To analyze the intrinsic thermodynamic stability of these vacancy-ordered perovskites, the CompetingPhases module of the doped 51 defects analysis package was used to query the Materials Project and identify all compositions bordering the chemical stability region of the material within the given Cs -M -X (M = Sn, Ti; X = I, Br, Cl) chemical system. Within these bordering phases, all polymorphs with energies above hull less than 0.1 eV/atom according to the Materials Project 52 energies (doped default) were then re-relaxed using the HSE06 hybrid DFT functional including van der Waal's dispersion corrections (D3) with the same plane-wave energy cutoff as before (300 eV), and converged kpoint sampling determined using vaspup2.0 47 in each case. Notably, the polymorph was found to 𝑃𝑚3𝑚 be the groundstate for all CsX (X = I, Br, Cl) with our HSE06+D3 model in agreement with experiment, despite this phase being 0.03 eV/atom above the convex hull according to the Materials Project database, demonstrating the importance of considering all phases within a reasonable error range of the convex hull when pulling initial structures from databases. For the extrinsic decomposition pathways, a similar workflow was followed where all bordering phases in the Cs -M -X -H -O (M = Sn, Ti; X = I, Br, Cl) chemical system, with energies above hull < 0.1 eV/atom, were pulled from the Materials Project using doped, and re-relaxed with HSE06+D3 with the same energy cutoff and converged kpoint sampling. Additionally, using the ComputedReaction class in pymatgen, potential exothermic decomposition reactions of Cs 2 MX 6 (M = Sn, Ti; X = I, Br, Cl) in the presence of H 2 O (g) , O 2(g) and/or H 2(g) were screened using the Materials Project energies. For gaseous species (O 2 , Cl 2 , H 2 O, HBr, HCl, HI), energies were determined by computing the internal energy of an isolated molecule in a 30⨉30⨉30 Å 3 cell, and adding the thermal enthalpic and entropic contributions as computed using vaspkit. All gases were calculated in their magnetic groundstate (i.e. singlet (S = 0) in each case except for O 2 which has triplet (S = 1) spin in the groundstate). Partial pressure contributions to the internal energies of gaseous phases were computed using the ideal gas law relation: