To be safe, sleep spaces need all four sides up and must be one foot (12 cms) away from all furniture, including adult beds.
Co-sleeper bassinets are marketed as a “safe” way to bed share; they are not safe.
Babies can become trapped between the sleep space and the adult bed, or have their heads become stuck over the side. DIY cot to co-sleepers are another common, unsafe way of bed sharing.
If a co-sleeper bassinet has all four sides up, it may be safe if it meets Standards ASTM F2194.