The ACO Assignment Improvement Act of 2025 seeks to modify how beneficiaries are assigned within the Medicare Shared Savings Program . This legislation amends Section 1899(c)(1) of the Social Security Act to update the criteria for beneficiary assignment. Specifically, for performance years commencing on or after January 1, 2027 , the bill mandates that primary care services delivered by an ACO professional will be factored into the beneficiary assignment process. This adjustment aims to refine the attribution of patients to Accountable Care Organizations, potentially impacting how shared savings are calculated and distributed.