Standardize error handling so failures remain diagnosable and user-facing messages are complete.

Apply these rules: 1) Preserve diagnostic context when propagating/rethrowing

• If you rebuild or re-aggregate errors, do not unintentionally lose stack trace/throw location. Centralize error creation/throwing so the actual throw happens in a predictable place, and avoid patterns that always create a fresh error object without considering stack semantics.

2) Ensure every issue code is explicitly mapped

• When adding a new ZodIssueCode (or changing which codes are emitted), update defaultErrorMap and/or locale maps so users don’t see generic fallback messages.

3) Extract/format messages safely (no unsafe casts)

• Don’t assume catch (error) is an Error. Prefer instanceof Error or a shared helper that guarantees a string.

4) Don’t unintentionally suppress other validations

• If you use early returns in validation logic, verify you’re not preventing other expected errors from being added for the same input.

Example: safe message extraction

try {
  return JSON.parse(val);
} catch (e: unknown) {
  const message = e instanceof Error ? e.message : String(e);
  ctx.addIssue({
    code: ZodIssueCode.invalid_string,
    validation: "json",
    message,
  });
}

Example: stack-preserving handler shape

const zodErrorHandler = (messageOrObject: string | unknown) => {
  if (typeof messageOrObject === "string") {
    throw ZodError.fromString(messageOrObject);
  }
  throw messageOrObject; // keep original throw semantics where possible
};

Checklist for PRs in this area

• New/changed ZodIssueCode has explicit message mapping (default + locales if needed).
• Validation control flow (early returns) can’t hide required additional issues.
• No error as Error without instanceof or a safe conversion helper.
• Error propagation/rewrap logic reviewed for stack/throw-location implications.