If I'm sued along with others, does my insurer only have to pay its share of the defence costs in Ontario?

Your insurer's duty to defend generally runs only to you, its own insured - not to co-defendants who aren't covered under your policy, even if you're all named in the same lawsuit. In practice, your insurer typically funds your own defence in full, and other defendants look to their own insurers (if any) for theirs, rather than the costs simply being split proportionally among everyone named in the claim.

Where it gets more involved is when multiple defendants are insured under the same or related policies, or where a single insurer covers more than one defendant in the same lawsuit - there, questions about how defence costs are allocated between the co-insureds can arise, particularly if their interests start to diverge as the case develops. If you're facing a lawsuit alongside others and are unsure whether your insurer is treating your defence appropriately relative to co-defendants, it's worth reviewing your specific policy's wording on defence costs, since the answer depends on the actual policy language rather than a general rule about shared litigation.