Mo. Rev. Stat. § 429.310

Any answer, or other pleading, or motion, or entry of appearance followed by pleading in due course, filed or made in any such equitable action by any mechanic's lien claimant, within six months after the preliminary statement for the lien of such claimant has been filed in the proper office therefor, as required by law, or, when filed, or made, at the time required by law on a summons in such equitable action, issued before or within such six-months' period within which suits on mechanics' liens including this equitable action are required to be commenced, provided such equitable action is still pending, shall be deemed a commencement of an action by such mechanic's lien claimant.