And as far as I can tell, there is no way to have it validated in prod and temporarily disabled for testing like this? Even a subclass doesn't work? This seems… wrong? Am I missing something?

You're not missing something; There is no way to temporarily turn off validation for this. Litestar is rather strict about this (you might even say pedantic 🙂).

That being said, a subclass should work. The reason why it does not comes down to a limitation of msgspec. This for example fails as well, even though it would be fine for e.g. a type checker:

import msgspec

class Foo(msgspec.Struct):
    pass

class Bar(Foo):
    pass

class Container(msgspec.Struct):
    foo: Foo

msgspec.convert({"foo": Bar()}, type=Container, strict=False)

The other reason this doesn't work is because Litestar implicitly validates the injected dependencies. This wasn't always the case and not intentionally designed as a feature. What you're experiencing here is one of the drawbacks of this approach.

We have considered changing this behaviour in v3, but for now I'm afraid there's not much you can do other than providing the correct type in your tests as well or turn of validation for this dependency.

A completely different approach we've discussed a while ago (and might consider picking up again @litestar-org/maintainers?) would be to change how dependency overrides with the test client work. Currently, you can provide dependency providers. An alternative would be to allow providing the values, to be injected directly into the handler.