offset is acceptable for small admin-style or search flows.
• Frontend impact: thin domain wrapper in apps/chat/src/server-api/ that delegates to generated @epam/chat-api-client, shared type in libs/chat-shared/ if needed, and i18n strings for any errors surfaced in UI.
• Library impact: if a hand-authored lib needs the data/behavior, name the prop/callback/resolved value it receives (see lib-isolation note above).
• Generated client impact: expected SDK class, method name (derived from the controller handler name via operationIdFactory), request type, response type, and whether callers need Raw access for headers/status.

Versioning And Compatibility

• Non-breaking changes need no new version: adding optional query params, adding response fields, or adding new endpoints.
• Breaking changes require a new versioned controller or endpoint path. Keep the previous version until callers are migrated.
• Proposals/specs must call out frontend migration impact and whether shared types change.

OpenSpec Integration

When writing or reviewing an OpenSpec change that touches API behavior:

• Specs must include method, full versioned path, request body, response body, and error codes.
• Tasks must name concrete files, not vague "update API" wording.
• Include dedicated tasks for: Swagger DTO/annotation updates, npm run openapi, npm run openapi:check, chat-api-client build/lint, backend controller/service tests, generated API singleton updates in apps/chat/src/server-api/api-client.ts, and frontend wrappers using generated @epam/chat-api-client.
• Generated-client frontend usage is required for new or changed business REST endpoints. Direct base.ts get/post/put/del usage is allowed only for documented generator gaps, streaming calls, or infrastructure endpoints.
• If endpoint behavior is surfaced through a lib, tasks must keep the generated-client/server-api call in apps/chat and pass only resolved data, values, or callbacks into the lib.
• Changes under libs/chat-api-client must come from OpenAPI generation (npm run openapi, npm run openapi:sdk, or the documented check), not manual edits.
• Include cache requirements when relevant.

Avoid

• Returning 200 for error cases.
• Designing backend-only contracts without checking the frontend server-api layer.
• Adding handwritten frontend REST helpers over base.ts for business endpoints the generated client can represent.
• Hand-editing generated files under libs/chat-api-client/src/generated.
• Adding unversioned business endpoints.
• Introducing a generic { data } envelope unless the domain already uses it or the change explicitly standardizes responses.
• Mixing API redesign with unrelated implementation refactors.