Porcelain Gallery - cannot respond
I was playing against my opponent who had their board assembled with a variety of creatures. Alela, cunning conquerer being their commander. Most of which were tokens from Alela, I was waiting to echoing decay them but my opponent unlocked their Porcelain Gallery and it did not even give me the chance to reply, the effect hit the stack and took effect, so I responded with echoing decay and my opponent paused for a long time after, I believe as confused as I was.
Triton204#72547
shared this idea
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Rezzahan#77802
commented
The reason is, that unlocking a room is a special action, and as such does not use the stack, and so cannot be responded to. Special actions happen immediately upon the decision of the player to take them. Since Porcelain Gallery has a static ability, it applies right away. This is different from unlocking rooms that trigger upon being unlocked. Such triggers can be responded to, but even in those cases the room is already unlocked at the time.
709.5e A player who controls a permanent that has one or more locked halves may pay the mana
cost of a locked half of that permanent to give that permanent the appropriate unlocked
designation. This cost is referred to as an “unlock cost.” This is a special action (see rule 116).
A player can take this action any time they have priority and the stack is empty during a main
phase of their turn.116.1. Special actions are actions a player may take when they have priority that don’t use the stack.
These are not to be confused with turn-based actions and state-based actions, which the game
generates automatically. (See rule 703, “Turn-Based Actions,” and rule 704, “State-Based
Actions.”)