Create File-Based Instructions

This skill helps you scaffold VS Code file-based instructions (.instructions.md files) that provide background knowledge to Copilot about specific parts of the codebase. These files are applied automatically based on glob patterns or semantic matching, giving Copilot domain-specific context when working on matching files.

When Not to Use

• Setting project-wide instructions — use .github/copilot-instructions.md or AGENTS.md instead
• Creating reusable agent workflows with structured steps — use Agent Skills instead
• Adding instructions that need to be invokable on demand — use Agent Skills instead

Workflow

Step 1: Investigate the topic

Build understanding of the area the instructions should cover. Identify:

• ] What files or file patterns the instructions apply to
• ] Key conventions, patterns, or architectural rules for that area
• ] Common pitfalls that Copilot should avoid
• ] Non-obvious domain knowledge that isn't discoverable from code alone

If the scope is unclear or overlaps with existing instructions, ask the user for clarification.

Step 2: Choose the file location

Instructions files go in .github/instructions/ by default. Pick a descriptive filename:

.github/instructions/<topic>.instructions.md

Examples: csharp-style.instructions.md, query-pipeline.instructions.md, test-conventions.instructions.md

Step 3: Generate the file with YAML frontmatter

Create the file with the required YAML frontmatter header:

yaml
---
name: '<Display Name>'              # Optional. Display name in the UI. Defaults to filename.
description: '<Short description>'  # Optional. Shown on hover. Also used for semantic matching.
applyTo: '<glob pattern>'           # Optional. Glob pattern relative to workspace root. Omit to require manual attachment or rely on semantic matching only.
---

Common applyTo patterns:

• **/*.cs — all C# files
• test/** — files under a specific folder

Step 4: Write the body content

Write concise, actionable Markdown content. Follow these principles:

• Be concise — instructions share the context window; only include what the agent wouldn't already know
• Be specific — use concrete rules, not vague guidance
• Include examples — short code snippets showing preferred vs. avoided patterns are very effective
• Explain why — when a rule exists for a non-obvious reason, state it so the agent applies it correctly in edge cases
• Skip linter-enforced rules — don't repeat what formatters and linters already catch
• Use Markdown links to reference specific files or URLs for additional context

Recommended sections (adapt as needed):

1. <Title> — one-line heading describing the domain
2. Context paragraph — brief explanation of what this area is and why these rules matter
3. Guidelines / Conventions — bullet list of concrete rules
4. Examples — short code blocks showing do/don't patterns (optional)
5. Key Files — table of important files for orientation (optional)
6. Common Pitfalls — traps to avoid (optional)

Step 5: Validate

After creating the file, verify:

• ] File is in .github/instructions/ (or a configured instructions folder)
• ] Filename ends with .instructions.md and only contains lowercase letters, numbers, and hyphens
• ] YAML frontmatter is valid
• ] applyTo glob matches the intended files
• ] Content is concise (aim for under 500 lines or 5000 tokens) — long instructions dilute effectiveness
• ] No secrets, tokens, or internal URLs included
• ] Instructions don't duplicate what's already in .github/copilot-instructions.md or under .agents/skills/

Common Pitfalls

| Pitfall | Solution | |---------|----------| | applyTo too broad | Use specific globs; ** applies to every file and wastes context | | Missing applyTo | Without it, instructions won't auto-apply — they require manual attachment or semantic matching via description | | Vague guidance | Replace "write good tests" with something like "add both positive and negative test cases using [ConditionalFact] methods" |