Validation, Error Response Design, and API Contract Quality
Make your API easier to consume by designing validation feedback and error contracts clearly and consistently.
Inside this chapter
- Why Validation Is More Than Rejecting Bad Data
- A Helpful Error Payload
- Consistency Matters
- Domain Errors vs Technical Errors
- Real Example
Series navigation
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Why Validation Is More Than Rejecting Bad Data
Validation protects system integrity, improves client behavior, and prevents poor-quality data from reaching business logic or storage. Good validation responses also teach clients how to correct requests rather than leaving them confused.
A Helpful Error Payload
{
"error": "validation_failed",
"message": "One or more fields are invalid",
"details": {
"email": "Email format is invalid",
"password": "Password must be at least 12 characters"
}
}Consistency Matters
Clients should not have to guess whether one endpoint returns {"error":"bad"} and another returns a completely different structure. A consistent contract makes frontend, mobile, and partner integrations easier to maintain.
Domain Errors vs Technical Errors
Some errors come from invalid client data, while others come from domain rules such as insufficient balance or expired plans. Still others come from server or infrastructure issues. Good API design communicates those categories clearly.
Real Example
A booking API might reject a request because the date is invalid, because the room is already reserved, or because the payment service is currently unavailable. These failures are not the same, and clients should be able to react appropriately to each one.