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Egendefinerte systemprompter

English | 简体中文 | 繁體中文 | 日本語 | 한국어 | हिन्दी | Tiếng Việt | Français | Русский | Español | Português | Norsk | Svenska | Deutsch | Nederlands | Italiano

Denne guiden forklarer hvordan du tilpasser systemprompten som GAC bruker for å generere commit-meldinger, slik at du kan definere din egen commit-meldingsstil og konvensjoner.

Table of Contents

What Are System Prompts?

GAC uses two prompts when generating commit messages:

  1. System Prompt (customizable): Instructions that define the role, style, and conventions for commit messages
  2. User Prompt (automatic): The git diff data showing what changed

The system prompt tells the AI how to write commit messages, while the user prompt provides the what (the actual code changes).

Why Use Custom System Prompts?

You might want a custom system prompt if:

  • Your team uses a different commit message style than conventional commits
  • You prefer emojis, prefixes, or other custom formats
  • You want more or less detail in commit messages
  • You have company-specific guidelines or templates
  • You want to match your team's voice and tone
  • You want commit messages in a different language (see Language Configuration below)

Quick Start

  1. Create your custom system prompt file:

    # Copy the example as a starting point
cp custom_system_prompt.example.txt ~/.config/gac/my_system_prompt.txt

# Or create your own from scratch
vim ~/.config/gac/my_system_prompt.txt

  2. Add to your .gac.env file:

    # In ~/.gac.env or project-level .gac.env
GAC_SYSTEM_PROMPT_PATH=/path/to/your/custom_system_prompt.txt

  3. Test it:

    uvx gac --dry-run

That's it! GAC will now use your custom instructions instead of the default.

Writing Your Custom System Prompt

Your custom system prompt can be plain text—no special format or XML tags required. Just write clear instructions for how the AI should generate commit messages.

Key things to include:

  1. Role definition - What the AI should act as
  2. Format requirements - Structure, length, style
  3. Examples - Show what good commit messages look like
  4. Constraints - What to avoid or requirements to meet

Example structure:

You are a commit message writer for [your project/team].

When analyzing code changes, create a commit message that:

1. [First requirement]
2. [Second requirement]
3. [Third requirement]

Example format:
[Show an example commit message]

Your entire response will be used directly as the commit message.

Examples

Emoji-Based Commit Style

See custom_system_prompt.example.txt for a complete emoji-based example.

Quick snippet:

You are a commit message writer that uses emojis and a friendly tone.

Start each message with an emoji:
- 🎉 for new features
- 🐛 for bug fixes
- 📝 for documentation
- ♻️ for refactoring

Keep the first line under 72 characters and explain WHY the change matters.

Team-Specific Conventions

You are writing commit messages for an enterprise banking application.

Requirements:
1. Start with a JIRA ticket number in brackets (e.g., [BANK-1234])
2. Use formal, professional tone
3. Include security implications if relevant
4. Reference any compliance requirements (PCI-DSS, SOC2, etc.)
5. Keep messages concise but complete

Format:
[TICKET-123] Brief summary of change

Detailed explanation of what changed and why. Include:
- Business justification
- Technical approach
- Risk assessment (if applicable)

Example:
[BANK-1234] Implement rate limiting for login endpoints

Added Redis-based rate limiting to prevent brute force attacks.
Limits login attempts to 5 per IP per 15 minutes.
Complies with SOC2 security requirements for access control.

Detailed Technical Style

You are a technical commit message writer who creates comprehensive documentation.

For each commit, provide:

1. A clear, descriptive title (under 72 characters)
2. A blank line
3. WHAT: What was changed (2-3 sentences)
4. WHY: Why the change was necessary (2-3 sentences)
5. HOW: Technical approach or key implementation details
6. IMPACT: Files/components affected and potential side effects

Use technical precision. Reference specific functions, classes, and modules.
Use present tense and active voice.

Example:
Refactor authentication middleware to use dependency injection

WHAT: Replaced global auth state with injectable AuthService. Updated
all route handlers to accept AuthService through constructor injection.

WHY: Global state made testing difficult and created hidden dependencies.
Dependency injection improves testability and makes dependencies explicit.

HOW: Created AuthService interface, implemented JWTAuthService and
MockAuthService. Modified route handler constructors to require AuthService.
Updated dependency injection container configuration.

IMPACT: Affects all authenticated routes. No behavior changes for users.
Tests now run 3x faster with MockAuthService. Migration required for
routes/auth.ts, routes/api.ts, and routes/admin.ts.

Best Practices

Do

  • ✅ Be specific - Clear instructions produce better results
  • ✅ Include examples - Show the AI what good looks like
  • ✅ Test iteratively - Try your prompt, refine based on results
  • ✅ Keep it focused - Too many rules can confuse the AI
  • ✅ Use consistent terminology - Stick to the same terms throughout
  • ✅ End with a reminder - Reinforce that the response will be used as-is

Don't

  • ❌ Use XML tags - Plain text works best (unless you specifically want that structure)
  • ❌ Make it too long - Aim for 200-500 words of instructions
  • ❌ Contradict yourself - Be consistent in your requirements
  • ❌ Forget the ending - Always remind: "Your entire response will be used directly as the commit message"

Tips

  • Start with the example - Copy ../../examples/custom_system_prompt.example.txt and modify it
  • Test with --dry-run - See the result without making a commit
  • Use --show-prompt - See what's being sent to the AI
  • Iterate based on results - If messages aren't quite right, adjust your instructions
  • Version control your prompt - Keep your custom prompt in your team's repo
  • Project-specific prompts - Use project-level .gac.env for project-specific styles

Troubleshooting

Messages still have "chore:" prefix

Problem: Your custom emoji messages are getting "chore:" added.

Solution: This shouldn't happen—GAC automatically disables conventional commit enforcement when using custom system prompts. If you see this, please file an issue.

AI ignoring my instructions

Problem: Generated messages don't follow your custom format.

Solution:

  1. Make your instructions more explicit and specific
  2. Add clear examples of the desired format
  3. End with: "Your entire response will be used directly as the commit message"
  4. Reduce the number of requirements—too many can confuse the AI
  5. Try using a different model (some follow instructions better than others)

Messages are too long/short

Problem: Generated messages don't match your length requirements.

Solution:

  • Be explicit about length (e.g., "Keep messages under 50 characters")
  • Show examples of the exact length you want
  • Consider using --one-liner flag as well for short messages

Custom prompt not being used

Problem: GAC still uses default commit format.

Solution:

  1. Check that GAC_SYSTEM_PROMPT_PATH is set correctly:

    uvx gac config get GAC_SYSTEM_PROMPT_PATH

  2. Verify the file path exists and is readable:

    cat "$GAC_SYSTEM_PROMPT_PATH"

  3. Check .gac.env files in this order:

    • Project level: ./.gac.env
    • User level: ~/.gac.env

  4. Try an absolute path instead of relative path

Language Configuration

Note: You don't need a custom system prompt to change the commit message language!

If you only want to change the language of your commit messages (while keeping the standard conventional commit format), use the interactive language selector:

uvx gac language

This will present an interactive menu with 25+ languages in their native scripts (Español, Français, 日本語, etc.). Select your preferred language, and it will automatically set GAC_LANGUAGE in your ~/.gac.env file.

Alternatively, you can manually set the language:

# In ~/.gac.env or project-level .gac.env
GAC_LANGUAGE=Spanish

By default, conventional commit prefixes (feat:, fix:, etc.) remain in English for compatibility with changelog tools and CI/CD pipelines, while all other text is in your specified language.

Want to translate prefixes too? Set GAC_TRANSLATE_PREFIXES=true in your .gac.env for full localization:

GAC_LANGUAGE=Spanish
GAC_TRANSLATE_PREFIXES=true

This will translate everything, including prefixes (e.g., corrección: instead of fix:).

This is simpler than creating a custom system prompt if language is your only customization need.

Want to switch back to default

Problem: Want to temporarily use default prompts.

Solution:

# Option 1: Unset the environment variable
uvx gac config unset GAC_SYSTEM_PROMPT_PATH

# Option 2: Comment it out in .gac.env
# GAC_SYSTEM_PROMPT_PATH=/path/to/custom_prompt.txt

# Option 3: Use a different .gac.env for specific projects

Related Documentation

Need Help?

  • Report issues: GitHub Issues
  • Share your custom prompts: Contributions welcome!