What is ABS? How MLB's ball-strike challenge system will work in spring training

In recent years, we’ve seen MLB become more receptive to the technology at its disposal. The league expanded the replay challenge rules, added a pitch clock and introduced Statcast to elevate how fans follow the game. It was only a matter of time until that technology got applied to the strike zone.

Amid complaints from fans and many players about the umpires’ struggles with accurately calling the strike zone, MLB is testing its compromise to “robot umps” for the first time this spring. Using the automated ball-strike system (ABS), teams will have the option to challenge ball-strike calls in the majority of spring training games.

The league saw the challenge system as a middle ground from having every pitch called by computers, and we’ll get to see it in action in the coming days. This is how it will work.

What is ABS?

ABS is MLB’s name for an automated strike zone — also known as robot umps. In a full ABS game, there will be a human umpire behind the plate, signaling the calls communicated through ABS. We won’t be seeing that full system in action this spring. Instead, teams will have a limited amount of challenges.

How will the challenge system work?

Each team will have two ABS challenges per game — separate from the standard replay challenges. The team retains its challenges if the review is successful.

Instead of having challenges signaled from the dugout, ABS challenges can only be made by a hitter, pitcher or catcher. And it must be done immediately after the pitch. Again, the request cannot come from the dugout at all. We saw it in action during the Padres’ live batting practice, and it looked like this:

We just had a preview of the ABS Challenge system during Yu Darvish’s live BP!

A strike call overturned into a ball. Take a look: pic.twitter.com/XMA1BvRqus

— Sammy Levitt (@SammyLev) February 19, 2025

How long is the review process?

According to MLB, the review process took on average 17 seconds during its minor-league testing. It goes up on the videoboard in real time.

Will we see this in the regular season?

The system is still in the testing phase, so we will not see it for the 2025 regular season. There’s a possibility for 2026, though.