T12 was assigned to the Torpedo School in late 1943 and was then transferred to the Baltic Sea in mid-1944 where she escorted heavy cruisers as they bombarded Soviet positions.

The boat was allocated to the Soviet Union after the war and renamed Podvizhny (Russian: Подвижный, "Agile"), serving with the Baltic Fleet until she was seriously damaged in a boiler explosion.

Renamed Kit (Russian: Кит, "Whale") in 1954 for use as a vessel in simulated nuclear testing on Lake Ladoga, the boat was scuttled in 1959.

On 16 November, T12 and her sister ships T4 and T7, departed Copenhagen, Denmark, en route to Cherbourg, France, to meet the commerce raider Komet.

The boat was one of the escorts for the badly damaged Prinz Eugen from Trondheim to Kiel on 16–18 May (Operation Zauberflote (Magic Flute)), together with T11 and the destroyers Z25 and Z5 Paul Jacobi.

Although escorted by T12, T2, Falke and the torpedo boats T18 and T23, the Italian blockade runner Himalaya failed in her attempt to break out through the Bay of Biscay to the Far East when she was spotted by British aircraft in late March 1943.

Two months later, T12 was one of the escorts for Prinz Eugen as the latter ship supported a German counterattack against advancing Soviet forces near Cranz, East Prussia, on 29–30 January 1945.

After failed repair attempts, she was withdrawn from service on 8 April 1953, disarmed, and handed over to the central directorate of the Soviet Navy as an unpowered experimental vessel, being renamed Kit on 30 December 1954.

The boat was sunk in shallow water off the islands of Heinäsenmaa and Makarinsaari in northwestern Lake Ladoga after simulated nuclear testing in early 1959 and struck on 13 March of that year.