All three rating agencies (S&P, Moody`s, and Fitch) rate Multilateral Development Banks bonds AAA for a number of reasons. Strong capital adequacy, robust risk management and ample liquidity buffers are the common terms used to describe the sturdy MDB credit profile. MDB`s possess a "preferred creditor status", which essentially gifts them priority over other creditors regarding interest and capital repayment. Another key pillar is the member (developed nations) callable capital, which in the case of the IBRD covers 107 percent1 of the outstanding bond issuance at the time of writing.
The investment rationale is clear sovereign-like AAA risk combined with incremental yield enhancement over US Treasuries, combined with a social impact. Moving part of your government bond allocation within your portfolio to MDB bonds would be a logical step in any sustainability driven investment process.
1World Bank, 2019.