# 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:**
```bash
yoyo --system "You are a Rust expert. Focus on performance and safety."
```
**From a file:**
```bash
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
```text
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:
```bash
yoyo --system-file review-prompt.txt -p "review src/main.rs"
```