OpenAI released GPT-5.6 on July 9, 2026, featuring three tiers: Sol, Terra, and Luna, all optimized for agentic tool calling. Each tier offers a 1 million token context window and a 128K maximum output. The new models introduce Programmatic Tool Calling, enabling the AI to write JavaScript to chain tool calls, significantly reducing token usage and costs. GPT-5.6 also boasts an ultra multi-agent mode, running subagents in parallel to improve performance.Sol is the flagship for complex agentic work, Terra is balanced for high-volume tasks, and Luna is the most affordable for everyday use. Choosing the correct tier is crucial to avoid overpaying, with a recommendation to start with Luna and escalate if needed. Testing these models with MCP servers can be done for free in the browser using tools like MCP Agent Studio. GPT-5.6 excels in tool orchestration and agentic benchmarks, though Claude may still lead in raw code-fixing accuracy.Reliable tool calls depend on clear tool descriptions and schemas, and it is advisable to confirm write actions before execution. Server-side errors are distinct from model errors, and MCP servers should be scanned for security risks. MCP Playground allows for free side-by-side testing of GPT-5.6 tiers and other models without needing an API key. Ultimately, comprehensive testing is the best way to determine the optimal GPT-5.6 tier for specific MCP server workloads.