S.D. Codified Laws § 55-1-42
A beneficiary of a mandatory or a support interest has an enforceable right to a distribution pursuant to a court's review. A trustee's distribution decision may be reviewed for unreasonableness, dishonesty, improper motivation, or failure, if under a duty to do so, to act. This does not, however, raise the beneficiary's support interest to the level of a property interest.
If the trust contains a spendthrift provision, notwithstanding the beneficiary's right to force a distribution with regard to a mandatory or support interest, no creditor may force a distribution with regard to a mandatory or support interest. No creditor may reach present or future support distributions with regard to a mandatory or support interest.
Regardless of whether a beneficiary has any outstanding creditor, a trustee of a mandatory or a support interest may directly pay any expense on behalf of such beneficiary. No trustee is liable to any creditor for paying the expenses of a beneficiary of a mandatory or support interest.
SDCL 55-1-42