In astrological tradition, the Mercury governs thinking, speech, and how information is processed, while Capricorn is a earth sign read as ambitious, disciplined, responsible, and long-game minded.
So Mercury in Capricorn is often read as a placement where communication style and the rhythm of the mind takes on a ambitious, disciplined quality. Tradition holds that someone with this placement tends to gathers and shares ideas in a way that feels responsible — coloured by the earth-sign temperament of Capricorn rather than by any single rule.
None of this is a prediction or a fixed trait. These are tendencies astrologers have long associated with Mercury in Capricorn, and how they actually show up depends on the rest of the chart — the houses, the aspects, and the other placements that this guide cannot see.
A general guide to astrological tradition — tendencies, not predictions. It describes the placement in the abstract and is not about any specific person.