What's the difference between suing a professional for negligence versus breach of contract in Ontario?

When a professional does substandard work, Ontario law generally lets you sue in both negligence (a tort duty owed independently of any agreement) and breach of contract (the engagement letter or retainer itself) at the same time, choosing whichever framing suits your case better - this is called concurrent liability. The practical differences matter. A contract claim is measured against whatever the engagement actually promised, and any limitation-of-liability or exclusion clause in that contract can restrict what you recover. A negligence claim exists independently of the contract's wording and focuses on whether the professional met the standard of a reasonably competent practitioner.

The two can also have different timing consequences under the discoverability principle, since a contract breach may be identifiable at a different moment than when you discovered the resulting harm. Damages theoretically differ too - contract damages aim to put you where the agreement promised you'd be, while negligence damages aim to put you where you would have been but for the carelessness - though in professional services cases these often land at a similar number. Because the interplay affects strategy and what a defence can rely on, it's worth having both angles assessed together rather than picking one in isolation.