Depends on the framework you're using. I'm using Qwen Code Companion (via a VS Code IDE plugin) and it's got some kind of tiered system built into it for adding tool definitions to context, it's got a standard built-in set that's always mentioned but other stuff only gets added to the context under certain circumstances. I can't recall all the details, I found out about it when I was chatting with Qwen Code to ask it why it hadn't used a particular MCP I'd given it to solve a problem despite it seeming like a useful tool for the task and we got into a discussion about to configure it. As I recall it's something along the lines of most tools only being present in context by default as little one-liner stubs and the agent has to tell the framework "that one looks like it might be useful for what I'm doing" for the full definition to get loaded.

Most of the time I'm working on small scripts and specialized applications these days, and codebase memory is overkill for those so I haven't actually seen it in use much.