At the turn of the twentieth century, the young Swedish architect Per O. Hallman emerged as an advocate for artistic city planning approaches. Inspired by St & auml;dtebau philosophies, Hallman viewed city planning competitions as crucial platforms to advance these innovative ideas, advocating for them as a counterpoint to the prevailing chessboard planning methods driven by engineers and land surveyors. His ideas resonated beyond Sweden, also diffusing into Finnish professional circles. This resulted in competitions for the unplanned districts of T & ouml;& ouml;l & ouml; in Helsinki (1898-1902) and southern Gothenburg (1901) - the first occurrences in Finland and Sweden to adopt artistic principles. Although Hallman served as a key agent for diffusing Camillo Sitte's principles from the Continent, his contributions remain underexplored within international city planning scholarship. The article addresses this gap by examining Hallman's largely untapped archival materials, which shed new light on his cross-collaborations with other prominent figures, including Fredrik Sundb & auml;rg, Bertel Jung, Lars Sonck, and even Josef St & uuml;bben, with whom he worked closely and judged the second stage of the T & ouml;& ouml;l & ouml; competition. Through these collaborations, Hallman not only advanced his own planning approach but also facilitated the breakthrough and integration of artistic values into city planning across two of the Nordic countries.