Do I have to pay into both countries' social security systems?
That is what the agreement is designed to prevent, but it only prevents it if the paperwork exists. Social security is not covered by the tax treaty at all — a separate agreement decides which country's system a cross-border worker pays into, and it is evidenced by a certificate of coverage. With the certificate in place, contributions go to one system. Without it, both systems will bill, because each is simply applying its own domestic rules to an employer and a worker it can see.
Who applies for the certificate of coverage, me or my employer?
The application goes to the authority in the home country, and in employment cases it is normally the employer who makes it, because the employer holds the assignment terms the application is built from — start date, expected duration, which entity pays, and where the work is performed. The worker's details are part of it but not the whole of it. The timing matters more than the form: it is applied for before or at the start of the assignment, not at the point someone notices a second set of deductions.
We already paid into both systems — can we get contributions back?
Often yes, and the route is to establish which system the worker belonged to for the period and then apply to correct the other one. The work is mostly evidential: reconstructing the assignment timeline, showing which entity employed and paid the worker and where the duties were performed, and obtaining the certificate covering the period retrospectively where the authority will issue it. Recovery is slower than getting it right at the outset and it is not always complete, so treat it as a repair rather than an alternative.
Will my years abroad count towards my pension back home?
That is the other half of what these agreements do. As well as stopping contributions to both systems, they allow periods completed under each system to be aggregated when eligibility for a benefit is tested, so a working life split across two countries is not judged as though the years abroad never happened. It is a question of eligibility rather than a transfer of money between systems. The claim is made to the authority you are claiming from, and it needs a documented record of the periods worked under each.
Does the tax treaty cover social security contributions as well?
No, and this is the assumption that causes the most trouble. A treaty position on income tax says nothing about contributions. They are dealt with by a separate agreement, with its own scope, its own tests for which system applies, and its own evidence in the form of a certificate. Employers who have taken careful treaty advice on withholding and stopped there are exactly the ones who find a second set of contributions being billed, because the two questions were never connected.
The assignment has already started — is it too late to apply?
It is later than ideal, not necessarily too late. The certificate is meant to be applied for before or at the start of the assignment, and applying during it is common enough that authorities deal with it. What changes is the amount of repair work: contributions may already have been taken by the other system and have to be corrected, and payroll has to be reset from a specified date. Deal with it as soon as it is noticed, because each pay run adds to what has to be unwound.
What happens if two countries both say I am resident?
The treaty tie-breaker resolves it to one residence, applied in order: where your permanent home is, then your centre of vital interests, then your habitual abode, then nationality, with a competent-authority referral if all of those fail. It is an evidence exercise rather than an election — you document the home and the life around it. Getting a single residence settled is what makes every other position in both returns consistent. See the residency tie-breaker.
Is foreign pension income taxable in Canada?
Yes. A Canadian resident reports foreign pension income in Canadian dollars like any other income, and foreign tax withheld on it becomes a credit rather than a reduction of the amount reported. Where a treaty exempts part or all of it — some social security pensions are treated this way — the relief is claimed as a deduction on the return, not by leaving the pension off. Omitting it and claiming it was exempt are two very different filing positions. See the pensions and annuities article.
