The introduction of the Robo-Zone and its ABS challenge system has sparked an intriguing debate: Will it make catcher defense more valuable or less so? This is a question that has been on my mind for quite some time, and now, with the offseason upon us, I've delved into the numbers to find some answers. However, my initial assumptions might be flawed, and I'm here to share my findings and insights with you.
Let's begin with the data. I've gathered major league framing data, including called strike rates for catchers both inside and outside the strike zone, for both the majors and Triple-A. In 2025, Triple-A implemented the same challenge system that will be used in the majors in 2026. Interestingly, I found that catchers who played significant time in both leagues experienced a drop in their called strike rate within the zone by an average of 1.4 percentage points, and 1.7 points outside it. This suggests that pitch framing is indeed more challenging at the major league level.
The data on the challenge system's performance in the minors and spring training is limited, mostly coming from MLB press releases. However, Tom Tango, a renowned baseball analyst, has provided some insightful takeaways on his blog. As expected, players are more likely to challenge calls in high-leverage situations, later innings, and on outcome-deciding pitches. Tango also identified catchers and batters who excelled or struggled with challenging, and he even hinted at the introduction of challenge probability numbers next year, which will calculate the likelihood of a pitch being challenged and the success rate of those challenges.
These upcoming challenge probability metrics will offer a wealth of information. We'll be able to analyze whether catchers who are skilled challengers behind the plate also perform well when they're at bat. We can explore the correlation between plate discipline, framing value, and making successful challenge decisions. It will also allow us to examine if certain pitchers or pitch types result in more challenges or successful overturns. In short, we'll have the tools to assess the impact on framing value.
Currently, the data is limited, but here's what we know so far. During spring training, 2.6% of all ball-strike calls were challenged, with the offense initiating 4.4% and the defense 1.8%. Eighty percent of spring games saw five or fewer challenges, and 52.2% of challenges were successful. Batters had a 50% success rate, catchers 56%, and pitchers 41%, resulting in an overall success rate of 54.4% for the pitching team. These numbers are relatively close to 50%, except for the pitchers' success rate, which is likely due to their poor performance in challenging and the subsequent restrictions placed on them in the minors.
The Triple-A success rate in 2024 was 50.6%, and MLB is keen to promote the idea that the challenge system won't drastically change the game, citing this balance. Morgan Sword, MLB's executive vice president of baseball operations, stated, "In no strike zone that we've tried, in no format that we've tried, has the rate moved much above or below 50%, which is pretty interesting in that these are the subset of pitches that are most 'controversial' among players."
However, these numbers don't perfectly balance out, and if we apply them to a regular season scenario, the offense would come out ahead. In 2025, Triple-A saw an average of 4.2 challenges per game, which would equate to 10,206 challenges over a major league season. If the spring training percentages hold true, the offense would win 3,015 challenges, compared to 2,271 for the defense, resulting in a net gain of 744 extra balls and roughly 93 extra runs. This is significant when considering that major league umpires called only 236 strikes on pitches outside the shadow zone in 2025.
But there's reason to believe these numbers might not hold up in the majors. The Statcast zone is slightly tighter than the zone called by human umpires, which could lead to more called strikes. Additionally, a Reddit user, Avondice, analyzed the challenges from the 2025 Triple-A season and found an overall success rate of 49.5%, with the defense initiating 52.4% of challenges. If these numbers were to translate to the majors, the swing would favor the defense, resulting in a net gain of 670 extra strikes and erasing roughly 84 runs.
While spring training and Triple-A play differ from the regular season, it's important to note that major league umpires are generally more accurate, even without the challenge system. They make fewer big mistakes, and without those easy overturns, regular season challenges might be less successful than what we saw in spring training. Even small differences in challenge success rates can have a significant impact on the game.
The key takeaway is that the ABS challenge system, when considered separately from framing, will reduce the range of framing outcomes. However, we could also view the challenge system as an extension of framing, as it's still about earning strikes. Catchers are focused on "keeping strikes strikes," ensuring that when their pitcher throws a strike, it's called as such. The best pitch framers earn strikes on nearly 90% of shadow zone pitches within the zone, but outside the zone, that number drops to below 20%. The challenge system will further emphasize this aspect, and catchers who excel at both framing and challenging could be more valuable than ever.
In conclusion, while we're only talking about a few overturned calls per game, the impact on catcher defense could be significant. If batters get more value out of the challenge system, as seen in spring training, catcher defense could take a hit. However, if catchers make the most of the challenge system, combining great framing with excellent challenge decisions, they could become even more valuable, even if framing numbers take a slight dip. The debate continues, and only time will tell how the Robo-Zone and its ABS challenge system will shape catcher defense.
