He basically mentions in front of the entire Wizengamont that he can perform a patronus charm.
Lucius Malfoy’s eyes narrowed. “By the report I received, you cannot cast the Patronus Charm, and Dumbledore knows this. The power of a single Dementor nearly killed you. You would not dare venture near Azkaban in your own person—”
“That was in January,” said Harry. “This is April.”
He basically mentions in front of the entire Wizengamont that he can perform a patronus charm.