Casparius spent his first professional season with the Arizona Complex League Dodgers and Rancho Cucamonga Quakes, pitching in five games and allowing two earned runs in six innings.[5] He pitched 2022 with Rancho Cucamonga and the Great Lakes Loons, pitching 89+2⁄3 innings in 27 games (16 starts) for a 6.42 ERA.[5] The following season, with Great Lakes and the Tulsa Drillers he made 26 appearances (21 starts) and was 6–7 with a 5.27 ERA.[5][9] After the 2023 season, Casparius pitched in the Arizona Fall League for the Glendale Desert Dogs.[10] He started 2024 with Tulsa, making five starts with a 3.32 ERA[5] before being promoted to the Triple-A Oklahoma City Baseball Club.[11] In 16 games (14 starts) at AAA, he was 4–3 with a 3.36 ERA.[5]
Casparius was promoted to the major leagues for the first time on August 18, 2024,[12] but was optioned back to Oklahoma City on August 21, without appearing in a major league game.[13] Casparius was promoted to the major leagues for a second time on August 31[14] and made his debut that day in relief against the Arizona Diamondbacks, working a scoreless inning of relief to pick up his first career win.[15] His first strikeout was of Nolan Jones of the Colorado Rockies on September 22.[16] He pitched a total of 8+1⁄3 innings in three games, with two wins, nine hits and two runs allowed with 12 strikeouts.[17]