HTTP 422 Unprocessable Content

Read the error body

Validation frameworks tell you exactly which field failed. Here is a FastAPI response, trimmed:

curl -s https://api.example.com/items \
  -H 'Content-Type: application/json' \
  -d '{"name": "", "qty": -1}' | jq

{
  "detail": [
    { "type": "string_too_short", "loc": ["body", "name"],
      "msg": "String should have at least 1 character" },
    { "type": "greater_than", "loc": ["body", "qty"],
      "msg": "Input should be greater than 0" }
  ]
}

Next.js Route Handler: 400 for bad JSON, 422 for bad values

Parse first, then validate. This example uses Zod, but any schema library works the same way.

// app/api/items/route.ts
import { z } from 'zod';

const Item = z.object({
  name: z.string().min(1),
  qty: z.number().int().positive(),
});

export async function POST(request: Request) {
  let body: unknown;
  try {
    body = await request.json();
  } catch {
    return Response.json({ error: 'invalid_json' }, { status: 400 });
  }

  const parsed = Item.safeParse(body);
  if (!parsed.success) {
    return Response.json(
      { error: 'validation_failed', issues: parsed.error.issues },
      { status: 422 }
    );
  }

  const item = await createItem(parsed.data);
  return Response.json(item, { status: 201 });
}

Client: show errors next to the fields

Map each error's path (loc in FastAPI, path in Zod, keys of errors in Laravel) to the form input it belongs to. Don't retry automatically; the same data fails the same way.