How to Update System Prompt
Custom system prompts let you guide Claude's behavior for specific tasks. I am exploring using them to make Claude focus on particular aspects like security, algorithm performance, or response constraints depending on what I am working on.
Understanding System Prompts
What they control - System prompts add specific instructions to Claude's default behavior, letting you customize its expertise level and focus areas.
How it works - The --append-system-prompt option adds your custom instructions to Claude's existing helpful behavior rather than replacing it entirely.
Interactive mode support - As of v1.0.51, --append-system-prompt works in interactive mode, not just print mode.
Basic Usage
Print mode with custom focus - Add specific instructions for single-task sessions:
# Add security focus to code review
claude -p "Review this authentication code" --append-system-prompt "After analyzing the code, always check for common security vulnerabilities like SQL injection, XSS, and insecure token handling."
# Database expertise for design tasks
claude -p "Design a user system" --append-system-prompt "You are a database architect with 10 years PostgreSQL experience. Focus on proper normalization, indexing strategies, and query performance. Always consider data integrity and scalability."
Interactive mode with persistent focus - Maintain consistent behavior throughout your coding session:
# Interactive mode with algorithm performance focus
claude --append-system-prompt "When writing code, prioritize algorithm efficiency and performance optimization. Always analyze time and space complexity and suggest performance improvements."
# Start session with security mindset
claude --append-system-prompt "You are a senior backend engineer specializing in secure API development. Prioritize input validation, authentication, and proper error handling. Always include rate limiting and logging."
Practical Development Examples
Code review specialist - You could use this when you need thorough code analysis:
claude --append-system-prompt "You are conducting a senior-level code review. Focus on: 1) Performance bottlenecks, 2) Security vulnerabilities, 3) Code maintainability, 4) Test coverage gaps. Provide specific line-by-line feedback with improvement suggestions."
Legacy code modernization - Helpful when working with older codebases:
claude --append-system-prompt "You are modernizing legacy code. Prioritize backwards compatibility while introducing modern patterns. Always explain migration strategies and potential breaking changes."
When to Use System Prompts
Print mode scenarios:
- Single-task focus - Working on one specific type of problem that needs expert attention
- Code review sessions - Adding consistent review criteria for quality checks
- Specialized analysis - Security audits, performance optimization, or architecture review
Interactive mode scenarios:
- Long coding sessions - Maintaining consistent expertise and focus throughout development
- Performance optimization - Focusing on algorithm efficiency and performance improvements across an entire project
- Learning sessions - Having Claude teach specific concepts or patterns consistently
See Also: Configuration Guide