An insert (320, 420) for a tool (300, 400) and a tool for assembling a bone anchoring device are provided, the insert comprising a bone anchoring element (1) with a head (3) and a shaft (2) to be anchored in the bone and a receiving part. The insert (320, 420) is configured to be inserted into a longitudinal recess (302a, 402a) of a holder (302, 402) of the tool (300, 400). The insert (320, 420) is capable of receiving the bone anchoring element (1) and the insert (320, 420) has at least two recesses (322a, 322b, 322c, 422a, 422b) configured to receive different sized bone anchoring elements (1).