Length 4–7 mm.
The species is easy to recognise by the edentate posterior margin of T3 and the narrow, elongate body shape. Species of Chrysura, which are similarly coloured and also lack apical teeth, do not have the scapal basin or the dark apical rim, and are larger in size. The head and mesosoma are mainly blue or greenish with golden reflections, and the mesoscutum is medially contrastingly darker than laterally. The metasoma is completely golden red dorsally, but the apical rim is dark blue or blackish. The apical rim is wide, medially slightly undulating and laterally with angled margins. F2, F3 and F4 are ventrally slightly bulging in the male. The shape of the body is very slender and elongate in both sexes.
Estonia. Very rare. One female was collected on 14.VII.2015 in Reinu, southwestern Estonia (58.032°N, 24.747°E, leg. V. Soon). No other records are known from the Nordic and Baltic countries, but one female has been collected from Russia, close to the eastern border of Latvia (Pskov Oblast, Krasikovo, 23 km south of Sebezh, VII.1999, leg. A. Reschikov).
West Palearctic: Europe, northern Africa and Middle East (Linsenmaier 1997).
Habitat: various biotopes with sun-exposed dead wood (Kunz 1994, Linsenmaier 1996, Rosa 2006). Adults often fly on wooden poles (Rosa 2006) and occasionally visit flowers of Apiaceae and Euphorbiaceae (Linsenmaier 1997).
Flight period: June to August in Germany (Kunz 1994).
Host: species of Microdynerus Thomson (Vespidae) (Friese 1883, Enslin 1929, Benno 1950, Wickl 2001) and possibly also Trypoxylon clavicerum Lepeletier & Serville (Crabronidae) (Morgan 1984). The only Microdynerus species found in Estonia is M. parvulus (Herrich-Schäffer), which could be the host of C. gracillima at the northern limit of its distribution area.