A totalization agreement says you owe social security to only one country — but saying it isn't proving it. The Certificate of Coverage is the document that proves it. Without one, the host country's tax authority can simply demand contributions anyway.
A Certificate of Coverage is an official statement that a worker remains covered by one country's social security system and is therefore exempt from the other country's contributions for the period shown. Every one of the 30 U.S. agreements uses this mechanism.
1. The employer (or the self-employed worker) applies to the SSA's Office of Earnings and International Operations (OEIO) — the SSA provides an online request system at ssa.gov/international, plus mail options.
2. Provide the worker's details (name, SSN, date of birth), the employer's details, the country, and the assignment start/end dates.
3. Receive the certificate and keep it for the entire assignment — employer and employee should each hold a copy, ready to show either country's authorities.
Applying late. The exemption exists under the agreement either way, but untangling contributions already withheld by a foreign system is slow and painful. Get the certificate before payroll starts.
Assuming it covers income tax. It doesn't — it governs social security contributions only. Income tax is a separate question under the relevant income tax treaty.
Outliving the certificate. Assignments that stretch past the detached-worker limit (usually 5 years) shift coverage to the host country. Extensions require both countries to agree — don't count on one.
Not sure which system should cover you in the first place? Run the calculator for your country and scenario.
Processing by the SSA Office of Earnings and International Operations typically takes several weeks. Online requests are usually faster than mail. Apply as soon as the assignment is confirmed — the certificate can be issued before the work abroad begins.
For employees, the U.S. employer requests the certificate. Self-employed people request it themselves. If the host country's system should cover you instead, the request goes to that country's social security authority, not the SSA.
Employees generally keep it on file and present it if either tax authority asks. Self-employed workers exempt from U.S. self-employment tax under an agreement attach a statement (and keep the foreign certificate) when filing Schedule SE — follow the current IRS instructions.
Certificates are issued for the expected assignment period, typically up to the agreement's detached-worker limit (usually 5 years). If the assignment runs longer, extensions are possible only in limited cases by mutual consent of both countries — otherwise coverage shifts to the host country.