A fluid jet nozzle for a decoking tool, a decoking tool and method of operating same. The nozzle includes a nozzle assembly for use in a fluid jet decoking tool. The assembly includes a housing to hold one or more nozzles that are used to spray or otherwise distribute decoking fluid. An internal flowpath that extends from an inlet of the nozzle to an outlet of the nozzle defines a tapered shape such that when the decoking fluid passes through the nozzle, the flowpath produces a predominantly coherent flow pattern in the fluid.