Rubber Duck Thursdays - The one with custom instructions
This episode dives deep into Copilot custom instructions — what they are, how to write them, and why they matter for guiding AI-assisted development. Chris reviews the GitHub Changelog and then live codes a higher/lower game for a Go-based GitHub CLI extension using Agent Mode with detailed custom instructions.
Changelog Highlights
- Copilot Code Review now supports C, C++, Kotlin, Swift, and more
- CodeQL improves JavaScript and Ruby analysis in version 2.21.1
- GitHub Models playground adds a prompt improvement feature to refine prompts with AI
- Copilot Edits for JetBrains IDEs is now generally available
- Credential revocation API for exposed tokens is now GA — supports bulk revocation via REST API
- Task lists retirement on April 30th — code scanning alert references deprecated
- Mistral Small 3.1 replaces Mistral Small in GitHub Models
- Actions Runner Controller now supports running Dependabot on self-hosted Kubernetes runners
Live Coding — Custom Instructions Deep Dive
- Explored how Copilot custom instructions provide repository context, coding standards, and structural guidance to every Copilot request
- Demonstrated writing detailed instructions for a Go-based GitHub CLI extension — specifying test commands, linting rules, folder structure, and game design patterns using the Cobra library
- Live coded a higher/lower guessing game using Copilot Agent Mode with Claude 3.7 Sonnet, showcasing how rich instructions lead to higher quality scaffolded code
- Showed the Copilot Airways app built with Agent Mode from a screenshot, demonstrating vision capabilities and custom instruction-guided development
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