Strict mode is an optional validation feature that detects undeclared properties in API requests and responses, even when the OpenAPI schema would normally allow them via additionalProperties: true.
In standard JSON Schema validation, setting additionalProperties: true (or omitting it entirely) permits any extra properties beyond those explicitly defined.
Strict mode overrides this permissive behavior to enforce API governance, ensuring that clients only send properties explicitly documented in the OpenAPI specification.
It supports configurable ignore patterns (using glob syntax like $.body.metadata.*) to exclude specific paths, and has sensible defaults for common HTTP headers that are typically not documented in specs.
Strict mode respects readOnly and writeOnly semantics based on whether validation is for a request or response.
https://pb33f.io/libopenapi/validation/#strict-mode-validation