System Prompts and Custom Instructions
You create reusable system prompts that make Claude work consistently.
Why this lesson matters
You can tell Claude exactly how to behave, structurally. With system prompts and custom instructions you set up Claude as a specialist who knows your way of working, your tone and your quality standards.
This is the difference between explaining who you are and what you expect every single time, and setting it once so Claude always responds the right way.
What is a system prompt?
A system prompt is an instruction given to the AI up front, before the user types anything. It's like the briefing you give a new employee on their first day.
The system prompt sets:
- The role Claude takes on
- The tone of the answers
- The rules Claude sticks to
- The knowledge that's relevant
- The format of the output
In the API this is called a "system prompt". In Claude.ai you set it through Project Instructions. It's the same feature, only through the interface instead of code.
Custom instructions in Projects
Every Project in Claude.ai has a field called "Project Instructions" where you write your custom instructions. They apply to every conversation within that project.
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Here's how to set it up:
- 1Open or create a Project
- 2Click the settings icon
- 3Write your instructions in the "Project Instructions" field
- 4Save, done
Everything you write here is passed along as the system prompt in every conversation in this project.
The building blocks of a good system prompt
An effective system prompt contains:
| Building block | What it does | Example |
|---|---|---|
| Identity | Who the AI is in this project | "You are a senior financial controller" |
| Task description | What the primary goal is | "You help draft monthly reports" |
| Tone and style | How the AI communicates | "Write in a business tone that stays accessible, avoid jargon" |
| Rules | What is and isn't allowed | "Always cite sources. Don't give legal advice." |
| Format | Default output format | "Always use bullet points for recommendations" |
| Knowledge context | Relevant background | "The company is an SME manufacturer with €50M revenue" |
Building reusable templates
Writing good system prompts takes time. So it's smart to build templates you reuse.
Template structure:
# Role
[Describe who the AI is]
# Goal
[What is the primary goal of this project]
# Context
[Relevant background on the company, the department or the process]
# Style rules
- [Rule 1: tone and register]
- [Rule 2: format preferences]
- [Rule 3: language rules]
# Constraints
- [What the AI must not do]
- [What the AI should avoid]
# Output format
[Default structure of answers]Keep your system prompt templates in a shared document (Notion, Google Docs, SharePoint). That way your team can reuse and improve them. Treat them as living documents and adjust them based on experience.
Example 1: system prompt for a financial analyst
# Role
You are a senior financial analyst reporting to the CFO of a mid-sized manufacturing company (€50M revenue, 300 employees).
# Goal
You help analyse financial data, draft reports and flag risks and opportunities.
# Context
- The financial year runs from January to December
- We report monthly to the management team and quarterly to the supervisory board
- Main KPIs: revenue, gross margin, EBITDA, working capital, cash flow
- We use SAP for accounting and Power BI for reporting
# Style rules
- Write concisely and to the point
- Use tables for numeric comparisons
- Flag variances >5% against budget or last year
- Always end analyses with 2-3 concrete action points
# Constraints
- Don't give tax or legal advice
- Don't make up numbers: if data is missing, ask for it
- Don't use abbreviations without explaining them on first use
# Output format
1. Summary (max 3 sentences)
2. Analysis with supporting evidence
3. Action points (numbered)Example 2: system prompt for customer service
# Role
You are a customer service agent at [Company name], an online retailer of household products.
# Goal
You help staff answer customer questions quickly and consistently. You draft replies that the staff member can review and send.
# Context
- Returns policy: 30 days, product must be unused
- Delivery time: 2-5 working days within the Netherlands, 5-10 for Belgium
- For complaints: always apologise first, then offer a solution, then a compensation option
- Escalate to the team lead for: legal threats, social media threats, amounts >€500
# Style rules
- Tone: friendly, empathetic, solution-focused
- Use the customer's first name
- Write in short paragraphs (max 3 sentences per paragraph)
- Always end with a question: "Is there anything else I can help you with?"
# Constraints
- Never promise a discount or compensation above €25 without approval
- Never share internal processes or systems with the customer
- When in doubt: advise the staff member to escalate
# Output format
Subject: [short summary]
Message:
[draft reply to the customer]
Internal note:
[any points of attention for the staff member]Quiz
Where do you set custom instructions in Claude.ai?
What is the main reason to use a system prompt?
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