Student Guide
Module 05
System Prompts and Personas
Persistent system instructions that define Claude's identity, voice, constraints, and output standards. The difference between "be professional" and a system prompt that specifies banned words and required formats is the difference between generic output and consistently excellent work.
Charter Oak Strategic Partners · Claude Mastery Program
- What a system prompt is and why it changes everything
- The four sections of a system prompt: Identity, Voice, Constraints, Output Standards
- How to match a brand voice from writing samples
- How to build a system prompt for your organization
- The difference between content instructions and voice instructions
From Per-Message to Persistent
In Tier I, every prompt you wrote stood on its own. A system prompt changes that. It is a set of instructions Claude reads before you say anything — defining its role, voice, rules, and output standards. Every message you send benefits from those instructions. You write them once. They work forever.
Think of it as hiring someone. A job description does not tell the employee what to do each day. It tells them how to operate. System prompts work the same way.
System Prompt Anatomy
| Section | What It Controls | Example |
|---|---|---|
| Identity | Who Claude is | "You are a senior financial analyst specializing in manufacturing companies" |
| Voice | How it writes | "Direct, data-driven, no jargon, every statement backed by a number" |
| Constraints | What it avoids | "Never use the words synergy, leverage, or alignment" |
| Output Standards | What the result looks like | "End every response with a concrete next step. Use tables for comparisons" |
“Be professional” does nothing. “Never use the word synergy, always lead with data, and end every response with a specific next step” produces consistently better work. The specificity of your system prompt determines the quality of every response that follows.
The Three Fictional Companies
Direct, no-nonsense Midwest manufacturing. Short sentences. Blue-collar vocabulary. Confidence without flash. Sounds like a plant manager who respects your time.
Warm, knowledgeable, approachable wellness. Longer sentences. Empathetic tone. Scientific credibility with accessible language. Sounds like a trusted doctor who is also a good listener.
Sharp, technical, slightly aggressive. Punchy cadence. Industry-specific terminology used without apology. Sounds like a security analyst briefing a board after a near-miss.
Content describes what to write about: “You are an expert in cybersecurity.”
Voice describes how to write: “You write in punchy, three-to-five word sentences. You use technical terms without explanation. Your tone is that of someone who has seen the breach and is telling the board what to do next.”
Content follows naturally from the conversation. Voice must be defined in the system prompt.
Exercise: The Style Match
Instructions: Your group is assigned one of the three companies. Read all three writing samples for your company. Then:
- Identify the voice characteristics (sentence length, vocabulary, formality, emotional temperature)
- Write a system prompt that captures that voice
- Test it by generating new content the original company would publish
The Test: Groups swap outputs. Can the other groups identify which company voice your system prompt was supposed to match? If your output could pass for the original, your system prompt is working.
Three questions. Score each yes or no:
- Does it sound like the same company?
- Would it fit on that company’s website without edits?
- Does it avoid generic AI voice?
Two or three yeses = effective system prompt. Zero or one = needs more specific constraints.
Exercise: Start Your Own
Build a system prompt for your organization. Your role. What should Claude sound like when it writes for you or your team?
- Start with Identity and Voice
- Add Constraints as they come to mind
- Define Output Standards
You will not finish in ten minutes. The goal is to start. You will refine this over time and bring it to the Tier II capstone in Module 09.
Reflection
Questions to Consider
- What is the most common writing task on your team? What would the ideal system prompt for that task include?
- Which of the four system prompt sections (Identity, Voice, Constraints, Output Standards) would make the biggest difference for your work?
- Where does your team’s writing currently lack consistency? Could a shared system prompt fix it?
System Prompt
Instructions Claude reads before every message: Identity, Voice, Constraints, Output Standards.
The Key Insight
“Be professional” does nothing. Specific bans, formats, and standards produce consistent quality.
Content vs. Voice
Content describes what to write about. Voice describes how to write. Voice must be defined in the system prompt.
Where System Prompts Live
Claude Projects: Custom Instructions. Cowork: inside skills. API: system parameter with every request.