# System Prompts
yoyo has a built-in system prompt that instructs the model to act as a coding assistant. You can override it entirely via CLI flags or config file.
## 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 (CLI flag):**
```bash
yoyo --system "You are a Rust expert. Focus on performance and safety."
```
**From a file (CLI flag):**
```bash
yoyo --system-file my-prompt.txt
```
**In config file (`.yoyo.toml`):**
```toml
# Inline text
system_prompt = "You are a Go expert. Follow Go idioms strictly."
# Or read from a file
system_file = "prompts/system.txt"
```
If both `system_prompt` and `system_file` are set in the config, `system_file` takes precedence (same as CLI behavior).
## Precedence
When multiple sources provide a system prompt, the highest-priority one wins:
1. `--system-file` (CLI flag) — highest priority
2. `--system` (CLI flag)
3. `system_file` (config file key)
4. `system_prompt` (config file key)
5. Built-in default — lowest priority
This means CLI flags always override config file values, and file-based prompts override inline text at each level.
## 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
- **Team defaults** — commit `.yoyo.toml` with `system_prompt` or `system_file` so every developer gets the same agent persona
- **Persona tuning** — make the agent more or less verbose, formal, etc.
## Viewing the assembled prompt
To see the full system prompt (including project context, repo map, skills, and any overrides), use:
```bash
yoyo --print-system-prompt
```
This prints the complete prompt to stdout and exits — useful for debugging or understanding exactly what context the model receives. It works with other flags:
```bash
# See what the prompt looks like with a custom system prompt
yoyo --system "You are a Rust expert" --print-system-prompt
# See the prompt without project context
yoyo --no-project-context --print-system-prompt
```
### Inspecting during a session
Once inside the REPL, use `/context system` to see the system prompt broken into sections with approximate token counts for each:
```
/context system
```
This shows each markdown section (headers like `# ...` and `## ...`), their line counts, estimated token usage, and a brief preview — without leaving the session.
## Automatic project context
In addition to the system prompt, yoyo automatically injects project context when available:
- **Project instructions** — from `YOYO.md` (primary), `CLAUDE.md` (compatibility alias), or `.yoyo/instructions.md`
- **Project file listing** — from `git ls-files` (up to 200 files)
- **Recently changed files** — from `git log` (up to 20 files)
- **Git status** — current branch, count of uncommitted and staged changes
- **Project memories** — from `memory/` files if present
Use `/context` to see which project context files are loaded.
## 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
# Via CLI flag
yoyo --system-file review-prompt.txt -p "review src/main.rs"
```
Or set it in your project's `.yoyo.toml`:
```toml
system_file = "review-prompt.txt"
```