prompt-file-design

Prompt File Design

Two-part reference for VS Code .prompt.md files: File Format Specification (syntax, variables, runtime behavior) and Design Principles (philosophy, quality tests, anti-patterns). Based on official VS Code documentation, GitHub best practices, and the three-layer agentic model.

Scope boundary: This skill covers .prompt.md file format and design. For prompt content patterns (XML sections, guards), apply prompting-guide. For context management within prompts, apply prompt-context-efficiency.

Part 1: File Format Specification

YAML Frontmatter & Variables

---
name: prompt-name                        # Used after typing / in chat
description: "One-line purpose"          # Shown in slash command menu
agent: "agent-name"                      # Optional: which agent executes
model: "Claude Sonnet 4.5"              # Optional: override default model
tools: ['read', 'search']               # Optional: restrict tools
---

Body Components

Dual-Runtime Compatibility

Prompt files can serve both VS Code Copilot (via agent: routing) and Claude Code (via .claude/commands/ symlink) from a single source file.

Architecture

.github/prompts/plan.prompt.md    ← Source of truth
       ↓ (read by VS Code)               ↓ (symlinked)
VS Code Copilot agent routing     .claude/commands/ → ../.github/prompts/

Runtime Behavior Matrix

Enrichment Pattern

VS Code prompts that only contain ${input:...} are "thin launchers" — they route to an agent that has the full methodology. Claude Code has no agent routing, so thin prompts produce poor results.

Solution: Enrich the body with a compressed summary of the agent's methodology. This serves as additive reinforcement in VS Code and primary instruction in Claude Code.

---
name: plan
agent: "plan"
description: "Create an implementation plan"
---

${input:request:What would you like to plan?}

## Context

You are an **Implementation Planner**. Create detailed, actionable plans without modifying code.

### Key Rules
- {3-5 agent-specific constraints, not generic project rules}
- Read `docs/DOCUMENTATION-GUIDE.md` to find relevant docs
- Use `make` targets — never raw `npm`, `poetry`, `pip`, or `python` commands

### Methodology
1. **{Phase}** — {compressed description}
2. **{Phase}** — {compressed description}
3. **{Phase}** — {compressed description}

### Output
{What to produce — format expectations}

### Skills
Apply these skills from `.github/skills/`: {skill1}, {skill2}

$ARGUMENTS

Key Principles

1. Header unchanged — VS Code frontmatter stays exactly as-is
2. Body = compressed agent — NOT a copy, but enough for standalone operation (~20-30 lines)
3. Both variable systems present — ${input:...} (VS Code) and $ARGUMENTS (Claude Code) coexist harmlessly
4. Single source of truth — .github/prompts/ is canonical; .claude/commands/ is a symlink
5. Still under 50 lines — enriched prompts should stay within the size limit

Symlink Setup

# Directory symlink — zero-maintenance on new prompts
cd .claude && ln -s ../.github/prompts commands

Claude Code invokes as /project:plan.prompt "add auth module". The .prompt suffix in the command name is a cosmetic tradeoff for zero-maintenance directory symlink.

Part 2: Design Principles

For design philosophy, quality tests, prompt types (routing vs task), templates, and anti-patterns, see design-principles.md.

Sources

• VS Code Prompt Files Documentation
• GitHub Blog — Better Prompts
• GitHub Copilot Chat Cookbook