6 month old suddenly has dry cough but does not look sick?

What the evidence says

A cough with no other symptoms in a baby who is feeding, playing, and breathing normally is usually not dangerous right away. Doctors say children with ordinary colds usually still have energy and want to play, unlike children with more serious infections who look sick and tired.

A dry cough can be the very first sign of a cold, before a runny nose or fever even shows up. It can also come from dry air, dust, or being near cigarette smoke. In a case description of a well infant with a dry, mildly barky cough and no fever, doctors noted no alarm signs were present, which is reassuring.

However, because your baby's cough started suddenly, doctors also want you to think about whether she could have breathed in or swallowed something small, like a piece of food, a bead, or part of a toy. Babies around this age put everything in their mouths. One documented case involved an eleven month old with a sudden dry cough and no fever who was first treated for a mild croup-like illness, but was later found to have a small object stuck in her airway.

Because of this, watch closely for the next day or two. See a doctor promptly if the cough gets worse, your baby seems to be working harder to breathe, makes a high-pitched noise when breathing in, refuses feeds, runs a fever, or if you ever saw or suspect a choking episode. Get emergency care right away if she turns blue, cannot cry or make sounds, or seems to be struggling badly to breathe.

If none of that is happening and she otherwise seems like herself, it is reasonable to keep watching her at home, use a cool mist humidifier if the air is dry, and keep her away from smoke. Most simple coughs like this settle within one to three weeks as part of a mild cold.