Working OpenAPI spec parser/linter?

What tools are people using to validate their OpenAPI specs before uploading?

I have a spec that's considered valid by two separate tools but is being rejected when I upload to Readme.

The error is useless: "Unknown error (Cannot read property 'slug' of null)."

Ideally there is a tool out there which matches ReadMe's quirks so I can develop my APIs locally and know that when I push the code I will also be able to push the API updates.