Custom instructions enable you to define common guidelines and rules that automatically influence how AI generates code and handles other development tasks. Instead of manually including context in every chat prompt, specify custom instructions in a Markdown file to ensure consistent AI responses that align with your coding practices and project requirements.
There are lots of different ways of providing instructions at different levels, from system-wide, to project-specific, and depending on your mental model, even something like [[20260306121850-ai-agent-skills]] can be considered 'instructions'.
[[20260612013413-agents-md]] has emerged as the main source, as of 2026.