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Write test assertions that are resilient to implementation changes by focusing on behavior rather than implementation details. Avoid exact matches when partial or behavioral assertions would suffice.

Key practices:

1. Use flexible matchers instead of exact equality
2. Focus on behavior over implementation details
3. Test for presence of required properties rather than exact object shape

Example - Instead of brittle exact matching:

// ❌ Brittle - breaks if new options are added
expect(options).toEqual({ 
  preview: false, 
  viewColumn: vscode.ViewColumn.Active 
})

// ✅ Resilient - verifies required properties
expect(options).toEqual(expect.objectContaining({ 
  preview: false,
  viewColumn: vscode.ViewColumn.Active 
}))

For token counting or similar approximate values:

// ❌ Brittle - breaks if tokenizer changes slightly
expect(tokenCount).toBe(14)

// ✅ Resilient - verifies reasonable range
expect(tokenCount).toBeGreaterThan(12)
expect(tokenCount).toBeLessThan(16)

This approach:

• Reduces test maintenance as implementation details change
• Focuses tests on behavioral requirements
• Makes tests more reliable and meaningful
• Avoids over-mocking that tests the mocks rather than the code

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