Alternative diagnostic diagrams for GS-z9-0. The left and middle panels leverage the very high-ionisation (∼97 eV) [Ne V]λ3426 emission line, and the same set of models from Gutkin et al. (2016) and Feltre et al. (2016) as in Fig. 8. The diagram in the left panel (C3He2-Ne5C3) is sensitive to the C/O abundance, while the diagnostic considered in the middle panel (O3HB-Ne53) is not. Here, the location of GS-z9-0 is more in line with AGN-powered line ratios, especially in the [O III] λ5007/Hβ vs. [Ne V]/ [Ne III] diagram, where we show also the empirical demarcation lines between SF, AGN, and IMBH/PopIII models from Cleri et al. (2023). The right panel shows the C III] λ1909/He II λ1640 vs. N III] λ1750/O III] λ1666 diagram. The location of GS-z9-0 is compared with the set of photoionisation models for star formation (solid lines) and AGN/NLR (dashed lines) ionisation from Ji et al. (2024a).