The opposite of issue QTBUG-122163, instead of generate C++ code from OpenAPI Specs (Design-First), Qt HTTP server should be able to generate OpenAPI JSON and/or YAML specs from code (Code-First).

Generating OpenAPI specs from code functionality is available in some of web framework both built-in or 3rd party solution, for example:
C++: oatpp
C#: .NET
Python: FastAPI, Litestar
Rust: Poem, Salvo
etc

Generally it's done by looking for path, query, request body, and response body model of route implementation and if something is missing for OpenAPI specs generation, it will be annotated.