Dear Querist,
With due respect to all experts, I differ and express my views as under:-
Since the contractor has provided construction service (without material i.e. material supplied by you), so it is a case of pure labour and pure labour cannot be classified under the category, "Works Contract Service". 100% ST is to be paid by the contractor after collecting from you under Section 68(1) of the Finance Act,1994. If the contractor has not exceeded the threshold exemption of ₹ 10 lakhs, no service tax is payable.No RCM is applicable. No direct liability upon you.If the Contractor has crossed threshold exemption limit, he will collect ST from you and deposit into Central Govt. after getting registration.
This service cannot be classified under, "Manpower Supply & Recruitment Agency" because hired labourers remained under the supervision and control of the Contractor and not under you supposing that there is no agreement between you and the contractor which may prove 'Manpower' remained under your control.
This service falls under the category of 'Other Than Negative List' or under erstwhile "BAS".