swagger.io/docs

There is openapitools/openapi-diff. To use, for example,

docker run -v $HOME/src/tmp/specs:/opt/specs openapitools/openapi-diff /opt/specs/old-spec/api.yml /opt/specs/new-spec/api.yml

Attention The tool might not yield diffs as it is not able to parse the specs properly.

Alternatively, use

docker run -v $HOME/src/tmp/specs:/opt/specs openapitools/openapi-generator-cli generate --config /opt/specs/config.yml -i /opt/specs/new-spec/api.yml -g openapi -o /opt/specs/new-spec/json

and compare using jsondiff.com.