Kari Zev's Expertise — ruling 1

The mana value of a split card is determined by the combined mana cost of its two halves. If an expertise spell allows you to cast a split card, you may cast either half or, if that split card has fuse, both halves.

Kari Zev's Expertise — ruling 2

If the Expertise spell you cast has any targets, and those targets become illegal before the spell resolves, the spell doesn't resolve and none of its effects happen. You won't get to cast a free spell.

Kari Zev's Expertise — ruling 3

A card's mana value is determined solely by the mana symbols printed in its upper right corner. The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost {1}{U}{U} has mana value 3. Ignore any alternative costs, additional costs, cost increases, or cost reductions that could apply to it. A card with no mana cost has a mana value of 0.

Kari Zev's Expertise — ruling 4

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Kari Zev's Expertise — ruling 5

Kari Zev's Expertise can target any creature or Vehicle, even one that you already control or that is already untapped.

Kari Zev's Expertise — ruling 6

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as emerge costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Cathartic Reunion, you must pay those to cast the card.

Kari Zev's Expertise — ruling 7

While you're casting your free spell, the Expertise spell is still on the stack. It will be put into its owner's graveyard after the free spell is cast. The free spell can't target the Expertise card in your graveyard. It can target the Expertise spell on the stack, but the Expertise spell will become an illegal target before the free spell resolves.

Kari Zev's Expertise — ruling 8

Any triggered abilities that trigger while performing the Expertise spell's first effect won't be put onto the stack until after you're done casting your free spell. They're put onto the stack at the same time as any abilities that triggered while casting that spell regardless of the order in which those abilities triggered.

Kari Zev's Expertise : règles, fonctionnement et interactions (MTG)

Kari Zev's Expertise 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

Gain control of target creature or Vehicle until end of turn. Untap it. It gains haste until end of turn. You may cast a spell with mana value 2 or less from your hand without paying its mana cost.

Complete guide

Kari Zev's Expertise is a versatile sorcery that combines the power of a temporary theft effect with a free spell cast. This Aether Revolt favorite allows you to swing the tempo of the game by stealing an opponent's best threat and immediately following up with another play from your hand. Whether you are snatching a creature or a Vehicle, this card provides an explosive turn for just three mana.

How it Resolves

When Kari Zev's Expertise resolves, you first take control of the target creature or Vehicle, untap it, and give it haste. Immediately after this happens, and while the Expertise is still on the stack, you may choose a card in your hand with mana value 2 or less to cast without paying its mana cost. If you cast a spell this way, it is placed on the stack on top of the Expertise. Once the free spell is cast, Kari Zev's Expertise finishes resolving and is put into your graveyard. Note that if the initial target becomes illegal (for example, if it is destroyed in response), the entire spell fails and you do not get to cast a free spell.

Navigating Mana Values

The free spell must have a mana value of 2 or less. This value is determined solely by the mana symbols printed in the top right corner of the card. Additional costs, like those found on Cathartic Reunion, must still be paid, while cost reductions do not change the mana value. If you cast a spell with {X} in its cost, X must be 0. Interestingly, you can cast both halves of a split card with Fuse as long as the total mana value of the card is 2 or less, though most split cards will exceed this limit.

Strategic Timing

Because this is a sorcery, you'll generally use it during your main phase to clear a blocker or steal a high-power attacker. It is particularly effective against Vehicles, as you can take control of them even if they aren't currently creatures. Since any triggered abilities that occur during the resolution wait to be put on the stack until after you've finished casting your free spell, you can carefully sequence your plays to maximize your board presence before your opponent can react to those triggers.

Interaction examples

Stealing a Blocker and Drawing Fresh Cards

Wheel of Fortune

You cast Kari Zev's Expertise targeting your opponent's creature. As it resolves, you untap that creature and gain haste. Then, because Wheel of Fortune has a mana value of 3, you actually cannot cast it for free with this Expertise, as the limit is mana value 2. You must choose a different card from your hand that meets the requirement, such as a low-cost creature or removal spell.

Generating Mana for a Big Turn

Jeska's Will

You cast Kari Zev's Expertise targeting an opponent's creature. While the Expertise is resolving, you check your hand for a spell with mana value 2 or less. Since Jeska's Will has a mana value of 3, it cannot be cast for free. If you had a card like Shock or a 2-mana creature, you could cast that instead to further develop your board while keeping your stolen creature ready to attack.

The Importance of Valid Targets

If you cast Kari Zev's Expertise targeting a 5/5 creature and your opponent responds by using a removal spell to destroy that creature, Kari Zev's Expertise will have no legal targets when it tries to resolve. The entire spell is removed from the stack, and you will not get to untap anything or cast a free spell from your hand.

Guide written with AI from the official text and rulings. Official English text prevails.

How it works : Kari Zev's Expertise

Original English text — Wizards of the Coast / Scryfall

#1April 18, 2017· Wizards of the Coast
The mana value of a split card is determined by the combined mana cost of its two halves. If an expertise spell allows you to cast a split card, you may cast either half or, if that split card has fuse, both halves.
No other card is named in this ruling.
#2February 9, 2017· Wizards of the Coast
If the Expertise spell you cast has any targets, and those targets become illegal before the spell resolves, the spell doesn't resolve and none of its effects happen. You won't get to cast a free spell.
No other card is named in this ruling.
#3February 9, 2017· Wizards of the Coast
A card's mana value is determined solely by the mana symbols printed in its upper right corner. The mana value is the total amount of mana in that cost, regardless of color. For example, a card with mana cost has mana value 3. Ignore any alternative costs, additional costs, cost increases, or cost reductions that could apply to it. A card with no mana cost has a mana value of 0.
No other card is named in this ruling.
#4February 9, 2017· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
No other card is named in this ruling.
#5February 9, 2017· Wizards of the Coast
Kari Zev's Expertise can target any creature or Vehicle, even one that you already control or that is already untapped.
No other card is named in this ruling.
#6February 9, 2017· Wizards of the Coast
If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as emerge costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Cathartic Reunion, you must pay those to cast the card.
No other card is named in this ruling.
#7February 9, 2017· Wizards of the Coast
While you're casting your free spell, the Expertise spell is still on the stack. It will be put into its owner's graveyard after the free spell is cast. The free spell can't target the Expertise card in your graveyard. It can target the Expertise spell on the stack, but the Expertise spell will become an illegal target before the free spell resolves.
No other card is named in this ruling.
#8February 9, 2017· Wizards of the Coast
Any triggered abilities that trigger while performing the Expertise spell's first effect won't be put onto the stack until after you're done casting your free spell. They're put onto the stack at the same time as any abilities that triggered while casting that spell regardless of the order in which those abilities triggered.
No other card is named in this ruling.
#9February 9, 2017· Wizards of the Coast
Effects that allow you to "cast" a card don't allow you to play a land card.
No other card is named in this ruling.