A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Can you even use Kaiu Smithy to discard a card if you don't have any non-unique weapons in the discard pile to target?
I would have assumed that similar to Falling Leaf Strike, if you can't fulfill all the required targets on the card that you couldn't take the action.
So, can you use Kaiu Smithy if there's no non-unique weapons in your discard pile?
Can you use Kaiu Smithy to discard a non-unique weapon and put it back in your hand?
Thanks
Can you even use Kaiu Smithy to discard a card if you don't have any non-unique weapons in the discard pile to target?
No, the non-Unique Weapon in your discard pile is a required target of the Limited action on Kaiu Smithy. See Explicit performing and required targeting.
I would have assumed that similar to Falling Leaf Strike, if you can't fulfill all the required targets on the card that you couldn't take the action.
Indeed, this is true for all actions.
So, can you use Kaiu Smithy if there's no non-unique weapons in your discard pile?
No, for the reason given above.
Can you use Kaiu Smithy to discard a non-unique weapon and put it back in your hand?
No, because required targets must be chosen before mandatory costs are paid. See Abilities and actions.