Conditional Instruction Management with GitHub Copilot applyTo Patterns¶

Q: What should I check if applyTo is not working?

Verify that chat.instructionsFilesLocations is correctly configured in settings.json, verify that the file extension is .instructions.md, and verify that the glob pattern uses recursive specification like **/*.tsx.

GitHub Copilot Complete Guide

Target Audience

Those who want to use Copilot differently across projects with mixed technology stacks

📍 Position within the Instruction Hierarchy¶

The applyTo pattern serves as the "Path-specific rules" layer within GitHub Copilot's instruction hierarchy.

Layer	File	When Applied
Repository-wide	`.github/copilot-instructions.md`	All requests (always)
Path-specific	`.instructions.md` + `applyTo`	When path matches (automatic)
Agent Skills	`.github/skills/*/SKILL.md`	When agent determines need (on-demand)
AGENTS.md	`AGENTS.md`	On agent startup (shared across multi-tool)

Choosing between applyTo and Agent Skills

applyTo = Automatic application based on file paths. The right rules take effect without developers needing to think about it
Agent Skills = Procedural knowledge loaded on-demand when the agent determines it's needed
Example: React component coding conventions -> applyTo; Terraform plan review procedures -> Skills

For a detailed overview, see the Custom Instructions Complete Guide.

Key Points¶

Conditional Instructions
Automatic rule application based on file extensions and naming patterns
Context Optimization
Focused delivery of only highly relevant information within the 64k token limit
Multi-Technology Stack Support
Optimized development assistance in mixed environments with React/Python/YAML and more
Team Workflow Standardization
Automatic naming conventions and formatting when creating meeting notes, PRs, and specifications

Quick Start (3 Minutes)

// 1. Add to .vscode/settings.json
{
  "chat.instructionsFilesLocations": {
    ".github/instructions/**/*.instructions.md": true
  }
}

// 2. Create .github/instructions/frontend.instructions.md
---
applyTo: "**/*.tsx,**/*.ts"
---

# Frontend Rules
- React Hooks: always specify dependency arrays
- TypeScript strict mode
- Component names: PascalCase

Open a .tsx file and check that the instruction appears in Copilot Chat References.

📖 What Are applyTo Patterns?¶

If the path matches, it takes effect automatically

GitHub Copilot's applyTo pattern is a feature in custom instruction files that applies instructions only to files matching specific conditions.

While traditional custom instructions apply to all files, using applyTo enables:

Dedicated rules by file extension
Automatic formatting by naming pattern
Optimization of context usage

⚙️ Prerequisites: settings.json Configuration¶

Important: Will not work without configuration

To use applyTo patterns, you must enable loading of multiple instruction files in VS Code's settings.json.

Required Settings¶

{
  "chat.instructionsFilesLocations": {
    ".github/instructions/**/*.instructions.md": true
  }
}

Add the configuration as a project setting in .vscode/settings.json to standardize it across the entire team:

{
  "chat.instructionsFilesLocations": {
    ".github/instructions/**/*.instructions.md": true,
    "docs/instructions/**/*.instructions.md": true
  }
}

🏗️ Basic File Structure¶

project-root/
├── .github/
│   ├── copilot-instructions.md         # Base rules (always loaded)
│   └── instructions/
│       ├── frontend.instructions.md    # For React/TypeScript
│       ├── backend.instructions.md     # For Python/API
│       ├── meeting.instructions.md     # For meeting notes
│       └── docs.instructions.md        # For documentation
├── .vscode/
│   └── settings.json                   # Project settings
└── docs/
    └── instructions/
        └── style.instructions.md       # For style guide

YAML Frontmatter Properties¶

The following properties can be used in the YAML Frontmatter at the top of each .instructions.md file.

Property	Required	Description
`description`	Recommended	Summary of the instruction (used by Copilot for loading decisions)
`applyTo`	Yes	Glob pattern(s) for target files (multiple patterns can be separated by commas)
`excludeAgent`	-	Exclude application to specific agents

Application Control with excludeAgent¶

By specifying excludeAgent, you can prevent instructions from being applied to specific agents.

---
description: "テスト実装ガイドライン"
applyTo: "**/*.test.ts,**/*.spec.ts"
excludeAgent: "code-review"
---

excludeAgent Value	Excluded Target
`coding-agent`	GitHub Copilot Coding Agent (automatic Issue processing)
`code-review`	Copilot Code Review

Relationship with Agent Mode

excludeAgent does not apply to Agent Mode (the chat agent within VS Code). excludeAgent targets Coding Agent and Code Review, which operate on GitHub.com.

🔧 Practical Implementation Examples¶

1. Instructions by Technology Stack¶

For Frontend (React/TypeScript)¶

---
description: "React/TypeScript専用のコーディング規則"
applyTo: "**/*.tsx,**/*.ts"
---

# React/TypeScript開発ガイドライン

## 基本ルール
- React Hooksの依存配列を必ず適切に設定
- TypeScriptは strict モードで使用
- コンポーネント名はPascalCase
- カスタムHooksは "use" プレフィックス必須

## インポート順序
1. React関連
2. 外部ライブラリ
3. 内部コンポーネント
4. 型定義
5. スタイル

## 命名規則
- コンポーネントファイル: `ComponentName.tsx`
- カスタムHooks: `useFeatureName.ts`
- 型定義: `types/FeatureName.ts`

For Backend (Python/FastAPI)

---
description: "Python/FastAPI専用のコーディング規則"
applyTo: "**/*.py"
---

# Python/FastAPI開発ガイドライン

## 基本ルール
- Type hints必須
- Black フォーマッター使用
- docstringはGoogle形式
- 例外処理は必ず実装

## ファイル構成
- APIエンドポイント: `routers/feature_name.py`
- データモデル: `models/feature_name.py`
- ビジネスロジック: `services/feature_name.py`

## 命名規則
- 関数名: snake_case / クラス名: PascalCase / 定数: UPPER_SNAKE_CASE

For Meeting Notes

---
description: "会議議事録作成時の自動フォーマット"
applyTo: "meeting-*.md,**/meetings/*.md"
---

# 会議議事録作成ガイドライン

## ファイル命名規則
- 形式: `meeting-YYYY-MM-DD-[会議名].md`

## 必須構成
1. **会議情報** - 日時・参加者・会議種別
2. **議題** - 各議題に番号を付与
3. **決定事項** - 何が決定されたか＋影響範囲
4. **アクションアイテム** - `[ ] タスク（担当者：@username、期限：YYYY-MM-DD）`

For Pull Requests

---
description: "PR作成時の自動テンプレート適用"
applyTo: "pr-*.md,pull_request_template.md"
---

# プルリクエスト作成ガイドライン

## 必須項目
- 📋 変更内容（新機能/バグ修正/リファクタリング/ドキュメント更新）
- 🔍 テスト（単体/結合/手動/エラーケース）
- 📝 チェックリスト（レビュー依頼/CI通過/ドキュメント更新/影響確認）

## ブランチ命名規則
- feature/機能名 / bugfix/バグ修正内容 / hotfix/緊急修正内容

For Technical Specifications

---
description: "技術仕様書作成時の構成とフォーマット"
applyTo: "**/*-spec.md,**/specs/*.md"
---

# 技術仕様書作成ガイドライン

## 必須構成
1. **概要** - 機能の目的・想定ユーザー・成功基準
2. **技術仕様** - アーキテクチャ図・DB設計・API仕様
3. **実装方針** - 使用技術・設計パターン・セキュリティ考慮
4. **テスト仕様** - 単体/結合テスト範囲・性能要件

💡 Strategy for Combining with Default Loading¶

Design Philosophy for the Base Configuration¶

# .github/copilot-instructions.md（基本ルール - 確実読み込み）

## 全体共通ルール
- エラーハンドリング必須
- ログ出力の統一
- セキュリティ原則の遵守

## ファイル作成時の命名規則
- 議事録: `meeting-YYYY-MM-DD-[会議名].md`
- PR: `pr-[機能名]-YYYY-MM-DD.md`
- 仕様書: `[機能名]-spec.md`
- テスト: `[対象].test.[拡張子]`

## 技術スタック別の詳細ルール
フロントエンド: [.github/instructions/frontend.instructions.md]
バックエンド: [.github/instructions/backend.instructions.md]
インフラ: [.github/instructions/infrastructure.instructions.md]

This design achieves the following: - Base rules: Reliably loaded via the default file - Detailed rules: Automatically applied based on conditions via applyTo - Loading reliability: Critical information is always communicated

The window is finite — choose your knowledge wisely

🚀 Benefits of Context Optimization¶

Before (No applyTo)After (With applyTo)

Loading all instructions:   15,000 tokens
Actually relevant info:      3,000 tokens
Efficiency:                     20%

Loading matched instructions: 4,000 tokens
Actually relevant info:       3,500 tokens
Efficiency:                    87.5%

1. Optimized Token Usage¶

Traditional approach:
- Loading all instructions: 15,000 tokens
- Actually relevant information: 3,000 tokens
- Efficiency: 20%

applyTo approach:
- Loading only matching instructions: 4,000 tokens
- Actually relevant information: 3,500 tokens
- Efficiency: 87.5%

2. Improved Response Accuracy¶

Because conditional branching ensures that only highly relevant information is included in Copilot's context:

Reduced noise from irrelevant information
Suggestions specialized to specific technology stacks
More concrete and practical code generation

3. Reduced Cognitive Load¶

Depending on the file being worked on, developers benefit from: - Appropriate rules being applied automatically - No need to think about "which rules should I follow right now" - Minimal interruption to the development flow

⚠️ Operational Considerations¶

1. Common Configuration Mistakes¶

// ❌ Missing extension
"chat.instructionsFilesLocations": {
    ".github/instructions/**/*.md": true
}

// ✅ Correct extension
"chat.instructionsFilesLocations": {
    ".github/instructions/**/*.instructions.md": true
}

2. Mistakes in applyTo Pattern Syntax¶

# ❌ Incorrect syntax
applyTo: "*.tsx"

# ✅ Correct syntax
applyTo: "**/*.tsx"

3. Over-Segmentation Leading to Management Complexity¶

❌ Structure to avoid:
├── instructions/
│   ├── react-hooks.instructions.md
│   ├── react-context.instructions.md
│   ├── react-components.instructions.md
│   └── react-routing.instructions.md

✅ Recommended structure:
├── instructions/
│   ├── frontend.instructions.md
│   ├── backend.instructions.md
│   └── docs.instructions.md

🔍 How to Measure Effectiveness¶

1. Reference Verification¶

Check the "References" shown in Copilot Chat responses to verify: - Whether the appropriate instruction files are being referenced - Whether unnecessary files are being loaded

2. Response Quality Assessment¶

Periodically verify the following: - Whether code generation matches expectations - Whether technology stack-specific suggestions are included - Whether naming conventions are being applied correctly

3. Team Feedback¶

Survey developers on their experience: - Perceived improvement in development efficiency - Appropriateness of suggestions - Ease of configuration

❓ Frequently Asked Questions (FAQ)¶

Q: Can I specify multiple extensions in an applyTo pattern?

Yes. You can specify multiple patterns separated by commas. Example: applyTo: "**/*.tsx,**/*.ts,**/*.jsx". Each pattern is evaluated as an OR condition.

Q: Can applyTo be used alongside copilot-instructions.md?

Yes. copilot-instructions.md serves as the always-applied base rules, while applyTo provides conditional detailed rules. Both sets of instructions are included in the context.