How Major League Baseball players can turn to a robot to challenge balls, hit this year
Listen to this article
Average 5 minutes
The audio version of this article was created by AI-based technology. Mispronunciations may occur. We are working with our partners to continuously review and improve the results.
Robot referees are coming to the big stage this year.
The Automated Ball-Strike (ABS) Challenge System will be introduced in the form of a challenge system where the home plate umpire makes each call, with teams able to invoke the computer system.
Robotic umpires have been tested in the minor leagues since 2019, with the latest tests in Triple-A starting in 2022, during MLB spring training last year and at the 2025 All-Star Game in Atlanta.
Here’s what you need to know about MLB’s robotic umpires.
How does the ABS system work?
Fields are equipped with cameras that track each pitch and determine if it crossed home plate within the strike zone. In early tests, referees would wear earplugs and hear “ball” or “strike,” then relay the call to players and fans with traditional hand signals.
The challenge system adds a wrinkle. Human referees call the entire field, but each team can challenge two calls per game. Teams that lose both challenges get one extra challenge for each extra inning. The team retains its challenge if the appeal is successful, similar to the video review rules in the major leagues, which began being used for home runs in August 2008 and expanded to a wider range of games in the 2014 season.
Only the batter, pitcher or catcher who may challenge the call, signs by tapping the helmet or cap. Help from the dugout is not allowed. The challenge must be made within two seconds, and an image of the pitch and strike is displayed on the scoreboard and broadcast feed. The referee then announces the revised count.
Challengers during spring training last year averaged 13.8 seconds.
What is technology?
The cameras’ Hawk-Eye tracking system tracks pitches and determines whether they fall within the strike zone based on the height of each batter, measured without shoes. Each player will be measured for their strike zone between 10 a.m. and noon continuously during spring training to maintain consistency, because the length can decrease over time. The data will be verified by the Southwest Research Institute. MLB has limited the scoring process to less than one minute per player.
While the strike zone called by major league umpires is usually oval, the ABS strike zone is rectangular, just like in the rulebook.
Defining the computer-generated strike zone was one of the biggest challenges in developing the system.
So what is the MLB strike zone?
MLB has changed the structure of the ABS strike zone several times.
It started at 19 inches in diameter in 2022, then was reduced to 17 inches, matching the width of home plate. Reducing the area resulted in more walks and only minor changes in strike rates.
The maximum strike zone was set at 51 percent of batters’ lengths in 2022 and 2023, then increased to 53.5 percent in 2024 after pitchers complained. The lowest strike zone has been 27 percent since 2022, after it was set at 28 percent. The shape of the batter is ignored.
ABS makes its ball or strike decision in the center of the plate — 8½ inches from front to back. That differs from the rulebook area called the umpire, which treats the strike area as a cube and defines a strike as any pitch that crosses any part of it. Major league umpires call about 94 percent of games correctly, according to UmpScorecards.
Where is ABS tested?
ABS, which uses Hawk-Eye cameras, has been tested in the minor leagues since 2019. The independent Atlantic League tested the system in the 2019 All-Star Game, and MLB installed the technology for that year’s Arizona Fall League featuring top prospects. ABS was used in eight of nine games in the Low-A Southeast League in 2021, then moved up to Triple-A in 2022.
At the start of the 2023 Triple-A season, half of the games used robotic umpires to make ball calls, while the other half had a human umpire making ABS-inducing calls.
MLB switched to Triple-A’s all-challenge system on June 26, 2024, and used the challenge system last year for 13 spring training ball games involving 19 teams for a total of 288 exhibition games. Teams won 52.2 percent of their field goal challenges, winning 617 of 1,182 appeals.
In last year’s MLB All-Star Game, four out of five calls by umpire Dan Iassogna were successful.
How successful are the teams in the challenge?
The success rate has increased by about 50 percent for children.
In Triple-A last season, the success rate dropped to 49.5 percent from 50.6 percent. The defense—usually the catch—was more successful, winning 53.7 percent of the challenges compared to 45 percent for the hitters. Challenges also increased to 4.2 per game from 3.9.
In 2024 at Triple-A, only 1.6 percent of first pitches were challenged. That rose to 3.9 percent on two-strike pitches, 5.2 percent on three-ball pitches and 8.2 percent of the total count.
Challenge levels were also higher later in the games. While 1.9% of pitches were challenged in the first three innings, that rose to 2.5 percent from the fourth through sixth, 2.8 percent in the seventh and eighth and 3.6 percent in the ninth.
