With hand-collected data from over one thousand corporate green bond issuers from 42 countries, this paper builds the first comprehensive dataset for green bond assurances and disclosure files. I find that committing to assurances before issuing a green bond is linked to reduced borrowing costs in the primary market. The impact of assurances is particularly stronger for assurances for green bond framework, as opposed to assurances for use of proceeds and impact reports. In the secondary market, the release of assurance documentation is also associated with decreases in daily yield spreads. Further evidence suggests that peer influence significantly shapes issuers' decisions to obtain assurances. The findings of this paper highlight the important role of external verification in enhancing transparency and reducing financing costs in the green bond market.