VoidActionError.message ignores the server's message, though the docs show it to users #680

Summary

When an action fails, VoidActionError.message is a status line such as Action failed with 404 Not Found. A 429's usage-limit message is the only exception. The handler's own message, for example from HTTPException, ends up only in error.body.

The action() guide shows the message to users:

if (!result.ok) {
  showToast(result.error.message);
}

Followed as written, that toast says "Action failed with 404 Not Found" instead of what the action said.

Reproduction

// pages/notes.server.ts
import { HTTPException } from 'hono/http-exception';
import { defineHandler } from 'void';

export const action = defineHandler(async (c) => {
  const status = Number(c.req.query('status'));
  throw new HTTPException(status as 404, { message: 'That note was deleted.' });
});

// pages/notes.tsx
const result = await action(`/notes?status=${status}`, { data: {} });
// For 403 and 500, action() throws, so read the caught error instead.

The server answers with the message as text/plain. On the client:

The message is built in VoidActionError's constructor (pages/action-result.ts), which reads body only for a 429's usage limit.

Why it matters

• The documented pattern shows a status line. An app that follows the guide shows users text written for developers.
• Every app writes the same helper. To show the action's message, each app checks that body is a non-empty string and falls back otherwise, and has to know that Void stores a text body there.
• Thrown errors have no guard. 401, 403 and 5xx reach the error boundary, where the only exported guard, isCallSiteActionError(), is false for them. Apps check error.name === 'VoidActionError' by hand, as the guard itself does, because instanceof isn't reliable when the class is bundled more than once.

Proposal

1. Use the server's text for 4xx. When the status is 4xx and body is a non-empty string, set message to it, as the constructor already does for a 429's limit message. Keep the status line for 5xx, where the text may not be meant for users, and when there's no body. status and statusText still carry the status.
2. Export isVoidActionError(), true for every status, so error boundaries can recognise action errors.
3. At minimum, fix the docs. Say what message holds, and change the action() example to read error.body with a fallback.

Prior art

• Better Auth, which Void bundles: a failed call's error.message is the server's message.
• SvelteKit: error(404, 'That note was deleted.') reaches the page as page.error.message.

Workaround

// By name: instanceof fails across bundled copies.
function isActionError(error: unknown): error is VoidActionError {
  return error instanceof Error && error.name === 'VoidActionError';
}

function getErrorMessage(error: unknown): string {
  if (isActionError(error) && error.status < 500 && typeof error.body === 'string' && error.body) {
    return error.body;
  }
  return 'Something went wrong, please try again';
}

Environment

• void 1.5.0
• @void/react 1.5.0