Build a resilient API client

The checklist

Every strong client does these five things:

1. Set a request timeout so a hung call fails fast.
2. Retry only 429 and 5xx, with jittered backoff.
3. Honour Retry-After on a 429.
4. Attach an idempotency key to every write.
5. Branch on error.code, not the HTTP status alone.

Wire it together

A thin wrapper composes them:

async function call(method, path, body, key) {
  return withRetry(() => fetch(BASE + path, {
    method, signal: AbortSignal.timeout(8000),
    headers: { 'Content-Type':'application/json',
               ...(key ? {'Idempotency-Key': key} : {}) },
    body: body && JSON.stringify(body),
  }));
}

Branch on the outcome

Map error.code to user-facing behaviour using the error code catalogue. Fix-the-request codes surface a clear message; transient codes are handled silently by the retry layer.