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)

Domain entity types and their schemas live with their feature module (modules/{feature}/types/) or a shared contract package; the client is generic over T.

2. One client, the only fetch boundary

Every verb returns the unwrapped data payload typed by the caller, and throws an ApiError on any failure. Components never see envelopes or raw responses.

// shared/api-client/client.ts (essence)
export const apiClient = {
  get<T>(path: string, options?: RequestOptions): Promise<T> {
    return request<T>("GET", path, undefined, options);
  },
  post<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
    return request<T>("POST", path, body, options);
  },
  patch<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
    /* … */
  },
  put<T>(path: string, body?: unknown, options?: RequestOptions): Promise<T> {
    /* … */
  },
  delete<T>(path: string, options?: RequestOptions): Promise<T> {
    /* … */
  },
} as const;

export type ApiClient = typeof apiClient;

// CORRECT — a feature hook wraps the client, typed by the caller
const invoice = await apiClient.get<Invoice>(`/invoices/${id}`, { signal });

// WRONG — a raw fetch in a component bypasses the boundary entirely
const res = await fetch(`/api/invoices/${id}`); // untyped, unhandled errors, no envelope

Hard rules:

• No fetch/axios/XMLHttpRequest outside shared/api-client/ — enforce with an ESLint no-restricted-imports/no-restricted-globals rule.
• The client is framework-free: no toasts, no router, no React. Side effects (toasts, redirects) live in the query layer's onError (see §6).
• Pass AbortSignal through RequestOptions so the query layer can cancel (wired by TanStack Query).

3. Parse, don't validate (the boundary transform)

"Validate" leaves you with the same untyped value and a boolean. "Parse" returns a new, typed value — so downstream code is guaranteed correct by the type system. Run a schema parse at the boundary; after that, the value is trusted.

// modules/invoice/types/invoice.schema.ts
import { z } from "zod";

export const invoiceSchema = z.object({
  id: z.string().transform(toInvoiceId), // brand it (see §5)
  number: z.string(),
  status: z.nativeEnum(InvoiceStatus),
  total: z.number().int(), // minor units — never float money
  issuedAt: z.string().datetime(),
});
export type Invoice = z.infer<typeof invoiceSchema>;

// the client (or a thin per-entity wrapper) parses at the edge
const raw = await apiClient.get<unknown>(`/invoices/${id}`, { signal });
return invoiceSchema.parse(raw); // throws on contract drift → surfaces as a typed failure

Why this matters: a backend that renames a field or sends a null it shouldn't is caught at the boundary, with a clear error, instead of producing undefined three components deep where the stack trace is useless. Components downstream never write invoice?.total ?? 0 defensively.

Validation library is your choice — Zod, Valibot, ArkType, io-ts. The rule is constant: a parse step converts unknown wire data into a typed domain value at one boundary.

4. One response envelope

Mirror the backend's single envelope in the client and unwrap it once.

// shared/api-client/types.ts
export interface ApiSuccessEnvelope<T> {
  data: T;
}
export interface ApiErrorEnvelope {
  error: ApiErrorBody;
}
export type ApiEnvelope<T> = ApiSuccessEnvelope<T> | ApiErrorEnvelope;

export function isApiErrorEnvelope<T>(
  e: ApiEnvelope<T>,
): e is ApiErrorEnvelope {
  return typeof e === "object" && e !== null && "error" in e;
}

export interface ApiErrorBody {
  code: ServerErrorCode; // machine-readable, stable
  message: string; // server message (NOT shown to users directly)
  fields?: Record<string, string[]>; // per-field validation errors
}

The parse step handles every shape: 204 No Content → undefined; { error } → throw; non-2xx with no well-formed envelope → synthesize an error; { data } → return data. The caller only ever sees a typed payload or a throw.

5. Branded (nominal) identifiers

Strings are interchangeable; domain IDs are not. Brand them so the compiler stops you passing a CustomerId where an InvoiceId is required.

// shared/types/id.ts
declare const brand: unique symbol;
export type Brand<T, B extends string> = T & { readonly [brand]: B };

export type InvoiceId = Brand<string, "InvoiceId">;
export type CustomerId = Brand<string, "CustomerId">;

export const toInvoiceId = (s: string): InvoiceId => s as InvoiceId;
export const toCustomerId = (s: string): CustomerId => s as CustomerId;

function loadInvoice(id: InvoiceId) {
  /* … */
}
loadInvoice(customerId); // ❌ compile error — exactly the bug you want caught
loadInvoice(invoiceId); // ✅

Brand IDs at the parse boundary (§3) so every ID in the app is already nominal. The runtime cost is zero — brands erase at compile time.

6. One normalized error type

Collapse every failure mode into a single ApiError so callers handle one shape. It carries a machine code, the HTTP status, optional per-field errors, and a stable key for localized messages (it does not localize — that's the UI's job).

// shared/api-client/error.ts (essence)
export class ApiError extends Error {
  readonly code: ServerErrorCode;
  readonly status: number; // 0 when no response (network/abort)
  readonly fields?: Record<string, string[]>;

  get isNetworkError() {
    return this.status === 0;
  }
  get hasFieldErrors() {
    return !!this.fields && Object.keys(this.fields).length > 0;
  }
  /** Stable key under an `errors` i18n namespace — never a raw server string. */
  get messageKey() {
    if (this.isNetworkError) return "network";
    return ERROR_CODE_MESSAGE_KEYS[this.code] ?? "generic";
  }

  static fromEnvelope(body: ApiErrorBody, status: number) {
    /* server { error } */
  }
  static fromNetwork(cause: unknown) {
    /* offline / CORS / abort → status 0 */
  }
}

6.1 Where side effects live

The client throws; the query layer decides what the user sees. Keep toasts/redirects out of the client.

// a TanStack Query mutation maps the typed error to a localized toast
useMutation({
  mutationFn: (input) => apiClient.post<Invoice>("/invoices", input),
  onError: (error: ApiError) => notifyError(error), // looks up error.messageKey in i18n
});

6.2 Per-field errors → form fields

Server validation (fields) maps straight onto form-field errors — one place, typed.

if (error.hasFieldErrors) {
  for (const [field, messages] of Object.entries(error.fields!)) {
    form.setError(field as Path<FormValues>, { message: messages[0] });
  }
}

Hard rules:

• Never throw new Error(string) from the data layer — always ApiError with a code.
• Never show error.message (a server/dev string) directly to users — resolve messageKey through i18n.
• A 2xx with an unparseable body is a contract violation → throw INVALID_RESPONSE, don't silently return undefined.

7. Library adapters

The discipline is constant; the data-fetching library only changes where onError/parsing hangs.

For React Native, the client is unchanged — fetch and AbortController exist in RN. Only credentials: "include" (cookie auth) may need swapping for a token header depending on your auth.

8. Conventions checklist (enforce in review)

• Exactly one apiClient in shared/api-client/; no fetch/axios anywhere else (lint-enforced).
• The client is framework-free — no toasts, router, or React inside it.
• Responses are parsed into typed domain values at the boundary (parse, don't validate).
• One { data } / { error } envelope, unwrapped once in the client.
• Every failure becomes one ApiError (code + status + optional fields); no bare throw new Error.
• Domain IDs are branded; IDs are branded at the parse boundary.
• error.messageKey resolves through i18n — server message is never shown to users.
• Per-field server errors map onto form fields via the typed fields map.
• AbortSignal flows through RequestOptions for cancellation.
• A 2xx with a malformed body throws a contract-violation error, not undefined.

9. How to apply this skill

Adding the boundary to a project: create shared/api-client/ with client.ts, error.ts, types.ts, config.ts. Add the lint rule banning fetch/axios elsewhere. Define your envelope to match the backend, and your ApiError codes.

Adding a new entity: define its schema (Zod/Valibot) and z.infer type in the feature module, brand its ID at parse time, and wrap apiClient in typed query/mutation hooks — never call the client from a component.

Debugging "undefined three components deep": add/repair the boundary parse so contract drift fails loudly at the edge with a typed error, instead of leaking undefined downstream.

Reviewing the data layer: run the checklist in §8. The highest-value catches are raw fetch in components (boundary bypass) and throw new Error(string) from the data layer (untyped failures).

Publishing / installing this skill

This skill follows the Anthropic SKILL.md format and is portable across agents.

1. Keep it under skills/frontend-data-contracts/SKILL.md in a public GitHub repo.
2. Keep the frontmatter name and high-signal description — discovery indexes match against it.
3. Install with: npx skills add <org>/<repo> --skill "frontend-data-contracts".
4. Non-SKILL.md agents can be pointed here from AGENTS.md / CLAUDE.md; Kiro can mirror it as a steering file.