GPT-5.6 Sol Terra Luna Explained: Which OpenAI Model Should You Use?

What GPT-5.6 means inside Microsoft 365 Copilot

The GPT-5.6 Microsoft 365 Copilot rollout is a separate buyer consequence from the model-family launch. OpenAI says GPT-5.6 will become the preferred model family in Copilot for Word, Excel, PowerPoint, Chat, and Cowork. Microsoft says it is rolling out now and may be selected automatically when Copilot decides the task fits; where model selection is available, a user can choose GPT-5.6 directly.

Preferred does not mean exclusive. It does not prove that every Copilot request uses GPT-5.6, that every tenant can select it today, or that Microsoft exposes Sol, Terra, and Luna as three ordinary Office buttons. The product can route models behind the Copilot experience, and Microsoft says availability can vary by region and tenant configuration.

The practical app map is straightforward. Microsoft and OpenAI describe Word as the drafting and editing surface, Excel as the analysis surface, PowerPoint as the presentation surface, Chat as the place for ambiguous questions and planning, and Cowork as the longer multi-step work surface. Those are provider descriptions, not BTI benchmark results.

Enterprise administrators also have a real rollout decision. Microsoft’s current Learn page says OpenAI-operated models are initially disabled unless an eligible organization enables the OpenAI subprocessor setting. Microsoft plans to enable that setting by default for eligible commercial customers on July 24, 2026 unless an administrator opts out. Government and sovereign-cloud exclusions apply at launch. Check the current Microsoft documentation before changing a tenant policy because this schedule and eligibility can change.

Where Grok 4.5 fits in this model update

xAI’s release notes say Grok 4.5 was added to the xAI API on July 8, 2026. xAI’s docs frame Grok 4.5 as a frontier model for coding, agentic tasks, and knowledge work, with configurable reasoning effort. That makes it part of the same buyer question: which model should run which job?

The clean BTI answer is not “Grok 4.5 beats GPT-5.6” or “GPT-5.6 beats Grok 4.5.” We do not have BTI benchmark evidence for that, and public model comparisons can change quickly. The useful answer is to test Grok 4.5 when your stack is already using xAI, when its API shape fits your workflow, or when you want to compare code and agent runs against a real task set.

For a serious team, the model update watchlist should track five things: official release notes, model names, product availability, reasoning controls, and migration risk. A model can be impressive and still be wrong for a production workflow if the access path, policy, latency, cost, or observability does not fit.

BTI model picker: the simple rule

Start with the outcome, then choose the model. If the reader is making a decision, ask whether the work needs depth, balance, or speed. Depth points to Sol. Balance points to Terra. Speed and cost point to Luna. Cross-provider API comparison points to a test lane that can include Grok 4.5.

Here is the workflow version. Use Sol for the final hard pass, Terra for normal production work, Luna for intake and high-volume steps, and Grok 4.5 as an xAI comparison or xAI-native build option. Then measure the result on your own prompts. Public launch posts are a starting point, not a substitute for a test set.

For BTI readers who only use ChatGPT, the key detail is now plan-specific. Free and Go are moving to Luna as the default and are scheduled to gain unlimited text chats plus a Think button next week, with tool limits still in place. Plus and Pro get updated Sol plus the thinking-time slider. Work and Codex Sol do not change in this Chat-focused release.