You’ll need to ensure that the attribute is accepted by the action you are submitting. Try adding accept [:the_attribute] to the action

Hmm…yeah, so AshPhoenix passes the skip_unknown_inputs option with all of the fields it submits, so that you don’t get errors on unknown inputs. The reason it does this is because often in a form there is “extra data” used to do UI related stuff, and we don’t want that to cause an error. I think adding some kind of warning or error would make sense, but there are some edge cases where you actually can’t do anything about the extra keys in the form, and you wouldn’t be able to clear that warning or error…

I’ll have to think about it clearly a bad DX there, but its one of those things that is more complex than it seems to address (within forms, if you passed these inputs in outside of a form when calling an action, you’d have gotten a clear error message).