Strictly speaking, a complete form stroke or thrust can be a telling blow even if it is made with a kodachi, but fencers can hardly make a complete form stroke or thrust with a kodachi because its ideal distance to an opponent is shorter, and also the strokes and thrusts made by a kodachi are judged more strictly than the strokes and thrusts by a tachi.
厳密に言えば、完璧な形で決まれば小太刀の打突も有効打となるが、間合いの短い小太刀で完璧な打突は決まる事がほとんど無いうえ、太刀での打突に比べて小太刀での打突は判定自体も厳しくなる。