Know of any similar frameworks that work the other way around? Where you can Keep an openapi definition as the source of truth and validate that your server follows it, I mean.

I agree. API-first is the way! Change your schema, auto-generate types into your code and use them for your server definition. It's just faster and more secure this way. Use api-fiddle.com or spotlight.io to work with the schemas (branching, sync with Github).

In a fully typesafe world, it should be pretty hard to derive from the shema this way.

As others suggested, I would diff against a generated one, then potentially treat the generated one as source of truth in the future... Then diff accordingly as it changes.

Rswag is still my favorite openapi-related project. You write integration tests against the server and get an openapi spec as output. But thats for rails.

Just have to be a bit careful mentioning it in front of non tech folks who'll inevitably ask "what's arsewag?"

You can simply diff it. Define the OpenAPI spec, have the framework generate a spec, compare.

You could generate a spec for the service and then diff to the expected perhaps.