GendoIkari is right.
Intrigue rules second edition, page 4 (emphasis added)
Diplomat: When playing this, you get +2 Cards, then count your cards in hand, and if you have 5 cards or fewer, you get +2 Actions. So, for example if you play this from a hand of 5 cards, you will put it into play, going down to 4 cards, then draw 2 cards, going up to 6 cards, and that is more than 5 cards so you would not get the +2 Actions.
Diplomat can also be used when another player plays an Attack card, if you have at least 5 cards in hand. Before the Attack card does anything, you can reveal a Diplomat from your hand; if you do, you draw 2 cards, then discard 3 cards (which can include the Diplomat). If you still have at least 5 cards in hand after doing that (such as due to Council Rooms), you can reveal Diplomat again and do it again. You reveal Reactions one at a time; you cannot reveal two Diplomats simultaneously. You can reveal a Moat from your hand (to be unaffected by an Attack) either before or after revealing and resolving a Diplomat (even if the Moat was not in your hand until after resolving Diplomat).
So long as the condition that triggers a Reaction remains true, you may reveal a single Reaction an infinite number of times in response to its trigger.
- Moat, Secret Chamber, and Trader avoid the problem by having no effect when revealed multiple times.
- Diplomat avoids the problem by reducing your hand-size by 1 each time it is used, eventually disabling its trigger.
- Watchtower avoids the problem by the lose-track rule.
- EDIT: Tunnel avoids the problem by not being revealed from your hand.
- Horse Traders, Fool's Gold, Beggar, Market Square, Caravan Guard, and Faithful Hound are all set aside, trashed, or discarded by their Reactions, thereby making it impossible to use their Reaction multiple times.
You can fix Firth Guild's Reaction either by making it only give +Coffers until you have a fixed number of tokens on your Coffers mat, or by removing Firth Guild from your hand when you use it (discard, top deck, trash, set aside, etc.).
theres so many rules we can’t memorize them all. So I’m just going to ignore this one. None of my friends are going to notice it anyway. When making cards all we can do is do our best and tweak them if too strong or too weak. But a rule I’m not aware or even understand is not my concern. If someone I don’t know has a problem with it then they can change it themselves for their own personal use.
It is not unreasonable to expect each player of Dominion to know the rules of Dominion. It is a fairly simple game. Card tracking and Reactions are the only mildly complex effects. We are not taking an adversarial stance in this regard. We are simply enforcing the rules as written.
Changing the card for personal use will necessarily create a significantly different effect of the card. If you set it aside, it will no longer be in your hand for the purpose of resolving an Attack, such as other Firth Guilds.