Which middleware is the feature for?

@hono/zod-openapi

What is the feature you are proposing?

The README says that with request.body.required unset, "a request with no body reaches the handler and c.req.valid('json') is {}". That {} is injected without running the schema, but it's typed as the schema's output, so an absent body and an explicit {} behave differently:

const route = createRoute({
  method: "post", path: "/x",
  request: { body: { content: { "application/json": {
    schema: z.object({ limit: z.number().default(20) }),
  } } } },
  responses: { 200: { description: "ok" } },
});
app.openapi(route, (c) => c.json(c.req.valid("json")));

await app.request("/x", { method: "POST" });                       // 200 {}          (no default)
await app.request("/x", { method: "POST",
  headers: { "content-type": "application/json" }, body: "{}" });  // 200 {"limit":20}

With z.object({ name: z.string() }), an absent body reaches the handler as {} typed { name: string }, while an explicit {} is a 400.

Proposal: when an optional body is absent, either run the schema on {}, so defaults apply and errors are reported, or pass undefined and type c.req.valid('json') as T | undefined. Either way, the type matches what the handler actually gets.

Workaround we use: route middleware that, when there's no body and no Content-Type, rewrites the request to content-type: application/json with body {}, so the validator runs.

Related: #891 and #2087 (the 415 for an undeclared content type).