Why This Matters

Copilot Chat doesn't have a separate system-prompt slot, but you can emulate one. This gives you persistent instructions across turns without re-stating them — useful for complex tasks with consistent conventions.

Concept Explained

A system-style prompt is a prefix block at the start of a thread that sets persistent instructions: 'You are a senior TypeScript engineer. Follow these conventions: ...'. Subsequent turns inherit these instructions.

How It Works

Start a new thread. First message: 'SYSTEM: You are [role]. Follow these rules: [list]. Apply these to all subsequent messages.' Then ask your actual question. The 'SYSTEM:' framing biases the model to treat it as persistent context.

Step-by-Step Tutorial

1. Start new thread

2. Write system prefix

3. Ask your question

4. Verify persistence

5. Reset when needed

Real-World Example

A team used system-style prompts for code review threads. First message: 'SYSTEM: You are a security reviewer. Check for: input validation, auth, secrets in code, unsafe deserialization. Output as numbered list with severity.' Every review in that thread followed the format consistently — no need to re-state per review.

Example Prompts / Commands / Code

SYSTEM: You are a senior TypeScript engineer reviewing code for production readiness.

Rules:
1. Flag any `any` types.
2. Flag missing error handling.
3. Flag direct DOM manipulation in React components.
4. Suggest improvements as numbered list.
5. For each item: severity (high/med/low), issue, suggested fix.

Apply these rules to all subsequent code I paste.

---

Now review this function:
[paste function]

# .github/copilot-instructions.md (repo-level system prompt):

You are an engineer at Acme Corp. Follow these conventions:
- Use TypeScript strict mode.
- Prefer functional style; avoid classes unless stateful.
- All async functions must have explicit return types.
- All public functions must have JSDoc.

# Copilot reads this automatically for all Chat in this repo.
# Combine with per-thread SYSTEM: prefix for additional rules.

Common Mistakes

• System prefix too long — eats context budget.
• Not starting a new thread — prior context conflicts.
• Forgetting to verify persistence — rules drift silently.
• Mixing system rules with task instructions — confusing.

Best Practices

• Start new thread for system-style prompts.
• Keep system prefix concise (<200 tokens).
• Use 'SYSTEM:' label for clarity.
• Verify rules persist across turns.
• Combine with .github/copilot-instructions.md for repo-wide rules.

Troubleshooting

Practical Exercise

Professional Challenge

Frequently Asked Questions

Further Reading

• Custom instructions for Copilot

Official References

• Copilot Chat in the IDE

Related lessons: IN-04, IN-39