A medical device system comprising: a therapy delivery circuit configured to generate an electrical stimulation therapy for delivery to a patient's heart; a sensing circuit configured to receive at least a first cardiac electrical signal via a sensing electrode vector; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: determine whether first criteria for detecting a ventricular tachyarrhythmia are met by the first cardiac electrical signal; determine a plurality of features from each one of a plurality of cardiac signal segments of the first cardiac electrical signal; in response to the first criteria being met, determine whether a first portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy monomorphic waveform criteria; determine whether a second portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy supraventricular beat criteria; determine whether second criteria for detecting the ventricular tachyarrhythmia are met; withhold detecting of the ventricular tachyarrhythmia in response to both the monomorphic waveform criteria and the supraventricular beat criteria being satisfied; and detect the ventricular tachyarrhythmia and controlling the therapy delivery circuit to deliver the electrical stimulation therapy in response to the first criteria and the second criteria being met and at least one of the monomorphic waveform criteria not being satisfied or the supraventricular beat criteria not being satisfied. a therapy delivery circuit configured to generate an electrical stimulation therapy for delivery to a patient's heart; a sensing circuit configured to receive at least a first cardiac electrical signal via a sensing electrode vector; and a control circuit coupled to the sensing circuit and the therapy delivery circuit and configured to: determine whether first criteria for detecting a ventricular tachyarrhythmia are met by the first cardiac electrical signal; determine a plurality of features from each one of a plurality of cardiac signal segments of the first cardiac electrical signal; in response to the first criteria being met, determine whether a first portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy monomorphic waveform criteria; determine whether a second portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy supraventricular beat criteria; determine whether second criteria for detecting the ventricular tachyarrhythmia are met; withhold detecting of the ventricular tachyarrhythmia in response to both the monomorphic waveform criteria and the supraventricular beat criteria being satisfied; and detect the ventricular tachyarrhythmia and controlling the therapy delivery circuit to deliver the electrical stimulation therapy in response to the first criteria and the second criteria being met and at least one of the monomorphic waveform criteria not being satisfied or the supraventricular beat criteria not being satisfied. determine whether first criteria for detecting a ventricular tachyarrhythmia are met by the first cardiac electrical signal; determine a plurality of features from each one of a plurality of cardiac signal segments of the first cardiac electrical signal; in response to the first criteria being met, determine whether a first portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy monomorphic waveform criteria; determine whether a second portion of the plurality of features determined from each one of the plurality of cardiac signal segments satisfy supraventricular beat criteria; determine whether second criteria for detecting the ventricular tachyarrhythmia are met; withhold detecting of the ventricular tachyarrhythmia in response to both the monomorphic waveform criteria and the supraventricular beat criteria being satisfied; and detect the ventricular tachyarrhythmia and controlling the therapy delivery circuit to deliver the electrical stimulation therapy in response to the first criteria and the second criteria being met and at least one of the monomorphic waveform criteria not being satisfied or the supraventricular beat criteria not being satisfied.