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Design effective system prompts for custom agents. Use when creating agent system prompts, defining agent identity and rules, or designing high-impact prompts that shape agent behavior.
npx skill4agent add melodic-software/claude-code-plugins system-prompt-engineering| Aspect | System Prompt | User Prompt |
|---|---|---|
| Scope | All conversations | Single task |
| Persistence | Affects everything | Per-request |
| Impact | Orders of magnitude higher | Lower blast radius |
| Sections | Purpose, Instructions, Examples | All sections |
## Purpose
You are a security expert specializing in code review.
Your role is to identify vulnerabilities and suggest fixes.
You focus on OWASP Top 10 and secure coding practices.## Instructions
- Focus only on security concerns
- Always explain the reasoning behind findings
- Prioritize issues by severity (Critical, High, Medium, Low)
- Do not modify code without explicit permission
- When uncertain, ask clarifying questions## Examples
### Example 1: SQL Injection Finding
**Input:** "Review this login function"
**Output:** "Found SQL injection vulnerability at line 42. The user input is directly concatenated into the query string. Recommendation: Use parameterized queries instead."
### Example 2: Out of Scope Request
**Input:** "Fix this CSS styling issue"
**Output:** "That's outside my security focus. I specialize in security vulnerabilities, not styling. Consider consulting a frontend expert for CSS issues."
### Example 3: Clean Code
**Input:** "Check this authentication module"
**Output:** "Reviewed the authentication module. No security vulnerabilities found. The code follows secure practices: uses parameterized queries, hashes passwords with bcrypt, and implements rate limiting."| Avoid | Why | Instead |
|---|---|---|
| Detailed workflows | Reduces autonomy | High-level guidelines |
| Dynamic variables | System prompt is static | Use user prompts |
| Prescriptive formats | Over-constrains | Flexible guidelines |
| Everything "just in case" | Context bloat | Only essentials |
---
name: agent-name
description: When to use this agent (for auto-delegation)
tools: [minimal tool set]
model: sonnet
color: blue
---
# Agent Name
## Purpose
[Identity and role definition]
## Instructions
[Rules and constraints]
## Examples
### Example 1: [Scenario]
**Input:** [typical input]
**Output:** [ideal output]
### Example 2: [Edge Case]
**Input:** [edge case input]
**Output:** [handling output]
### Example 3: [Boundary]
**Input:** [out-of-scope request]
**Output:** [how to decline/redirect]## System Prompt Design
**Agent Name:** [name]
**Domain:** [expertise area]
**Model:** [sonnet/opus/haiku]
### Purpose
[2-3 sentences defining identity]
### Instructions
- [rule 1]
- [rule 2]
- [rule 3]
### Examples
**Example 1:** [scenario]
- Input: [input]
- Output: [output]
**Example 2:** [scenario]
- Input: [input]
- Output: [output]
### Validation
- [ ] Purpose is specific
- [ ] Instructions are actionable
- [ ] Examples are diverse
- [ ] Boundaries are clear## Purpose
You are an expert in [domain] with deep knowledge of [specifics].## Instructions
- Validate all inputs against [criteria]
- Block requests that [conditions]
- Log suspicious activity## Examples
### Input Format
[format A]
### Output Format
[format B]"System prompts are orders of magnitude more important than user prompts. They run once and affect everything."