I'm one of the builders of an open source project (https://buildwithfern.com/docs) to improve API codegen. We built Fern as an alternative to OpenAPI, but of course we're fully compatible with it.
We rewrote the code generators from scratch in the language that they generate code in (e.g., the python generator is written in python). We shied away from templating - it's easier but the generated code feels less human.
I think it's worth pointing out that a lot (most?) of Fern's complaints against OpenAPI are really complaints against JSON Schema. There have been talks before about allowing other schema systems in OpenAPI - I wouldn't be incredibly surprised to see such things come up for Moonwalk. JSON Schema is not not a type definition system https://modern-json-schema.com/json-schema-is-a-constraint-s....
It's also worth noting that most JSON Schema replacements I've seen that prioritize code generation are far less powerful in terms of runtime validation (I have not examined Fern's proposal in detail, so I do not know if this is true for them).
The ideal system, to me (speaking as the most prolific contributor to JSON Schema drafts-07 through 2020-12), would have clearly defined code generation and runtime validation features that did not get in each other's way. Keywords like "anyOf" and "not" are very useful for runtime validation but should be excluded from type definition / code generation semantics.
This would also help balance the needs of strongly typed languages vs dynamically typed languages. Most JSON Schema replacements-for-code-generation I've seen discard tons of functionality that is immensely useful for other JSON Schema use cases (again, I have not deeply examined Fern).
Just wanted to chime in and say we are a big fan of Fern! It makes developing our APIs 10x easier as we get full end-to-end type safety and code completion, but the really great part is we get idiomatic client and server SDKs as well as OpenAPI output that we use to autogen documentation. Our small team of two engineers are able to ship multiple client facing SDKs because we are built on Fern!
Speed of development on these guys is huge, and have enjoyed using their SDKs. Community is getting involved in building things like auto-retry with backoff and other pretty helpful features in SDKs. Big fan of these guys!
The generators are open source: https://github.com/fern-api/fern
We rewrote the code generators from scratch in the language that they generate code in (e.g., the python generator is written in python). We shied away from templating - it's easier but the generated code feels less human.
Want to talk client library codegen? Join the Fern Discord: https://discord.com/invite/JkkXumPzcG