Before revision:  In the offline setting , the current best approximation ratio for pairwise spanners with uniform lengths and directed Steiner forests with uniform costs are both O(n^{ 3 / 5 + \epsilon}), due to Chlamtac, Dinitz, Kortsarz, and Laekhanukit (TALG 2020). We observe that a small modification of the online covering framework by Buchbinder and Naor implies a polynomial-time primal-dual approach with separation oracles, which a priori might perform exponentially many calls to the oracle .
After revision:  In the offline setting , the current best approximation ratio for pairwise spanners with uniform lengths and directed Steiner forests with uniform costs are both O(n^{ 3 / 5 + \epsilon}), due to Abboud and Bodwin (SODA 2018). A small modification of the online covering framework by Buchbinder and Naor implies a polynomial-time primal-dual approach with separation oracles, which a priori might perform exponentially many calls to the oracle .