While replacing a custom implementation of Problem Details by the new one that came with Spring 6 I was puzzled by how content negotiation is implemented. I'm currently using Spring Boot 3.0.0 with Spring 6.0.2.

I'm getting a Content-Type application/problem+json as expected when sending no accept header or Accept: */* or Accept: application/problem+json, application/json.

I also expected to receive a Problem Details JSON with Content-Type application/problem+json when sending Accept: application/json. But that might be correct - it doesn't seem to be clearly defined by RFC 7807.

Clearly a bug, seems that I'm still getting Content-Type application/json when sending Accept: application/json, application/problem+json. See also:

Message converters and encoders indicate a preference for application/problem+json when ProblemType is serialized. This ensures application/problem+json is preferred when the client is flexible or has no preference.

from: #28189 (comment)