System Prompts

yoyo has a built-in system prompt that instructs the model to act as a coding assistant. You can override it entirely.

Default behavior

The default system prompt tells the model to:

• Work as a coding assistant in the user’s terminal
• Be direct and concise
• Use tools proactively (read files, run commands, verify work)
• Do things rather than just explain how

Custom system prompt

Inline:

yoyo --system "You are a Rust expert. Focus on performance and safety."

From a file:

yoyo --system-file my-prompt.txt

If both --system and --system-file are provided, --system-file takes precedence.

Use cases

Custom system prompts are useful for:

• Specializing the agent — focus on security review, documentation, or a specific language
• Project context — tell the agent about your project’s conventions
• Persona tuning — make the agent more or less verbose, formal, etc.

Example prompt file

You are a senior Rust developer reviewing code for a production system.
Focus on:
- Error handling correctness
- Memory safety
- Performance implications
- API design

Be concise. Point out issues with line numbers.

Save as review-prompt.txt and use:

yoyo --system-file review-prompt.txt -p "review src/main.rs"