Resolving a Contract Dispute With a Supplier or Customer in Ontario

When a business relationship breaks down, the contract is your roadmap. Whether a supplier failed to deliver what was promised or a customer refuses to pay what they owe, knowing how to navigate a contract dispute with a supplier in Ontario can mean the difference between recovering your losses and writing them off.

Ontario law gives businesses real tools to enforce their agreements — but those tools work best when you act quickly, document everything, and understand your options before tensions escalate. This article walks you through the most common scenarios and how to move from the first sign of trouble to a resolution.

Common Scenarios That Lead to a Dispute

Commercial disputes rarely arrive without warning signs. The most frequent situations we see include:

• Goods not delivered or delivered late — A supplier misses a delivery deadline and your production line or customer commitments are affected.
• Services not rendered or cut short — A contractor or service provider fails to complete the work scope, stops part-way through, or provides work that falls well short of what was agreed.
• Non-payment by a customer — You delivered exactly what was promised, but invoices go unpaid, disputed, or partially offset without a valid reason.
• Defective goods or materials — Products arrive damaged, fail to meet the agreed specifications, or break down prematurely — causing loss well beyond the original purchase price.
• Substandard workmanship — A renovator, IT firm, or professional services provider delivers work that doesn't meet the standard promised in the contract or reasonably expected in the industry.

Each scenario follows a similar legal path: establish what was promised, show how that promise was broken, and quantify the resulting loss.

First Steps: Get Your House in Order

Before you send any message or make any call, take these foundational steps.

Review the Contract

Read the agreement carefully. Look for:

• The exact obligations of each party and the timeline for performance
• Any notice requirements before terminating or escalating
• Dispute resolution clauses (some contracts require mediation before litigation)
• Limitation of liability or exclusion of consequential damages clauses

If the contract was informal — a series of emails, a purchase order, or a handshake — gather every piece of written communication that shows what was agreed.

Document the Breach

Write down a clear timeline: what was promised, when it was supposed to happen, and what actually occurred. Photograph defective goods. Screenshot late delivery confirmations. Save every email and text message. Good documentation is often what separates a strong claim from an unwinnable one.

Preserve Evidence

Do not discard defective goods, delete email threads, or alter any records. In litigation, evidence that disappears after a dispute arises can become a problem of its own.

The Demand Letter: Your First Move

A well-crafted demand letter does more than signal that you're serious — it often resolves disputes without the cost and delay of court. The letter should:

• Identify the specific breach in plain terms
• Reference the contract or agreement
• State the amount owed or the remedy you're seeking
• Set a reasonable deadline for a response (10 to 21 days is common)
• Make clear that you will pursue legal remedies if the matter is not resolved

The tone should be firm but professional. Avoid threats you don't intend to follow through on, and avoid language that could be read as harassment. Many suppliers and customers respond seriously to a demand letter — especially one that clearly comes from or is being reviewed by a lawyer.

Escalation: What Happens if the Letter Doesn't Work

Negotiation and Mediation

If the parties are still talking, direct negotiation is worth exploring. When direct talks stall, a neutral mediator can help both sides find a resolution they can live with, at a fraction of the cost of going to court. Some contracts require mediation as a first step — check yours.

Small Claims Court

Ontario's Small Claims Court handles civil disputes up to a monetary threshold — as of writing, that limit is $35,000 (verify the current limit at ontario.ca, as it can be adjusted by regulation). The process is designed to be accessible without a lawyer, though legal advice before you file can help you avoid procedural missteps. Judgment timelines and costs are generally lower than Superior Court.

Ontario Superior Court of Justice

For disputes above the Small Claims threshold, or where the issues are complex, the Superior Court of Justice is the appropriate forum. Commercial litigation here can involve document production, examinations under oath, and expert witnesses. It is more expensive and time-consuming than Small Claims — but it is also the venue that can handle injunctions, declaratory relief, and larger damage awards.

The Sale of Goods Act

If your dispute involves goods rather than services, Ontario's Sale of Goods Act may work in your favour even if your contract is silent on quality. The Act implies certain conditions into contracts for the sale of goods — including that goods must be of merchantable quality and fit for the purpose for which they were sold. Importantly, these implied conditions cannot always be contracted out of, particularly in consumer-facing transactions. If you received goods that were defective or unfit for purpose, this legislation may give you rights that go beyond what the contract itself says.

Damages and the Duty to Mitigate

If you succeed in a contract dispute, the usual goal of damages is to put you in the position you would have been in had the contract been performed — these are called expectation damages. You can recover the difference between what you received and what you were entitled to, plus foreseeable losses that flowed from the breach.

However, Ontario law also requires the innocent party to mitigate their losses — meaning you must take reasonable steps to limit your damages. If a supplier fails to deliver materials and you sit on your hands while losses mount, a court may reduce your damages to the amount you would have suffered had you acted reasonably. Source a replacement supplier. Stop work on a defective project rather than continuing to spend money. Document every step you take.

Limitation Period

In Ontario, most contract claims must be commenced within two years of the date you discovered (or reasonably ought to have discovered) the breach — as of writing, verify the current limitation period under Ontario's Limitations Act, 2002, as exceptions and special rules can apply. Missing the deadline can bar your claim entirely, regardless of how strong it is. If you suspect a breach has occurred, do not delay in getting legal advice.

Frequently asked questions