Perhaps "before implementing a new utility or helper function, ask the not-invented-here tool if it's been done already in the codebase"
Of course, now I have to check if someone has done this already.
I'd bet that most the improvement in Copilot style tools over the past year is coming from rapid progress in context engineering techniques, and the contribution of LLMs is more modest. LLMs' native ability to independently "reason" about a large slushpile of tokens just hasn't improved enough over that same time period to account for how much better the LLM coding tools have become. It's hard to see or confirm that, though, because the only direct comparison you can make is changing your LLM selection in the current version of the tool. Plugging GPT5 into the original version of Copilot from 2021 isn't an experiment most of us are able to try.
Just like with humans it definitely works better if you follow good naming conventions and file patterns. And even then I tend to make sure to just include the important files in the context or clue the LLM in during the prompt.
It also depends on what language you use. A LOT. During the day I use LLMs with dotnet and it’s pretty rough compared to when I’m using rails on my side projects. Dotnet requires a lot more prompting and hand holding, both due to its complexity but also due to how much more verbose it is.
We started with building the best code retrieval and build an agent around it.