OpenAI Launches GPT-5.6 as a Three-Tier Model Family with Programmatic Tool Calling in the Responses API

OpenAI has released GPT-5.6 as a structured family of three models — Sol, Terra, and Luna — each targeting different capability-cost tradeoffs, from lightweight tasks to frontier-level reasoning. A key developer-facing addition is programmatic tool calling support directly in the Responses API, enabling more reliable and structured agentic workflows without brittle prompt engineering. The three-tier architecture gives developers a clear upgrade path and lets them match model size to task complexity programmatically within a single API surface. This is a meaningful shift for teams building multi-step agents, as structured tool calling reduces failure modes in function dispatch and output parsing. Developers can start integrating via the OpenAI Responses API today, and the model is already live as the default in Microsoft 365 Copilot.