See IRS Publication 597 Information on
the United
States–Canada
Income Tax
Treaty page 3 - https://www.irs.gov/pub/irs-pdf/p597.pdf#page=3
U.S. social security benefits paid to a resident of Canada are taxed in Canada as if they were benefits under the Canada Pension Plan, except that 15% of the amount of the benefit is exempt from Canadian tax.
You would not include your US Social Security benefits on a US federal tax return when those benefits are being reported on the Canadian tax return.
