Card Interaction Rules
Official answers for edge cases. These are settled decisions; if anything changes, we'll announce it ahead of time.
Battle Card interactions
A piece with an active card is captured. Losing the piece costs the card nothing. Its charges belong to the card for the whole match, so it can still fire on another piece of that type. Whether the game uses up one of the card's uses depends only on the game, never on what happened to the piece. You never lose a card just because a piece was captured.
A Pawn with a card promotes. Nothing moves, because charges belong to the card, not to one piece: your Pawn card keeps working on your other Pawns. The promoted piece is a real Queen (or Rook, Bishop, Knight) from then on, so your cards for that piece apply to it, and any Pawn-only bonus it carried, like Vanguard's extra attack, ends.
Both players have the same ability active. Abilities don't cancel each other out. Both can trigger independently on their own turns — there is no counter-ability system.
Outpost (Knight, Defensive). Outpost is never played: it waits face down in your loadout and fires on its own. When an enemy piece tries to capture your Knight, or an enemy card's strike hits it, while the Knight stands in the center 16 squares (c3 to f6), Outpost flips over and the Knight gains +1 SHLD right before the hit: an extra point that any damage breaks first. It fires at most once per enemy turn. It doesn't stack: if the Knight already has a shield bonus, Outpost doesn't fire and keeps its charge. Leaping Strike's capture doesn't set it off. A capture of your Knight when it's at 0 shield does: Outpost fires first and gives it 1 shield to stand on. The attacker's rarity makes no difference. 2 charges a match.
The shaded squares, c3 to f6, are where Outpost works. The Knight on d4 is inside them, so when the Bishop takes a swing at it, Outpost flips and the Knight gets +1 SHLD just before the hit. On b5 or g4 it would get nothing.
Several cards ready at once. Any number of your cards can be ready in the same turn, but you activate only one ability per turn, in every club. You choose which one before your move.
A freeze would leave you no legal move. If Royal Decree, Absolute Pin or En Passant Lock would leave you no legal move, the freeze lifts for that turn and the game tells you: the lock mark comes off the piece and a line on your board says it is free this turn. This only happens when you are not in check. In check a freeze never lifts, so checkmate stays checkmate. If you have any other legal move the freeze holds, and declaring a card does not change whether you count as stuck.
Black to move, not in check. The King on h8 has nowhere to go (g8 and g7 are covered by the White King, h7 by the Queen), and Royal Decree has frozen the Rook on a5. With no legal move left, the freeze lifts for this turn: the lock comes off and the Rook may move.
A protection would leave you no legal move. A capture onto a protected piece (Leaping Strike's Knight, the squares Fortify holds) is refused and you choose another move; no turn is spent. A protection gives way first: if you are not in check and capturing the protected piece is your only legal move, that capture is allowed. A freeze lifts only if you're still stuck after that.
Black to move, not in check. The King is boxed in, the Pawn on b6 is blocked, and its only capture is the Knight on c5, which Leaping Strike is protecting. Because capturing it is Black's only legal move, the protection gives way: b6 takes c5 is allowed, and the fight resolves as normal.
Your King's only move would lose the fight. In ChessVS, a capture your King would not survive is treated the way chess treats a capture of a defended piece: it never counts as a move when the game asks whether you still have one. If a King attacks a piece and loses the fight, the King is eliminated (the Honeypot), and if both pieces would fall the King still falls, so that capture does not count either. So if it is the only move you have left and you are not in check, the position is a stalemate, a draw. The game checks for it right after each move: if your opponent's move leaves you boxed in that way, it is a draw at once; if you box yourself in, your opponent gets one more move first and can still deliver checkmate instead. In check, the same position is checkmate. A capture your King wins is a normal move. If you play a losing capture anyway while you have another move, your King falls; that is the Honeypot. And a freeze or protection never counts that losing capture as your move: the card gives way instead, as above.
Black to move, and the King is down to 1 SHLD from an earlier fight. In chess, King takes g8 would be legal. Here the Knight's 3 ATK beats the King's 1 SHLD, so the capture would end the King, and it does not count as a move. Black has no other move and is not in check: stalemate, a draw.
A card's move gets you out of check, or out of a stalemate. A card's move is a chess move. If the move a card makes answers a check (Warp Strike carrying your Knight across the board, say, or a shove that clears the line), it is a legal escape, and the game counts it when it asks whether you are checkmated. The same goes for stalemate: if a card gives you a move, you are not stalemated. Chess still decides the game; the card was part of the position all along.
A card you declare would leave you no move. The game refuses the declaration and you keep the card, charge and all. Choose another card or play without one. If the reason is a hidden enemy card (a face-down ward whose extra shield, or a Tower Guard whose block, the game had to count), that card flips face up for both players, and it keeps its charge too.
When hidden cards show themselves. The face-down wards (Outpost, Sanctum, Stonewall, Bulwark, Home Court, Light Court, Sortie, Nightwatch and Rampart), Tower Guard, Royal Escort and Royal Guard stay hidden until they fire. Two things reveal one early. If it causes a refusal, because your move or your declaration would be illegal once its shield or block is counted, it is shown face up to both players and keeps its charge, and a card you had declared for that move goes back to your hand. And if a checkmate or stalemate exists only because of a card still face down, the game-over line names that card and reveals it to both players, unspent.
Gambit Card interactions
Gambit Cards are arriving in a later season. Interaction rules for them will be published here when they go live.
Tension Gauge interactions
Etheryst's First Light. Arriving in a later update. Once per match, the first ability an Etheryst card uses spends no charge. Everything else about that activation is normal, and every club, Etheryst included, activates one card ability per turn.
The FL3 ELO gate. FL3 cards unlock only when both conditions are met: the in-match Tension Gauge is at least 15 and your account ELO is at least 1,400. A high Tempo (once it arrives) with a low ELO still cannot access FL3. The ELO gate is permanent and unaffected by seasonal resets.
The golden rule
When any interaction is unclear, the principle is always the same: chess wins. Cards add depth and drama, but they never break the rules of chess. If a card's move answers a check, the position was never lost: the card was part of it all along.