Came across a difference of opinion with some people last night and could use some clarification.
Let's say you have played Forager, trashed a copper and it is the only treasure in the trash pile. Forager is now valued at +1.
Now you enter the buy phase of that same hand and Counterfeit some other treasure card. There are now 2 differently named treasure cards in the trash.
We have always calculated the value of Forager in the buy phase, choosing to calculate it after the Counterfeit action, hence +2.
The players last night believed that the value of Forager is fixed at the moment it is played.
Neither the instructions nor the wiki addressed it precisely. Your explanation is much appreciated!
You're somewhat misrepresenting Forager in terms of a "value". It is helpful to think of having a "Money" counter, which is a running counter during your entire turn that counts how much coin you have in your pool. Playing a card that says "+1 Coin" (e.g., Market) adds +1 to the Money counter. Playing a Silver adds +2 to the money counter. The counter runs through your entire turn, not just the Buy phase.
Your turn starts with
Money: 0
If you, for instance, played a Fishing Village last turn, then it would take effect at the start of your turn and you would have
Money: 1
If you now play, say, a Market, you would have
Money: 2
If you now enter your buy phase and play a Gold and a Copper, you would have
Money: 6
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Take another scenario, to maybe emphasize why this is important. You start your turn with
Money: 0
Pretend you (for some reason) played six Fishing Villages last turn, so you start with
Money: 6
and let's also say you previously used Prince on a Smithy so that you have eight cards. Let's say your hand is Poor House and 7 Coppers.
If you play Poor House, you first get +4 coins, so
Money: 10
Now, continuing the resolution of Poor House, you get -1 for each Treasure in your hand, which is a total of -7, so you have
Money: 3
This is all still during your Action phase. Now during your Buy phase you can play your 7 Coppers and get to
Money: 10
Note that if you had not played Poor House and chosen to instead go to your Buy phase, then playing the 7 Coppers would leave you with
Money: 13
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The short of it is, Forager (or any other card that gives or takes away Money) affects the Money counter when it is played, as part of its resolution. Even though in "real life" playing you usually don't count all them up until later (when you go to Buy something).
You should think of Actions and Buys in the same way. You start with Actions: 1 and Buys: 1, and playing an Action costs 1 from the Action counter, Buying a card costs 1 from the Buy counter.
In other words, the way you see it on Goko or previous electronic implementations of Dominion.