How to Use errors.New and fmt.Errorf in Go

Use errors.New for simple, static error messages and fmt.Errorf when you need to format variables or wrap existing errors with context.

// Simple static error
var ErrNotFound = errors.New("item not found")

// Formatted error with context
func processFile(path string) error {
    if _, err := os.Stat(path); err != nil {
        return fmt.Errorf("failed to access %s: %w", path, err)
    }
    return nil
}

Use errors.New when you just need to say something went wrong with a fixed message. Use fmt.Errorf when you need to include specific details like file names or IDs, or when you want to attach a new error message to an existing one so the full history is preserved. Think of errors.New as a standard warning sign and fmt.Errorf as a detailed incident report.