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If the expected and actual values make a longer message, the error message is truncated.

const fruits = ['Apples', 'Oranges', 'Pears', 'Kiwi']

cy.wrap(fruits).should('deep.equal', [
  'Oranges',
  'Grapefruits',
  'Plums',
  'Kiwi',
])

You can control the truncate limit, it is part of the chai.config object

chai.config.truncateThreshold = 200
cy.wrap(fruits).should('deep.equal', [
  'Oranges',
  'Grapefruits',
  'Plums',
  'Kiwi',
])

The full error message shows the two arrays now

Watch the video Increase Chai Truncate Threshold To Show More Information (opens new window)