Function createApiErrorCaches

Create a QueryCache and MutationCache pre-wired with the library's error policy. Pass them to your QueryClient constructor:

const { queryCache, mutationCache } = createApiErrorCaches({
  onError(error, ctx) {
    // observe-only: the promise always rejects afterwards.
    showErrorBanner(ctx.operationId)
  }
})
const queryClient = new QueryClient({ queryCache, mutationCache })
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Observe-only. opts.onError is a side-effect hook — the promise always rejects regardless of what the callback does or returns.

error is unknown, not AxiosError, because cache-level onError fires for ANY rejection, including non-axios throws (e.g. errors synthesised by request interceptors). Narrow with isAxiosError inside the policy.

The returned caches own config.onError. A consumer that needs an additional cache-level onError must compose it inside opts.onError, not replace the caches.