I never liked GraphQL to TypeScript types codegen. I always felt it should be done the other way around yet existing solutions still approached it from GraphQL end. My idea was to use bi-directional introspectable schema to encode GraphQL typesystem and derive GraphQL schema from it. Unlike other code-first GraphQL schema type systems it allows for reusing existing well-known library for defining your GraphQL Data Transfer Objects with all benefits of encoding / decoding, JIT-compiled parsers and other goodness.
It provides decent level of type-safety, even for convoluted GraphQL Interface use cases.
API should be considered work-in-progress

Basic E2E example can be found here E2E test

Type test for subtyping GraphQL interface can be found here type test

Ideas and contributions are welcome!