If a courtesy runner pinch-hits for another player during the same half-inning, what is the ruling?

When a courtesy runner pinch-hits for another player during the same half-inning, this action makes the courtesy runner an illegal substitute. Courtesy runners are permitted in certain situations, primarily for the pitcher or catcher when they reach base, but their role is strictly limited to running the bases. The rules do not allow a courtesy runner to take on a different role, such as pinch-hitting for another player, within the same half-inning.

This ruling is designed to maintain the balance and integrity of the game. A courtesy runner is expected to fulfill a specific purpose, and changing their role mid-inning violates the structured approach to player participation and strategy in the game. Therefore, the ruling that the courtesy runner becomes an illegal substitute is accurate and aligns with the regulations governing player roles in baseball.