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'.
AGENTS.md is the common open standard for project-wide and system-wide instructions, basically anything that provides non-specific information. There's other competing standards, like CLAUDE.md and GEMINI.md.
AGENTS.md is made to be machine-readable, keeping README.md as human-readable. It exists at the root of the repository and should cover things like the project overview, code style guidelines, build/test commands, etc.
As always, consider [[20260318091849-ai-context-tokens]]. This file is loaded on every request, so keep it lightweight. As a rule of thumb, it should only contain guidance that is relevant to every single task in the repository.