Why This Matters

Built-in slash commands are useful but generic. Custom commands encode your team's conventions and reusable prompts. This is the foundation of team-wide Copilot consistency.

Concept Explained

Custom slash commands are defined via .github/copilot-instructions.md (repo-level) or prompt files (workspace). They appear in the slash command picker like built-ins.

How It Works

Create .github/copilot-instructions.md with system-style instructions. Or create .github/prompts/*.md files for custom commands. Copilot reads them and offers in the picker.

Step-by-Step Tutorial

1. Create instructions file

2. Add conventions

3. Create prompt files

4. Use in Chat

5. Iterate

Real-World Example

A team added .github/copilot-instructions.md with: 'Use TypeScript strict mode. Prefer functional style. No classes. Use early returns.' All Copilot Chat in the repo followed these conventions. Code review consistency improved dramatically.

Example Prompts / Commands / Code

# .github/copilot-instructions.md

You are an engineer at Acme Corp. Follow these conventions:

## Code style
- TypeScript strict mode.
- Prefer functional style; avoid classes.
- Use early returns; max 3 nesting levels.
- All async functions must have explicit return types.
- All public functions must have JSDoc.

## Don'ts
- Don't use lodash (use native methods).
- Don't use try/catch (use Result type from neverthrow).
- Don't use classes (use functions and closures).
- Don't use any (use unknown and narrow).

## Patterns
- Repository pattern for data access.
- Result type for error handling.
- Zod for runtime validation.

Apply these to all code suggestions.

# .github/prompts/security-review.md

Act as a security reviewer.
Check for:
- Input validation
- Authentication/authorization
- Secrets in code
- Unsafe deserialization
- SQL injection
- XSS

Output as numbered list with severity (high/med/low), issue, fix.

Code to review:
$selection

Common Mistakes

• Instructions too long — eats context budget.
• Not committing instructions — team doesn't benefit.
• Not iterating — first version rarely best.
• Confusing instructions (system) with prompt files (commands).

Best Practices

• Keep copilot-instructions.md concise (<500 tokens).
• Commit to repo so team benefits.
• Use prompt files for custom commands.
• Iterate based on output quality.
• Document the layering: instructions (persistent) + prompt files (commands).

Troubleshooting

Practical Exercise

Professional Challenge

Frequently Asked Questions

Further Reading

• Custom Copilot instructions

Official References

• Copilot Chat in the IDE

Related lessons: IN-05, IN-39