Override Error Messages

You can override the default message for a validation on a per property basis. Here is how you configure the extend call:

ko.validation.rules['mustEqual'] = {
	validator: function (val, otherVal) {
		return val === otherVal;
	},
	message: 'The field must equal {0}'
};
ko.validation.registerExtenders();

//if not modifying the default message, you would call like this:
// viewModel.PercentSum.extend({ mustEqual: 100 });

//if you want to override the message, you need to move your validation parameter into the "params" field
viewModel.PercentSum.extend({ mustEqual: { params: 100, message: "Percentages must add to 100" } });