defineHandler(middleware, handler) breaks page loaders and actions: 500 on GET, empty 204 after an action, validator types dropped #564

Description

defineHandler(middleware..., handler) is documented for per-route middleware, and the loader and action guides say they use "the same defineHandler API as server routes". In routes/ it works. In a page's .server.ts it breaks both loaders and actions, even with a middleware that only calls next():

• A loader with middleware makes the page fail with a 500.
• An action with middleware returns an empty 204 instead of the re-rendered page, and a validation failure returns the raw 400 JSON instead of the page with field errors.
• The action's validator types are lost, so useForm('/db?create', …) accepts any body.

Steps to reproduce

// pages/db.server.ts
import { defineHandler, defineMiddleware } from 'void';
import * as z from 'zod';

const passThrough = defineMiddleware(async (_c, next) => {
  await next();
});

// Case A: loader with middleware
export const loader = defineHandler(passThrough, async () => {
  return { notes: await getNotes() };
});

// Case B: action with middleware (test with a plain `defineHandler(async () => …)` loader, since case A fails first)
export const actions = {
  create: defineHandler(
    passThrough,
    defineHandler.withValidator({
      body: z.object({ title: z.string().min(1) }),
    })(async (_c, { body }) => {
      await createNote(body);
    }),
  ),
};

# Case A
curl -i http://localhost:5173/db
curl -i -H 'X-VoidPages: true' http://localhost:5173/db

# Case B
curl -i -X POST 'http://localhost:5173/db?create' -H 'Origin: http://localhost:5173' \
  -H 'Content-Type: application/json' -H 'X-VoidPages: true' -d '{"title":"a"}'
curl -i -X POST 'http://localhost:5173/db?create' -H 'Origin: http://localhost:5173' \
  -H 'Content-Type: application/json' -H 'X-VoidPages: true' -d '{"title":""}'

Actual behavior

Types: ResolveActionBody<'/db?create'> falls back to Record<string, unknown>. { title: 1 } type-checks and form.data.title is unknown.

Expected behavior

The middleware form behaves like the plain form: the loader renders the page, an action re-renders it with fresh props, a validation failure renders it with errors, and the validator's body type reaches void/routes.

Cause

packages/void/src/runtime/handler.ts (main @ 6ba3dbcc3a, lines 288–303): the composed handler stores the final result in c.res and returns c.res:

} else {
  c.res = convertReturnValue(await handler(c));
}
// …
return c.res;

Hono's c.res setter sets c.finalized = true, and Hono's compose only adopts a Response returned by a later handler while finalized === false. Page code in packages/void/src/pages/protocol.ts then reads that Response and returns a new page response, which Hono drops and replaces with c.res:

• Loader: runLoader() → extractProps() reads c.res's JSON body, so Hono sends a body that was already read → 500.
• Action that returns nothing: c.res is the 204 from convertReturnValue(undefined), and that is what's sent.
• Validation failure: c.res is withValidator's 400. Since fix(pages): deferred props, action responses and dev rescan errors #517 it is read via clone(), so it's sent as is. On 1.1.12 this was a 500 (This ReadableStream is disturbed).

In routes/ the returned Response is the final answer anyway, so nothing is lost there.

Types: the middleware overloads return TypedHandler<{}, …>, so the __validators of a withValidator handler passed last are dropped. #490 fixed __output for loaders, not the validators.

Possible fix

• Runtime: have the composed handler return the handler's raw result (or a middleware's short-circuit Response) instead of assigning it to c.res, so page code gets the same value it gets without middleware. Middleware that edits c.res after next() (e.g. Hono's etag()) should keep working in routes/, so the convertReturnValue step may need to move to the route adapter.
• Types: an overload that keeps the last handler's type, e.g. defineHandler<V, R>(mw: MiddlewareFn, handler: TypedHandler<V, R>): TypedHandler<V, R>.

Workaround we use for now: a small withMiddleware(...middleware, handler) that runs the chain itself, returns the handler's result untouched, and is typed as the handler (H extends TypedHandler).

Related

• InferProps is unknown for a defineHandler loader with middleware #489 / fix(handler): infer output type for defineHandler with middleware #490: the loader output type with middleware (fixed in 1.2.0). InferProps is unknown for a defineHandler loader with middleware #489 notes that the runtime already unwraps a loader's return value as props, but on 1.2.1 the page itself fails (case A).
• fix(pages): deferred props, action responses and dev rescan errors #517: the clone() change for 400 responses from actions.

Environment

• void 1.2.1, @void/react 1.2.1 (the action cases also reproduce on 1.1.12)
• hono 4.13.13, zod 4
• vite-plus 1.3.0, Node 22.23.3, macOS 26.6