Converting geometry to WKT format by using QGIS Expression

WKT is case-insensitive for definition of geometry (Reference) and QGIS accepts the WKT string without the first and last point being the same.

For example:

geom_from_wkt( 'Polygon ((27 52, 31 37, 18 21))' )

and

geomWkt = QgsGeometry.fromWkt('Polygon ((27 52, 31 37, 18 21))')

work.

From documentation:

Note that unlike some programs, QGIS will close the ring for you so there is no need to duplicate the first point as the last.

The geometry type does not need to be written in capital letters. But of course, the wkt string should have closing coordinates. Seems like a bug to me as on Windows 64bit version of 3.16.2 and 3.10.12 I get closing coordinates when using geom_to_wkt($geometry).

However, as stated by Kadir Şahbaz, QGIS should be able to work without. However, just to complete this in case you need the closing coordinate, here is a fancy workaround:

'Polygon((' || -- start wkt string
array_to_string(
array_foreach( -- Go throuth every element of the following array
    string_to_array(geom_to_wkt(nodes_to_points($geometry)),','), -- Create the array of WKT information splitted by comma [ 'MultiPoint ((0.2974026 51.8987013)', '(0.00909091 51.50… ]
        x(make_point( -- Create a point geometry for every array content
            to_real(if(
                length(regexp_substr( -- Only create X coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)'))>0, -- everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' before the whitespace '0.2974026' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)')), -- Extract the X coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 0.2974026
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            )),
            to_real(if(
                length(regexp_substr( -- Only create Y coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*'))>0, --  everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' after the whitespace '51.8987013' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*')), -- Extract the Y coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 51.8987013
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            ))
        ))
|| ' ' || -- separator between x and y coordinates
        y(make_point( -- Create a point geometry for every array content
            to_real(if(
                length(regexp_substr( -- Only create X coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)'))>0, -- everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' before the whitespace '0.2974026' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'([^ ]+)')), -- Extract the X coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 0.2974026
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            )),
            to_real(if(
                length(regexp_substr( -- Only create Y coordinate if
                    regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*'))>0, --  everything between the Brackets of 'MultiPoint ((0.2974026 51.8987013)' after the whitespace '51.8987013' is not empty like ''
                    to_real(regexp_substr(regexp_substr(@element,'\\d.*\\d.[0-9]'),'(?<=\\s).*')), -- Extract the Y coordinate of 'MultiPoint ((0.2974026 51.8987013)' --> 51.8987013
                    NULL -- If invalid coordinate use NULL, maybe replace with 0
            ))
        ))
)
,', ') -- Split node coordinates from the array to a string separated by comma and whitespace
|| ', ' || x(point_n($geometry,1)) || ' ' || y(point_n($geometry,1)) -- re-add the first vertex-coordinates and add it as closing-coordinates
|| '))' -- Add closing brackets