so... maybe something like:

const CraftFromInventory = struct {
	dest: InventoryAndSlot,
	item: Item,
	amount: u16 = 1,
	sources: []const InventoryAndSlot

?
The workbench would then be client side and the client would only send what it wants to craft and what sources to use and where it should go. Maybe we could even send which recipe to use so the server doesn't have to figure it out itself. But I don't know if we can be sure that the client / server list of recipes will be in the same order (if we go by index).

or maybe:

const CraftFrom = struct {
	destinations: []const Inventory,
	item: Item,
	amount: u16 = 1,
	source: Inventory // for the future maybe also directly []Inventory

or simpler:

Yeah I'd prefer that, the explicit slots would be too fragile I think.

But I don't know if we can be sure that the client / server list of recipes will be in the same order (if we go by index).

The index is not stable, but you can just send the whole thing, if it ever becomes a performance problem, we can just turn the linear search into a hashmap.