Abstract The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-resolution spectral catalog of 45 local high- z analogs in the ultraviolet (UV; 1200–2000 Å) to investigate their stellar and gas properties. Here we present a toolkit of UV interstellar medium (ISM) diagnostics, analyzing the main emission lines of CLASSY spectra (N iv λ λ 1483,87, C iv λλ 1548,51, He ii λ 1640, O iii λ λ 1661,6, Si iii λλ 1883,92, C iii λ 1907,9). Specifically, our aim is to provide accurate diagnostics for the reddening E ( B − V ), electron density n e , electron temperature T e , metallicity 12+log(O/H), and ionization parameter log( U ), taking the different ISM ionization zones into account. We calibrate our UV toolkit using well-known optical diagnostics, analyzing archival optical spectra for all CLASSY targets. We find that UV density diagnostics estimate n e values that are ∼1–2 dex higher (e.g., n e (C iii λ λ 1907,9) ∼ 10 4 cm −3 ) than those inferred from their optical counterparts (e.g., n e (S ii λ λ 6717,31) ∼ 10 2 cm −3 ; n e (Ar iv λ λ 4714,41) ∼ 10 3 cm −3 ). T e derived from the hybrid ratio O iii λ 1666/ λ 5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O iii λ 4363/O iii λ 5007 within ∼ ±0.3 dex. We also investigate the relation between the stellar and gas E ( B − V ), finding consistent values at high specific star formation rates (sSFRs; log ( sSFR ) ≳ − 8 yr −1 ), while at low sSFRs we confirmed an excess of dust attenuation in the gas. Finally, we investigate UV line ratios and equivalent widths to provide correlations with 12+log(O/H) and log( U ), but note that there are degeneracies between the two. With this suite of UV-based diagnostics, we illustrate the pivotal role CLASSY plays in understanding the chemical and physical properties of high-z systems that JWST can observe in the rest-frame UV.