Maybe it's because of the example, but if the LLM knows the GitHub CLI and I bet it knows it, shouldn't it be able to run the commands (or type them for us) to count the open PRs on foo/bar?

However I see the potential problem of the LLM not knowing an obscure proprietary API. The traditional solution has been writing documentation, maybe on a popular platform like Postman. In that case the URL of the documentation could be enough, or an export in JSON. It usually contains examples too. I dread having to write and maintain both the documentation for humans and the MCP server for bots.