The "Fit to Serve Act" amends title 10, United States Code, to establish a comprehensive prohibition on discrimination within the Armed Forces based on gender identity. This includes preventing the military from using an individual's gender identity, or a diagnosis of gender dysphoria, to prescribe service qualifications or involuntarily separate service members. The bill explicitly forbids denying medically necessary health care coverage or requiring a member to serve in the sex assigned at birth due to their gender identity. Furthermore, it ensures that accession, reenlistment, or continuation of service cannot be denied on this basis. The Act defines "gender identity" broadly to include an individual's gender-related identity, appearance, or characteristics, regardless of their sex assigned at birth.