The magic functions library uses a transformer to take the TypeScript types and port them to JSON schema, such that they're available during runtime. This JSON schema is then used to validate that the response from the LLM matches the expected type signature of the function (and err if it doesn't).

Because TypeScript doesn't support 3rd party "transformers" by default, you're forced to hack around it (via ttypescript or ts-patch). This is especially problematic when TypeScript has a major version change, as the workarounds need to be modified accordingly; this often takes significant time.

Here's the long-lived Github issue: https://github.com/microsoft/TypeScript/issues/14419

And here's the newest proposal to add official support: https://github.com/microsoft/TypeScript/issues/54276