Coin Metal Modification Authorization and Cost Savings Act of 2021 This bill authorizes the United States Mint to modify the metallic composition of circulating coins (including by prescribing reasonable manufacturing tolerances with respect to those coins) if a study and analysis conducted by the Mint indicates that the modification will reduce costs incurred by the taxpayers; be seamless, which shall be determined by verifying that the coins will work interchangeably in most coin acceptors using electromagnetic signature technology; and have as minimal an adverse impact as possible on the public and stakeholders. The Mint must notify Congress before making the modification and provide a justification for the modification.