GPT-5.6 system card details Sol, Terra, Luna models

GPT-5.6 family includes flagship Sol, mid-tier Terra, and low-cost Luna. Pricing: Sol $5/$30, Terra $2.5/$15, Luna $1/$6. New settings include Max and Ultra (sub-agents). Sol is a 'step function better' than GPT-5.5 but still short of Mythos.