The fetch client throws two distinct error types worth handling separately: ValidationError for failed validation before the request is sent, and standard errors for network or server failures.

pages/example/index.tsx

import fetchMap, { ValidationError } from "_/fetch";

const useFetch = fetchMap["users/[id]"];

try {
  const response = await useFetch.POST([userId], payload);
} catch (error) {
  if (error instanceof ValidationError) {
    // data didn't pass validation - no request was made
    console.error("Invalid data:", error.message);
  } else {
    // network error, server error, etc.
    console.error("Request failed:", error);
  }
}

Validation errors carry the same structured detail as server-side ValidationError instances - target, errors, errorMessage, errorSummary - so you can surface field-level feedback without waiting for a server response.

More on Error Details →