Based on the documentation for fullWsTransport it seems like you should be able to perform mutations over web sockets, but in practice this results in a "Method not allowed" error.

This is happening because of the check here: https://github.com/mercurius-js/mercurius/blob/master/index.js#L474

It checks whether the raw request is GET and doesn't allow mutations. But the raw request in the case of web sockets is the original GET request that receives a 101 Switching protocols / connection upgrade.

Is this intended behavior?

As a side note, this actually results in an invalid response that at least Apollo client doesn't like and errors with ApolloError: "error" message expects the 'payload' property to be an array of GraphQL errors, but got "Method not allowed"