How does CRA decide if my one-client incorporated consulting gig is a personal services business?

CRA applies the same substance-over-form factors used for any employee-versus-contractor question, control over how and when the work is done, who owns the tools and equipment, whether there's a genuine chance of profit or risk of loss, and how integrated you are into the client's operations, and then asks the specific PSB question on top: if your corporation didn't exist, would you reasonably be considered an employee of that one client? Having only a single client for an extended period is a meaningful risk factor pointing toward that conclusion, though it isn't automatically decisive on its own.

What matters alongside the single-client fact is how the relationship actually works day to day, whether the client sets your hours and supervises your work closely, whether you use your own equipment and bear real financial risk, and whether you have the kind of independence a genuine contractor would have. A one-client arrangement that otherwise looks like genuine independent contracting has a better chance of avoiding PSB status than one where the client also controls the details of how and when you work.

Because this determination turns on the specific facts of your arrangement, reviewing your actual working relationship against these factors, rather than relying on the invoice or contract alone, is the realistic way to assess your risk.