Amendment to Annual Report (Form 10-K/A)

This Amendment No. 1 on Form (this Amendment or Form to amend the Annual Report on Form of Arcellx, Inc., a Delaware corporation (referred to as "Arcellx," the "Company," "we," "us," or "our") for the fiscal year ended December 31, 2025, originally filed with the Securities and Exchange Commission (the SEC) on February 26, 2026 (the Original Report), is being filed for the purpose of including the information required to be disclosed by Part III of Form This information was previously omitted from the Original Report in reliance on General Instruction G(3) to Form which permits the information in the above referenced items to be incorporated in the Form by reference from our definitive proxy statement if such proxy statement is filed no later than 120 days after our fiscal The reference on the cover page of the Original Report to the incorporation by reference to portions of our definitive proxy statement into Part III of the Original Report has been deleted. This Amendment hereby amends and restates in their entirety the cover page and Items 10 through 14 of Part III of the Original Report.

As required by Rule under the Securities Exchange Act of 1934, as amended (the Exchange Act), in connection with this Form our Chief Executive Officer and Chief Financial Officer are providing Rule certifications as included herein. We are amending Item 15 of Part IV solely to reflect the inclusion of these certifications.

Except as described above, this Form does not modify or update disclosure in, or exhibits to, the Original Report. Furthermore, this Form does not change any previously reported financial results, nor does it reflect events occurring after the filing date of the Original Report. Information not affected by this Form remains unchanged and reflects the disclosures made at the time the Original Report was filed.