MLB's ABS Challenge System Explained: How Robot Umpires Actually Work

For more than a century, the home plate umpire's word on balls and strikes was final. Not anymore. Beginning with the 2026 regular season, Major League Baseball added the Automated Ball-Strike (ABS) challenge system, a hybrid model that keeps human umpires behind the plate while letting players appeal to technology on the calls that matter most.

How the Challenge System Works

The human umpire still calls every pitch. The difference is what happens next:

1. The batter, pitcher or catcher disagrees with a call.
2. Immediately, they signal a challenge by tapping their helmet or cap.
3. The ABS system checks the pitch location against the official zone.
4. The result appears on the stadium videoboard within seconds, and the call is confirmed or overturned.

No managers, no dugout signals, and no long delays. The challenge has to come from the players on the field, right away.

How Many Challenges Each Team Gets

Each team gets two challenges per game. A successful challenge is retained, so a team that is right can keep challenging. A failed challenge is lost. That creates real strategy: catchers and hitters with a sharp eye become even more valuable, and nobody wants to waste a challenge in the second inning.

The ABS Strike Zone

The robot zone is based on each batter's height rather than how they crouch. The top of the zone is set at a fixed percentage of the player's height and the bottom at a lower percentage, and the zone is measured at the middle of home plate. It is a two-dimensional box, which is why some pitches that look like strikes on the edge come back as balls, and vice versa.

Why MLB Chose Challenges Over Full Automation

MLB tested a fully automated zone in the minor leagues. Players and fans pushed back on losing the human element, including the art of pitch framing and the drama of an umpire's call. The challenge model was the compromise: keep umpires in charge, but give players a safety valve for the obvious misses.

What It Means on the Field

• Catchers matter more. They often have the best view of a pitch and frequently lead the decision to challenge.
• Late innings get tense. Teams try to save at least one challenge for high leverage at-bats.
• Umpire accuracy is public. Overturned calls show up in real time, which adds pressure and accountability.

Baseball has changed a lot in a few short years. If you missed the earlier updates, catch up with our guide to MLB's pitch clock rules.