The bonus-point victory puts Sale in second place in Pool Five behind leaders Brive, and their home clash with the French side already has the look of a group decider.
Fly-halves Valentin Courrent and Nick McLeod swapped early penalties before the home side took advantage of a yellow card shown to Agen lock Damien Lagrange.
The Sharks made their one-man advantage count in emphatic fashion with quick tries from Tom Brady and skipper David Seymour to stretch their lead to 20-6.
Courrent's third penalty on the stroke of half-time made it 20-9, but after the break Brady crossed for his second try before scrum-half Cillian Williams scored the home side's fourth try for the vital bonus point.
Agen's replacement centre Jamie Robinson, the former Wales international, and Tongan number eight Opeti Fonua then scored tries for the visitors to close the gap, but a late try from Scott Mathie sealed a comfortable Sale win.
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