agent-customization

Create a .instructions.md file in .github/instructions/ with a YAML frontmatter containing a keyword-rich description and an optional applyTo glob pattern. The body holds concise, actionable guidelines loaded when relevant files or tasks match.

Both appear as slash commands in chat. A prompt is a single focused task with parameterized inputs, while a skill is a multi-step workflow bundled with scripts, references, and assets in its own folder with a SKILL.md entry point.

Silent failures usually come from YAML frontmatter errors like unescaped colons, tabs instead of spaces, or a name that mismatches the folder. Also check that the description contains trigger keywords, since discovery depends on it.

Use hooks when behavior must be enforced deterministically, such as blocking dangerous commands or auto-formatting after edits. Instructions only guide agent behavior non-deterministically, while hooks run shell commands at lifecycle events like PreToolUse.

Use only one, not both. copilot-instructions.md in .github/ is recommended for project-wide standards, while AGENTS.md is an open standard supporting monorepo hierarchies with files in root or subfolders.