HTTP 400 Bad Request — Causes, Fix & API Validation Examples

When to Return 400

Use 400 for input validation failures: missing required fields, invalid field values, malformed JSON, invalid date formats, or business rule violations that the client could fix by changing the request.

Common Causes

• Malformed JSON body (parse error)
• Missing required request fields
• Invalid field values (wrong type, out of range)
• Invalid query parameter format
• Duplicate submission of a unique value

HTTP Response Example

HTTP/1.1 400 Bad Request
Content-Type: application/json

{"error": "Bad Request", "details": {"email": ["Invalid email address"]}}

Code Examples

import { z } from 'zod'

const CreateUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1),
})

app.post('/users', (req, res) => {
  const result = CreateUserSchema.safeParse(req.body)
  if (!result.success) {
    return res.status(400).json({
      error: 'Bad Request',
      details: result.error.flatten().fieldErrors,
    })
  }
  // proceed with result.data
})

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

const schema = z.object({ email: z.string().email(), name: z.string().min(1) })

export async function POST(request: Request) {
  const body = await request.json().catch(() => null)
  if (!body) return Response.json({ error: 'Invalid JSON' }, { status: 400 })

  const result = schema.safeParse(body)
  if (!result.success) {
    return Response.json(
      { error: 'Bad Request', details: result.error.flatten().fieldErrors },
      { status: 400 }
    )
  }
  // proceed
}

Related Status Codes