Rest-frame UV diagnostic diagrams. The location of GS-z9-0 is reported and compared to a sample of local, high-z analogues from the CLASSY survey (Berg et al. 2022; Mingozzi et al. 2024). Top panels: C III] λλ1907,1909/He II λ1640 vs. O III] λ1666/He II λ1640 (C3He2-O3He2, left) and C III] λλ1907,1909/He II λ1640 vs. C IV λ1550/C III] λλ1907,1909 (C3He2-C43, right) line ratios are compared with photoionisation models by Gutkin et al. (2016) and Feltre et al. (2016) for constant star formation (solid lines) and NLR of AGNs (dashed lines), respectively. We fix the dust-to-metal ratio to ξ_d = 0.1, while varying metallicity and the ionisation parameter (colour-coded), C/O abundance, and slope α of the AGN continuum (different symbols). GS-z9-0 occupies a region fully consistent with star formation driven ionisation in the C3He2-O3He2 diagram, whereas it sits in a region of overlapping star formation and AGN grids in the C3He2-C43 diagram. In the C3He2-O3He2 diagram the solid and dot-dashed black lines represent the empirical demarcation between SF-, AGN-, and shock-driven ionisation as proposed by Mingozzi et al. (2024), whereas in the C3He2-C43 diagram we report the demarcation lines between SF, AGN, and composite proposed by Hirschmann et al. (2019). Bottom panels: Same as upper panels, but for single stellar population models from Plat et al. (2019, 3 Myr and 1 Myr old for solid and dashed lines, respectively, with fixed ξ_d = 0.3, and C/O = 0.1C/O_⊙). The grids from 1 Myr stellar populations at low metallicity fully overlap those of AGN-like ionisation of higher metallicity and solar C/O (ξ_d = 0.1) in the C3He2-C43 diagram.