Mercury in Sagittarius is how you think and talk visible, warm, fast.
Sagittarius Mercury thinks fast and out loud. Ideas arrive as urges, not arguments. You convince by enthusiasm. Sometimes you arrive at the conclusion before the reasoning has caught up — and you are usually right anyway. Your mutable tends to adapt — to flex, to translate, to revise in motion, which colours how this Mercury actually shows up day to day.
The shadow: overthinking, gossip, sharp speech weaponized against tenderness. The work is befriending your specific kind of mind instead of fighting it.
Practice: notice the speed your mind likes. Do not borrow another sign's tempo. Read what your tempo reads best. Speak to who actually listens at your speed.