Cloud Key — ruling 1

The cost reduction applies only to generic mana in the cost of spells of the chosen type you cast.

Cloud Key — ruling 2

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Cloud Key's ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Cloud Key : règles, fonctionnement et interactions (MTG)

Cloud Key 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

As this artifact enters, choose artifact, creature, enchantment, instant, or sorcery. Spells you cast of the chosen type cost less to cast.

How it works : Cloud Key

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
The cost reduction applies only to generic mana in the cost of spells of the chosen type you cast.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of Cloud Key's ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.
No other card is named in this ruling.