I find great success in not relying on LLM's built-in knowledge, but giving it links to necessary docs/manuals and have it read that before doing anything.
If you have zig installed, you can run ‘zig std’ to see that.
You still have the limitations of attention etc…
Even zed’s agent will leverage that built in tarball, but it doesn’t solve the problem, especially as some of the languages killer features are unavailable in C and other languages.