Capacity to contract

What is the capacity of the following to contract in India? How do they differ from each other in that respect:
    a) Foreign sovereigns

    b) Accredited representatives of a foreign state

    c) Diplomatic staff of foreign state?

All the above persons enjoy diplomatic immunity and hence can not be persecuted in Indian courts, in normal circumstances. But please remember they do not suffer from any incapacity- as such; so, if they enter, as per their free will, into a contract in India that would be enforceable. The question is where? They can certainly be tried at an international court but that would put the other party into undue disadvantage. Hence, it is advisable to put a clause, in the contract, where it is explicitly mentioned that the disputes arising, on the basis of the contract, would be tried in an Indian court. If the person accepts such a clause - as per her free will- then subsequently she can not invoke her diplomatic immunity and seek trial elsewhere.