Can you respond to Mindsplitter being played with a spell if it has reaction speed?

You cannot respond to the act of playing the Mindsplitter unit itself, but you can respond to its "When you play me" triggered ability once it is on the chain.

Here is the breakdown of the interaction based on the game rules:

1. Playing the Unit

According to Rule 356.2 and the standard chain rules, when a player plays a unit, it enters the chain and resolves immediately. Priority does not pass, and there is no window to play a spell or reaction to "counter" or stop the unit from entering the board.

2. The "When you play me" Trigger

Once the Mindsplitter has entered the board, its "When you play me" ability triggers. This ability is a triggered ability that uses the chain (as confirmed by the chain rules).

• Sequence:
  1. The player plays Mindsplitter.
  2. Mindsplitter enters the board.
  3. The "When you play me" ability triggers and is placed on the chain as a pending item.
  4. At this point, the chain is populated, and players have an opportunity to play Reaction speed spells or abilities in response to that trigger.

3. Responding with a Spell

You can respond to the "When you play me" trigger with a spell, provided that spell has the Reaction keyword (or is otherwise playable at Reaction speed).

• Important Note: If your spell is only Action speed, you cannot play it in response to the trigger because the chain is currently "Closed" (a chain item is waiting to resolve). You can only play Action speed spells during an "Open State" (when the chain is empty).

Summary:

• You cannot stop the Mindsplitter from entering the board.
• You can respond to the "When you play me" ability once it is on the chain, provided you use a Reaction speed card.