4xx Client ErrorCommonly usedRFC 9110
400Bad Request
Server cannot process the request due to a client error.
What it means
The server cannot or will not process the request due to something wrong with the request itself — malformed syntax, invalid parameters, missing required fields, or invalid data. The client should not repeat the request without modification.
When to use it
- ✓Request body is malformed or invalid JSON
- ✓Required fields are missing
- ✓Field validation failed (wrong type, out of range)
- ✓Query parameters are invalid
Common causes
- →Sending invalid JSON in request body
- →Missing required fields
- →Wrong data types (string where number expected)
- →Invalid date formats
Code Examples
Express with Zod validation
typescript
import { z } from 'zod';
const schema = z.object({
email: z.string().email(),
age: z.number().min(18),
});
app.post('/users', (req, res) => {
const result = schema.safeParse(req.body);
if (!result.success) {
return res.status(400).json({
error: 'Invalid request',
details: result.error.flatten(),
});
}
// process valid data...
});Quick Facts
Code400
CategoryClient Error
SpecRFC 9110
CommonYes