System Prompts and Personas
80 minutes
Module 05
Tier II Begins
System Prompts
and Personas
From per-message instructions to persistent identity.
Charter Oak Strategic Partners
In Tier I, every prompt stood on its own.
System prompts change that. You write instructions once. Claude follows them for the entire conversation. Every message benefits.
A job description for Claude.
A job description tells an employee how to operate. What to prioritize. What voice to use. What standards to meet. A system prompt does the same thing. Write it once. It works forever.
“Be professional”
is a wish.
Specificity is an instruction.
Four sections.
Each one sharpens the output.
Identity
Who Claude is in this context. A senior financial analyst produces different work than a marketing coordinator.
Voice
How Claude should sound. Short sentences or long. Direct or diplomatic. Data-driven or narrative.
Constraints
What Claude should never do. Ban specific words by name. Set budget thresholds. Define escalation rules.
Output Standards
What finished work looks like. Tables for comparisons. Clear next steps. Word limits. Headers on every section.
A complete system prompt.
You are the senior communications director at Greenfield Manufacturing, reporting to the CEO. 15 years of experience in B2B industrial communications.
Short sentences. Active voice only. No corporate jargon. Confident and direct, like a founder talking to investors.
Never use the words "synergy," "leverage," "innovative," or "cutting-edge." Never open with a question. Maximum one exclamation mark per document.
Every deliverable: one-sentence summary at top, bullet points for key data, clear next action at bottom. Maximum 500 words unless specified.
The Difference
“Avoid jargon.”
“Be clear.”
“Write well.”
Claude guesses. Results vary.
“Never use: synergy, leverage, alignment, circle back, deep dive.”
“Short sentences for facts. Longer for explanation. End with an action item.”
Claude follows a blueprint. Consistent quality.
One concept. Three places it lives.
Claude Projects
Custom Instructions field
Available on claude.ai. Persists across every conversation in that project. The most accessible starting point.
Cowork
Inside skills
System prompts power every Cowork skill. Write the prompt once. The skill runs it automatically.
API
System parameter
Sent with every API request. The standard pattern for building Claude into products and workflows.
Live Demo
Voice Matching.
One paragraph from a fictional company. Build a system prompt that captures the voice. Test it live. Iterate until it sounds right.
Three Fictional Companies
Ironclad Industrial
Direct, no-nonsense. Short sentences. Blue-collar vocabulary. Confidence without flash. Sounds like a plant manager who respects your time.
Verdant Health
Warm, knowledgeable, approachable. Longer sentences. Empathetic tone. Scientific credibility with accessible language. Sounds like a trusted doctor who listens.
Vex Cybersecurity
Sharp, technical, punchy cadence. Industry-specific terminology used without apology. Sounds like a security analyst briefing the board after a near-miss.
Content describes
what to write about.
Voice describes how to write.
Sentence length. Vocabulary level. Emotional temperature. These are the levers a system prompt controls.
The Style Match
Three groups. Each gets one company. Read the samples. Identify the voice. Build a system prompt that captures it.
Generate new content with your prompt. Another group guesses which company voice it matches.
If the other groups recognize the voice, your system prompt is working.
Start Your Own
Your organization. Your role. What should Claude sound like when it writes for your team?
Start with identity and voice. Add constraints as they come to mind. You will refine this over time and bring it to the Tier II capstone in Module 09.
Debrief
The specificity of your system prompt determines the quality of every response that follows.
“Be professional” produces nothing useful. “Never use the word synergy, always lead with data, end every response with a specific next step” produces consistently better work.
Claude has an identity now. Next: teach it to show its reasoning.
Module 06 covers chain-of-thought and extended thinking. When to ask for the answer. When to ask for the reasoning behind the answer.