What should be sent to the client in response if the input data fails validation?

If the incoming data fails validation, the server should send a response to the client with an HTTP 400 Bad Request status code. The response body typically contains information about the validation errors so that the client can understand what exactly is wrong and correct the request.

Example response in ASP.NET Core:

if (!ModelState.IsValid)
{
    return BadRequest(ModelState);
}

Here, ModelState contains details of the validation errors, which will be serialized in the response and sent to the client.