Cobra Trap — ruling 1

You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.

Cobra Trap — ruling 2

Casting a Trap by paying its alternative cost doesn’t change its mana cost or mana value. The only difference is the cost you actually pay.

Cobra Trap — ruling 3

Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.

Cobra Trap — ruling 4

A spell or ability destroys a permanent only if that spell or ability specifically contains the word “destroy” in its text. If a spell or ability an opponent controls exiles a noncreature permanent you control, causes you to sacrifice a noncreature permanent, removes all loyalty counters from a planeswalker you control, or causes an ability you control to trigger (and then that ability destroys a permanent you control), Cobra Trap’s alternative cost condition hasn’t been met.

Cobra Trap : règles, fonctionnement et interactions (MTG)

Cobra Trap is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

If a noncreature permanent under your control was destroyed this turn by a spell or ability an opponent controlled, you may pay rather than pay this spell's mana cost. Create four 1/1 green Snake creature tokens.

How it works : Cobra Trap

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
Casting a Trap by paying its alternative cost doesn’t change its mana cost or mana value. The only difference is the cost you actually pay.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
No other card is named in this ruling.
#4October 1, 2009· Wizards of the Coast
A spell or ability destroys a permanent only if that spell or ability specifically contains the word “destroy” in its text. If a spell or ability an opponent controls exiles a noncreature permanent you control, causes you to sacrifice a noncreature permanent, removes all loyalty counters from a planeswalker you control, or causes an ability you control to trigger (and then that ability destroys a permanent you control), Cobra Trap’s alternative cost condition hasn’t been met.
No other card is named in this ruling.