frontend-data-contracts

Frontend Data Contracts (typed network boundary)

Portable skill — readable by Claude Code, OpenCode, Codex, Cursor, Windsurf, and others. This skill describes a discipline at the network edge — one client, one envelope, one error type, validated types — not a state library or a styling system. It pairs with the frontend-architecture skill (the client lives in shared/api-client/) and is the foundation the frontend-optimistic-mutations skill builds on.

The goal: the moment data crosses from the network into the app, it stops being any-shaped wire JSON and becomes a trusted, typed domain value — or it becomes a single, typed error. There is exactly one place this transformation happens, and nothing untyped escapes it.

0. The five core ideas

1. One client is the only fetch boundary. A single typed apiClient wraps fetch. Components and hooks never call fetch/axios directly — the boundary is enforceable in review and lint.
2. Parse, don't validate. Wire JSON is parsed into domain types at the boundary. After the client returns, the value is trusted everywhere downstream — no defensive ?. chains, no re-checking shapes in components.
3. One envelope. Every response is { data } on success or { error } on failure. The client unwraps data and throws on error, so callers receive the payload directly or a typed throw.
4. One normalized error type. Server error envelope, non-2xx status, malformed body, network failure, and abort all become a single ApiError with a machine code, status, and optional per-field errors. Callers handle one shape.
5. Identifiers are branded. Domain IDs are nominal types (InvoiceId, CustomerId) so the compiler rejects passing one where another is expected — the most common silent bug in data-heavy UIs.

1. Directory layout

The boundary is one folder in shared/ (per the frontend-architecture skill).

src/shared/api-client/
├── index.ts        ← barrel: apiClient, ApiError, types
├── client.ts       ← the fetch wrapper: buildUrl, headers, parse, verbs
├── config.ts       ← base URL resolution, default headers
├── error.ts        ← the ApiError class + code→message-key mapping
├── types.ts        ← envelope types, HttpMethod, RequestOptions, field errors
└── client.test.ts  ← boundary behavior tests (envelope, errors, network)