The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
36. (The soul is an agent) also because it is designated as such with regard to actions; if it were not such, there would be a change of designation.
The quality of being an agent belongs to the soul for that reason also that the sacred texts speak of its agency in sacred and secular actions. 'Understanding performs the sacrifice, it performs all acts' (Taitt. Up. II, 5).--But, an objection may here be raised, we have seen that the word 'understanding' applies to the buddhi; how then can it indicate the circumstance of the soul being an agent?--The soul only, we reply, is designated there, not the buddhi. If the soul were not meant to be designated, there would be a change in the designation, i.e. the passage would run, 'through understanding it performs,' &c. For we see that in another passage where the buddhi is meant the word 'understanding' is exhibited in the instrumental form, 'Having through the understanding (intelligence) of these senses taken all understanding' (Bri. Up. II, 1, 17). In the passage under discussion, on the other hand, the word 'understanding' is given in the case characteristic of the agent (viz. the nominative), and therefore indicates the Self which is distinct from the buddhi. Hence your objection is not valid.--Another objection is raised. If the soul in so far as distinct from the buddhi were the agent, it would, because it is independent, bring about exclusively what is pleasant and useful to itself, not the opposite. We, however, observe that it does bring about the opposite also. But such an unrestricted proceeding does not become
the independent Self.--To this objection the following Sûtra replies.