Mo. Rev. Stat. § 514.050: Issuance of writ without fees by clerk, when.

If any person file, before any clerk in vacation, an affidavit that he has a just and subsisting cause of action on which he proposes to bring a suit, and that he is unable to pay the costs, the clerk shall issue an original writ, without fees for the same. But the court may, in its discretion, refuse or permit the further prosecution of the suit.