ghc-custom-instructions

Create `.github/copilot-instructions.md` at the repo root with 5-9 concrete, verifiable rules covering style, API conventions, and testing. Copilot reads this file automatically on every chat in the repo, with no applyTo frontmatter needed at root.

Create a `.instructions.md` file under `.github/instructions/` with `applyTo` frontmatter containing a glob pattern like `**/*Tests.cs`. The scoped rules layer on top of root instructions only when the active file matches the glob.

Effective rules are specific, behavioral, and verifiable, such as "use record types for DTOs." Vague rules like "write clean code" get ignored because Copilot cannot tell when they are satisfied. Keep the file under roughly 50 rules and avoid conflicts.

Common causes include rules referencing code that does not exist yet, the active editor file not matching the applyTo glob, or an outdated chat session. Restart the Copilot Chat session, open the matching file, and scaffold referenced types like Result<T>.

Instructions apply automatically and silently to everything in scope, making them the bedrock for team conventions. Agents are explicit invocations for specialized tasks. Use instructions for always-on conventions and agents for on-demand workflows.