The "Fit to Serve Act" amends title 10, United States Code, to establish a comprehensive prohibition against discrimination based on gender identity within the Armed Forces. This legislation aims to ensure that all service members are treated equally, regardless of their gender identity, and defines gender identity broadly to include appearance, mannerisms, or other gender-related characteristics. Specifically, the bill prohibits the Secretary concerned from discriminating in various aspects of military service, including prescribing qualifications, involuntarily separating members, or denying medically necessary health care coverage. It also prevents requiring individuals to serve in the sex assigned at birth and prohibits denying accession, reenlistment, or continuation of service based on gender identity, including a diagnosis or potential diagnosis of gender dysphoria.