Hey there! I checked the logs for your Request ID, and this isn’t a Cursor-side problem with GPT-5.6 itself. It’s coming from a custom OpenAI Base URL override in your setup.
In Cursor Settings > Models, under the OpenAI provider, there’s a custom Base URL enabled that points to a third-party endpoint (tinker.thinkingmachines.dev). Because of that, every GPT/OpenAI model you pick, including GPT-5.6 Sol, gets routed to that endpoint instead of Cursor’s own infrastructure, and that endpoint rejects the request with the validation error you’re seeing:
"body -> tools -> 7 -> type", "Input should be 'function'"
That’s also why your other models (Claude, Gemini, Composer, etc.) keep working. The override only applies to OpenAI/GPT models.
To use Cursor’s native GPT-5.6, turn off the custom OpenAI Base URL override (or remove that OpenAI key) under Cursor Settings > Models, and GPT-5.6 will route through Cursor and work normally. If you set that endpoint up on purpose for a custom model, you can just leave the override off when you want Cursor’s built-in GPT models and turn it back on for your own.