Nono, you of course CAN teach tool use at inference, but it's different than doing so at training time, and the models are trained to call specific tools right now.

Also MCP is not an Agent protocol, it's used in a different category. MCP is used when the user has a chatbot, sends a message, gets a response. Here we are talking about the category of products we would describe as Code Agents, including Claude Code, ChatGPT Codex, and the specific models that are trained for use in such contexts.

The idea is that of course you can tell it about certain tools in inference, but in code production tasks the LLM is trained to use string based tools such as grep, and not language specific tools like Go To Definition.

My source on this is Dax who is developing an Open Source clone of Claude Code called OpenCode