GPT-5.6 vs GPT-5.5: In-Depth Comparison — 3 Models, Terra at 40% of the Price, Benchmarks & Migration
Arriving July 9, just two and a half months after April 2026's GPT-5.5, GPT-5.6 is not merely a performance bump — the biggest change is the reorganization from a single flagship into a three-model lineup: Luna/Terra/Sol. What matters most to many GPT-5.5 users is that the mid-tier Terra delivers GPT-5.5-class quality at a far lower price: it launched at half price, and the July 30, 2026 price cut took it to 40% of GPT-5.5's unit price ($2/$12). For those wanting more performance there is the top-tier Sol (same $5/$30, with SWE-Bench Pro rising an estimated 58.6→64.6%); for those wanting lower costs there is Terra. This article lays out, based on official announcements and independent analysis, what the three-model shift means, a spec-at-a-glance table, Terra's price-to-performance, generational benchmark gains (same-metric SWE-Bench Pro +6pt, but TerminalBench differs in version 2.0→2.1 and much general reasoning is undisclosed), 5.6's new features (Programmatic Tool Calling, max effort, ChatGPT Work, GPT-Live, GitHub Copilot support), real cost (roughly $1,100→$440 for 100M input/20M output a month), and which model you should migrate to (cut the bill by 60% with Terra, then move only performance-critical work to Sol).
