docs/data/reusables/copilot/custom-instructions-reference.md

Custom instructions are not visible in the Chat view or inline chat, but you can verify that they are being used by {% data variables.product.prodname_copilot_short %} by looking at the References list of a response in the Chat view. If custom instructions were added to the prompt that was sent to the model, the .github/copilot-instructions.md file is listed as a reference. You can click the reference to open the file.