A sleeve valve assembly featuring a cylindrical housing within which a sleeve valve is axially slidable within a radially expanded section of the housing. The radially expanded section of the housing presents an inwardly extending stop shoulder at one point along its length and an annular expansion notch at another point. The sleeve valve includes a radially projecting chamfered boss about its circumference. A portion of the sleeve valve is longitudinally slotted so as to form collets. An outwardly biased C-ring is disposed about the sleeve valve within the radially expanded section. The C-ring is initially disposed to be free to travel axially along the radially expanded section between the boss and the stop shoulder. As the sleeve valve is moved toward an open position, the boss, C-ring and stop shoulder engage each other such that the sleeve valve is releasably snagged against further axial movement toward the open position. A significant axial force upon the sleeve valve is required to effect unsnagging. Upon application of increased axial force, the collets of the sliding sleeve are forced radially inward to permit the boss to slip past the C-ring.An exemplary stimulation tool incorporating the sleeve valve assembly is described which permits acid to be selectively communicated into the surrounding formation. A stimulation tool constructed in accordance with the present invention is particularly useful for acid stimulation applications in horizontal well conduits as the snagging feature of the sleeve valve assembly provides a positive indication that lateral acid flow ports within the shifter tool have been placed adjacent complimentary flow ports in the surrounding housing.