Defences to Breach of Contract Claims in Ontario

Not every alleged breach leads to liability. Many Ontario cases turn on defences grounded in contract wording, timelines, performance issues, and damages proof.

Common defences (overview)

• No contract / uncertain terms: essential terms not agreed
• Interpretation: the contract doesn’t mean what the claimant says
• Performance issues: claimant didn’t perform or prevented performance
• Notice failures: required notice/cure steps weren’t followed
• Limitation issues: claim brought too late (fact-specific)
• Mitigation: claimant failed to reduce losses
• Damages proof: losses are speculative or not caused by the breach
• Contra Proferentem: if a clause in a contract is ambiguous the interpretation to be used is the one that favours the party that did not draft the contract. (I.e. interpret against the drafter)

If you’re responding to a claim, the first priority is often building a clean timeline, preserving communications, and identifying which defences are strongest on the documents.

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FAQ

What are common defences to breach of contract in Ontario?

Common defences include no contract or uncertain terms, contract interpretation disputes, performance/causation issues, notice failures, limitation issues, mitigation, and damages proof problems.

Can a claim fail even if there was a breach?

Yes. Claims can underperform or fail if damages aren’t proven, losses aren’t linked to the breach, or mitigation was not reasonable.

How does “failure to mitigate” work as a defence?

It can reduce damages if the claimant could have reasonably reduced losses but did not. The focus is on reasonableness and evidence.

Do notice provisions matter in a defence?

Often. If a contract requires notice and an opportunity to cure, non-compliance can affect termination rights and available remedies.

What should I do first when served with a contract claim?

Preserve all communications, gather the contract and key documents, build a timeline, and obtain advice promptly especially if deadlines are approaching.