It’s official: the robot overlords have made it to the major leagues. Back in September, MLB announced that the Automated Ball-Strike (ABS) challenge system would be in place for the 2026 season. More recently, they’ve confirmed this change and provided more clarity to the challenge process and the strike zone itself. The full list of rules and regulations around this can be found here; what follows is a summary of the most important aspects.
Fans of minor league baseball should be familiar with the ABS challenge system. In recent years, MLB has used its affiliates as testing grounds for procedural changes like the pitch clock and the extra innings “ghost runner” before implementing them in the major leagues. They’ve done the same with ABS and challenges, which have been used in the minor leagues in since 2022. The system started gaining more traction lately and was demoed in spring training last year. Now, it will be implemented in full for the 2026 MLB season.
It’s important to note that the ABS and challenge process will not be replacing umpires in full, although this technology is capable of doing so under future rules changes. Instead, the challenge system acts as a corrective process. Umpires call the game as usual, and if a player disagrees, they initiate a challenge. The rules surrounding the challenge process are quite interesting, in a legalese kind of way.
How challenges work
The first main rule is that only three people are ever allowed to challenge a pitch: the batter, catcher, and pitcher. No defender can initiate a challenge and neither can managers or coaches. Additionally, the player must initiate the challenge immediately and cannot receive any signal from a non-eligible player or coach before initiating. In fact, if the umpire suspects the initiating player received any external aid, he can reject the challenge.
The second major rule is that teams are allowed a finite number of challenges, in theory. Teams are allotted 2 challenges for a 9-inning ballgame, and an unsuccessful challenge (where the umpire’s call is confirmed) is lost. This does mean, however, that a successful challenge is retained, meaning an infinite number of correct challenges is theoretically possible. This is meant to encourage players to only challenge in situations where the call is either obviously wrong or so important that it’s worth risking a lost challenge for a slight chance of overturning the call. It will be interesting to see how that balances out. At the Triple-A level, many teams have instructed their players only to challenge in higher leverage situatioons so as not to potentially waste their challenges. Major league hitters may be allowed more leeway.
Furthermore, extra innings throw an extra wrinkle into the operation. If, at the end of 9 innings, a team has used and lost both of their challenges, they will be granted one bonus challenge for the 10th inning. If the team loses their bonus challenge and the game continues to the 11th inning, they will receive another bonus challenge, but they cannot stockpile bonus challenges. This means every team starts every inning after the 9th with at least one challenge. If the team saved both challenges from the original 9 innings, they’d keep both challenges into extras but couldn’t earn a bonus challenge until losing both of the originals. So, functionally, there’s no limit once you hit extra innings.
The rules include a handful of other procedures that are supposed to be exhaustive, but something will inevitably come up that isn’t strictly covered here. Teams are required to show how many challenges are left on the scoreboard, both the scoreboard and the broadcast will display a graphic showing the path of any challenged pitch with the strike zone overlaid, and if there happens to be a technical glitch, the challenge system can simply be prohibited until everything’s working again. It’s worth noting that ABS and challenges will be active in every MLB stadium and thus will be used for the playoffs, but not promotional games like the Field of Dreams game or the Mexico City games. The Tigers aren’t scheduled for any of those games, but it’s good to know.
What is the Automated Ball-Strike zone?
ABS itself is going to take some getting used to, for players and umpires as well as fans. The measurements for the automated strike zone are different than the zone that umpires are currently enforcing in a few ways.
For starters, the ABS zone is defined as a two-dimensional cross section down the middle of home plate, instead of a three-dimensional space like umpires call. The ABS zone is also slightly smaller. The top and bottom of the zone will be player-specific based on pre-season measurements. The lower limit is 27% of a player’s height and the upper limit is 53.5% of their height, standing straight up, without cleats. No more “knees to chest” or anything like that. Previously, umpires were instructed to call a zone that ranged from 24.2% to 55.6% of a player’s height, which is quite a bit larger than the ABS zone. The slightly more compact zone, customized to each players’ dimensions should lead to an adjustment period for veteran players and umpires.
It’s also very different than The Box we see on broadcasts. Almost every broadcast has a strike zone overlaid near home plate, but like the yellow first down marker, it’s just an approximation. Unless the producers adjust, this will be even more true now. The Box is set at the front of the zone and is widely understood to be a one-size-fits-all zone used for every player. Whether that’s updated likely depends on the specific broadcast.
This visualization of the called strike heat map compared to the current strike zone box gives you a pretty good idea how umpire accuracy has improved over the last 15 years. Essentially the middle of the zone used to be quite a bit wider than it should be, though the corners are still not called with much accuracy.
Personally, I’m glad to see the challenge system come to the major leagues. Umpires are quite accurate, having improved significantly with game by game feedback on their accuracy over the past decade. I’d hate to remove them entirely, but they’re still human, and even just sneezing at the wrong time can ruin a call. The actual amount of challenges may be lower than you’d expect. However, if teams do as expected and save them for high leverage scenarios and the late innings, their impact may on game outcomes may be fairly signficant. It’s going to take some getting used to, as called strikeouts in big spots in a game will see outsized use of the challenges. No doubt teams are furiously strategizing on how to instruct their players to use them.
The ABS challenge setup should eliminate those obvious mistakes without fundamentally changing how the game is played, and I think that’s a strong positive. Umpires will also be allowed to request in-game feedback from the ABS system to judge how well they’re calling the game. That type of feedback has already increased their accuracy in recent years.
How well it’s implemented won’t be known for a while, but the majority of players seem on board. I expect young players exposed to the system at the Triple-A level to have far more initial success than a veteran seeing this zone for the first time. The system has been tested at both the Single-A and Triple-A levels for several years now and will be active in Grapefruit League play again, so it shouldn’t take too long for everyone to adapt.