We use and recommend Zod for that:

https://github.com/colinhacks/zod#error-handling

Like any user input, you do runtime type-checking and validation. TypeScript is especially great for that because it has type narrowing [0].

[0] - https://www.typescriptlang.org/docs/handbook/2/narrowing.htm...

> Curious how you handle `request.formData()` in Form Actions and ensuring type safety there.

Hopefully the same way typed programming languages do it: parsing and type checking.