The warning message is expected based on your model schema and controller/action setting. Because, there's no element called 'GetTagMessages' in your model.

If you config the action in the Edm model builder, you get "405" is expected because OData routing builds a 'conventional' endpoint for 'GetTagMessage' controller method.

I create a sample for your reference and make the Edm action working without warning. See details at commit https://github.com/xuzhg/WebApiSample/commit/87cfed8981156ab2edde5618cb9f28eb4e6fc057

Please let me know your details requirements. You can file issue on the github or leave the comments here.

Thanks.