Wayne Pivac’s side took the bonus-point victory by virtue of a fifth try, but not before Connacht had thrown everything at the champions.
Although the visitors collected only one bonus point for their effort, it was the performance Kieran Keane had demanded. In a festival of try-scoring in the opening half, the champions led 26-22, having opened with the first try after eight minutes through Johnny McNicholl. Connacht hit back through Jack Carty who added the conversion.
Halfpenny tacked on an 11th minute penalty. Then left wing Steff Evans grabbed a second try after 17 minutes.
Connacht kept in touch with a Carty penalty, but the home side profited charging down Connacht’s restart, Patchell crossing out wide to stretch the lead to 19-10.
Connacht, silenced the 7,700 Parc Y Scarlets crowd with two tries in quick succession. Tiernan O’Halloran touched down out wide after Farrell had carved a gap, with Carty converting, and three minutes later Cian Kelleher took advantage of an overlap to dive over in the right corner.
The home side retook the lead after Connacht failed to protect possession, and Steff Evans supplied Halfpenny, who also added the extras.
The second half remained a loose open affair, punctuated by a Halfpenny penalty to extend Scarlets’ lead. However in the 74th minute Connacht No 8 Eoin McKeon set up Tiernan O’Halloran for a fourth try. Carty missed the conversion, and more heartbreak followed as ex-Leinster player Tadhg Beirne spoiled the party with a try, which Halfpenny converted at the death.
L Halfpenny, J Mcnicholl, J Davies, S Williams, S Evans, R Patchell, A Davies, R Evans, K Owens (cpt), W Kruger; J Ball, D Bulbring, A Shingler, W Boyde, J MacLeod.
Replacements, D Evan for R Evans (20m), T Beirne for Boyde (HT), L Rawlins for Bulbring (52m), J Evans for Davies (54m), E Phillips fort Owens and P Asquith for Patchell (both 64m), T Prydie for McNicholl (69m) for Patchell (both 64m), S Gardiner for Kruger (71m).
D Leader, CKelleher, T Farrell, B Aki, T Farrell, T O’Halloran, J Carty, C Blade, D Buckley, T McCartney (capt), C Carey, U Dillane, Q Roux, J Heenan, J Butler, E McKeon Replacements: K Marmion for Blade and E Griffin for Leader (both 52m), G Thornburgh for Dillane and F Bealham for Carey (59m), D Coulson for Buckley (65), S Delahunt for McCartney (70m), E Masterson for Heenan (79m).Referee: N Owens