An extra-cardiovascular implantable cardioverter defibrillator system, comprising: an electrical sensing circuit configured to receive a first cardiac electrical signal via a first extra-cardiovascular sensing electrode vector and sense cardiac events from the first cardiac electrical signal; a therapy delivery circuit configured to deliver anti-tachycardia pacing pulses to a patient's heart via an extra-cardiovascular pacing electrode vector different than the first extra-cardiovascular sensing electrode vector; and a control circuit coupled to the electrical sensing circuit and the therapy delivery circuit and configured to: detect tachycardia from the first cardiac electrical signal; determine a tachycardia cycle length from the first cardiac electrical signal; determine a first ATP interval based on the tachycardia cycle length; set a first extended ATP interval that is longer than the first ATP interval; control the therapy delivery circuit to deliver a first plurality of ATP pulses to the patient's heart via the extra-cardiovascular pacing electrode vector, the first plurality of ATP pulses comprising a first leading ATP pulse delivered at the first extended ATP interval after a cardiac event sensed by the electrical sensing circuit from the first cardiac electrical signal and at least a second ATP pulse delivered at the first ATP interval following the first leading ATP pulse. an electrical sensing circuit configured to receive a first cardiac electrical signal via a first extra-cardiovascular sensing electrode vector and sense cardiac events from the first cardiac electrical signal; a therapy delivery circuit configured to deliver anti-tachycardia pacing pulses to a patient's heart via an extra-cardiovascular pacing electrode vector different than the first extra-cardiovascular sensing electrode vector; and a control circuit coupled to the electrical sensing circuit and the therapy delivery circuit and configured to: detect tachycardia from the first cardiac electrical signal; determine a tachycardia cycle length from the first cardiac electrical signal; determine a first ATP interval based on the tachycardia cycle length; set a first extended ATP interval that is longer than the first ATP interval; control the therapy delivery circuit to deliver a first plurality of ATP pulses to the patient's heart via the extra-cardiovascular pacing electrode vector, the first plurality of ATP pulses comprising a first leading ATP pulse delivered at the first extended ATP interval after a cardiac event sensed by the electrical sensing circuit from the first cardiac electrical signal and at least a second ATP pulse delivered at the first ATP interval following the first leading ATP pulse. detect tachycardia from the first cardiac electrical signal; determine a tachycardia cycle length from the first cardiac electrical signal; determine a first ATP interval based on the tachycardia cycle length; set a first extended ATP interval that is longer than the first ATP interval; control the therapy delivery circuit to deliver a first plurality of ATP pulses to the patient's heart via the extra-cardiovascular pacing electrode vector, the first plurality of ATP pulses comprising a first leading ATP pulse delivered at the first extended ATP interval after a cardiac event sensed by the electrical sensing circuit from the first cardiac electrical signal and at least a second ATP pulse delivered at the first ATP interval following the first leading ATP pulse.