[simonw]

"Is gpt-3.5-turbo-instruct function calling a chess-playing model instead of generating through the base LLM?"

I'm absolutely certain it is not. gpt-3.5-turbo-instruct is one of OpenAI's least important models (by today's standard) - it exists purely to give people who built software on top of the older completion models something to port their code to (if it doesn't work with instruction tuned models).

I would be stunned if OpenAI had any special-case mechanisms for that model that called out to other systems.

When they have custom mechanisms - like Code Interpreter mode - they tell you about them.

I think it's much more likely that something about instruction tuning / chat interferes with the model's ability to really benefit from its training data when it comes to chess moves.