Jace's Sanctum — ruling 1

The first ability of Jace’s Sanctum can’t reduce the colored mana requirement of an instant or sorcery spell.

Jace's Sanctum — ruling 2

If there are additional costs to cast an instant or sorcery spell, apply those before applying cost reductions.

Jace's Sanctum — ruling 3

Jace’s Sanctum can reduce alternative costs such as miracle or overload costs.

Jace's Sanctum — ruling 4

Jace’s Sanctum’s scry ability will resolve before the instant or sorcery spell that caused it to trigger.

Jace's Sanctum : règles, fonctionnement et interactions (MTG)

Jace's Sanctum 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

Instant and sorcery spells you cast cost less to cast. Whenever you cast an instant or sorcery spell, scry 1.

How it works : Jace's Sanctum

Original English text — Wizards of the Coast / Scryfall

#1June 22, 2015· Wizards of the Coast
The first ability of Jace’s Sanctum can’t reduce the colored mana requirement of an instant or sorcery spell.
No other card is named in this ruling.
#2June 22, 2015· Wizards of the Coast
If there are additional costs to cast an instant or sorcery spell, apply those before applying cost reductions.
No other card is named in this ruling.
#3June 22, 2015· Wizards of the Coast
Jace’s Sanctum can reduce alternative costs such as miracle or overload costs.
No other card is named in this ruling.
#4June 22, 2015· Wizards of the Coast
Jace’s Sanctum’s scry ability will resolve before the instant or sorcery spell that caused it to trigger.
No other card is named in this ruling.