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Describe the Bug:
Cavern of Souls will trigger and tap itself even if its requirement is not met (colorless mana in cmc of chosen creature type)

Which card did behave improperly ?
Cavern of Souls

Which update are you using?(date,name)Which type(Duel,Gauntlet,Sealed Deck)
PDMNv2

What exactly should be the correct behavior/interaction ?
Cavern of Souls will not trigger and tap itself if its requirement is not met (colorless mana in cmc of chosen creature type)

Are any other cards possibly affected by this bug ?
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In the screenshot above, I cast Gravecrawler which mana cost is B by tapping a Orzhov Guildgate, the game trigger and autotap Cavern of Souls which has the Zombie type selected.

For info, the current ML3 version of Cavern of Souls is that it make cmc of creature chosen by player when playing Cavern of Souls cost less by 1

Approximation. If you play a spell that has the type selected for Cavern of Souls and the land is untapped, just tap & use it.

The issue is it doesn't work as you say when dual land or multicolor land such as Akoum Refuge are exclusively in play:







In the screenshot above, I try to cast Gravecrawler with some Akoum Refuge in play and Cavern of Souls with Zombie type.
When I try to click on Cavern of Souls, the latter does tap itself but only generate 1 instead of B and thus cannot validate the cmc.
If I try to cast Gravecrawler with Akoum Refuge, as you say Cavern of Souls automatically tap itself doing nothing.

In PTT you no longer have a choice, if you try to cast a spell of the type Cavern of Souls selected then Cavern of Souls will be used (even if you don't double-click). But it does produce the correct mana.

Sorry for posting in the archives but this bug still seems to be present. I just find Cavern of Souls very hit or miss.

In the attached screenshot, you can see I have a Master of the Pearl Trident in my hand, and my mana sources include an Island and a Cavern of Souls set to merfolk. You can see the card is uncastable.

EDIT: It definitely sometimes works however. I can cast a turn 1 Cosi's Trickster using just a Cavern of Souls (for example).

I wonder if we could hack it so it's only activateable while there's an appropriate creature card being paid for, and only for colors currently flagged as being charged in needed_mana_colors.

Pros:

• Card being cast seems to be easily accessible, whether mana is being charged normally or within EVENT_CAST_SPELL (e.g. for a hybrid card), by checking stack_size>0 && stack_data[stack_size-1].generating_event==EVENT_CAST_SPELL and then inspecting stack_cards[stack_size-1]
• each matching Cavern of Souls will be tappable for mana exactly once, will (probably) be found by autotapping, and can be chosen whether to tap and which (needed) color to tap for
• when a Cavern is tapped, we can make the card on the stack untargettable

Cons:

• It'll always have to declare available one mana of any color, so e.g. a Mind Funeral will highlight yellow with a Plains, a Swamp, and a Cavern of Souls in play
• If casting is cancelled, the colored mana will stay in your pool and the creature card will remain untargettable. It won't be impossible to fix that, but it'll be quite difficult.
• Creatures cast with something else as an intermediary (such as Haakon, Stromgald Scourge or Teferi, Mage of Zhalfir) probably won't be detectable, though I doubt they are with this implementation either.

Worth noting it sometimes gives you a discount on creatures. Here I am casting a True-Name Nemesis using just a Cavern of Souls and an Island .

After extensive playing I am consistently finding the same 3 problems:
a) Cavern of Souls auto-taps i.e. grants 2 mana (1 colourless 1 coulored) instead of 1; or
b) Cavern of Souls does not tap for coloured mana even if I have the chosen creature type to play; or
c) Cavern of Souls is not granting the "cannot be countered" skill to said spell.
(seems to be a summary of the bugs described above)
I cannot pinpoint how or when, as Harlequin said, it's very hit and miss.