Gnosis Safe API – Creating Safes and Executing Transactions on Mumbai via Transaction Service API

The MultiSend.sol library requires transactions to be successful, so yeah, if one of the underlying transactions fails, it should revert:

https://github.com/gnosis/safe-contracts/blob/main/contracts/libraries/MultiSend.sol#L58-L59

You can test if it reverts by estimating the gas for the transaction beforehand. It won't be able to estimate gas if the transaction is failing.

safe-core-sdk should throw an error if the transaction reverted too

With Safe transaction (similar as with Ethereum transaction) it is easily possible to estimate how much gas is required if the transaction fails.

The Safe service estimation endpoint returns an error, because it cannot get the Safe transaction to execute successfully and therefore cannot provide an estimate. You will see a similar behaviour from eth_estimateGas if you try to estimate a transaction that will always revert.

I took a look at your transaction. You want to perform an Ether transfer, an ERC20 transfer and a ERC721 safeTransferFrom within one multisend. The problem is your choice of ERC20 token (vCOW). This token is not transferable and therefore throws an error. With multisend the whole transaction will revert if one of the inner transactions reverts, therefore it is not possible to successfully execute this transaction.