Most teams write an OpenAPI spec after their API is built, as documentation. The spec-first approach inverts this: write the spec before the implementation, and use it to drive both code generation and security validation. When the spec is the source of truth, security properties defined at design time are automatically enforced at runtime.
Security Properties You Can Encode in OpenAPI
Field constraints
maxLength prevents oversized string payloads (relevant for prompt injection — a 100,000-character string in a request body is rarely legitimate). minimum and maximum bound numeric fields. pattern applies regex validation to string fields. enum restricts a field to a defined set of values.
Authentication requirements
The securitySchemes section defines your authentication methods. The security property at the operation level specifies which methods apply to each endpoint. An endpoint with no security property inherits global defaults — or is explicitly unauthenticated if you declare security: []. This makes auth requirements visible and auditable in the spec.
Response schema validation
OpenAPI 3.1 allows full JSON Schema for response bodies. Define what your API returns and validate responses against the schema in your test suite. A response that includes fields not defined in the schema — like an accidentally leaked internal field — is caught before it reaches production.
Schema Validation as a Security Gate
When your API gateway validates every request against the OpenAPI spec, the spec becomes a positive security model: anything not explicitly defined is rejected. This eliminates entire classes of injection attacks — a request with an unexpected parameter that your application might have processed vulnerably is rejected at the gateway before it reaches code.
G8KEPR loads your OpenAPI spec and enforces it as a request/response validation layer. Requests that violate the spec are rejected with a 400. Responses that violate the spec are flagged as anomalies. The spec is your security perimeter.