---
name: agent-development
description: This skill should be used when the user asks to "create an agent", "add an agent", "write a subagent", "agent frontmatter", "when to use description", "agent examples", "agent tools", "agent colors", "autonomous agent", or needs guidance on agent structure, system prompts, triggering conditions, or agent development best practices for Google Antigravity plugins.
---

# Agent Development for Google Antigravity Plugins

## Overview

Agents are autonomous subprocesses that handle complex, multi-step tasks independently. Understanding agent structure, triggering conditions, and system prompt design enables creating powerful autonomous capabilities.

**Key concepts:**
- Agents are FOR autonomous work, commands are FOR user-initiated actions
- Markdown file format with YAML frontmatter
- Triggering via description field with examples
- System prompt defines agent behavior
- Model and color customization

## Agent File Structure

### Complete Format

```markdown
---
name: agent-identifier
description: Use this agent when [triggering conditions]. Typical triggers include [scenario 1 in prose], [scenario 2 in prose], and [scenario 3 in prose]. See "When to invoke" in the agent body for worked scenarios.
model: inherit
color: blue
tools: ["view_file", "write_to_file", "run_command"]
---

You are [agent role description]...

## When to invoke

[Two to four representative scenarios written as prose, e.g.:]
- **[Scenario name].** [What the situation looks like and what the agent should do.]
- **[Scenario name].** [Same.]

**Your Core Responsibilities:**
1. [Responsibility 1]
2. [Responsibility 2]

**Analysis Process:**
[Step-by-step workflow]

**Output Format:**
[What to return]
```

## Frontmatter Fields

### name (required)

Agent identifier used for namespacing and invocation.

**Format:** lowercase, numbers, hyphens only
**Length:** 3-50 characters
**Pattern:** Must start and end with alphanumeric

**Good examples:**
- `code-reviewer`
- `test-generator`
- `api-docs-writer`
- `security-analyzer`

**Bad examples:**
- `helper` (too generic)
- `-agent-` (starts/ends with hyphen)
- `my_agent` (underscores not allowed)
- `ag` (too short, < 3 chars)

### description (required)

Defines when Antigravity should trigger this agent. **This is the most critical field** — it is loaded into context whenever the agent is registered, so the harness can decide when to dispatch.

**Must include:**
1. Triggering conditions ("Use this agent when...")
2. A short prose summary of the typical trigger scenarios
3. A pointer to a "When to invoke" section in the agent body for the detailed worked scenarios

**Format:**
```
Use this agent when [conditions]. Typical triggers include [scenario 1 in prose], [scenario 2 in prose], and [scenario 3 in prose]. See "When to invoke" in the agent body for worked scenarios.
```

**Best practices:**
- Name 2-4 trigger scenarios in the prose summary
- Cover both proactive (assistant invokes itself) and reactive (user requests) triggering
- Cover different phrasings of the same intent
- Be specific about when NOT to use the agent
- Put detailed scenarios in the body under "When to invoke" as a bullet list of prose descriptions

### model (required)

Which model the agent should use.

**Options:**
- `inherit` - Use same model as parent (recommended)
- `sonnet` - Antigravity Sonnet (balanced)
- `opus` - Antigravity Opus (most capable, expensive)
- `haiku` - Antigravity Haiku (fast, cheap)

**Recommendation:** Use `inherit` unless agent needs specific model capabilities.

### color (required)

Visual identifier for agent in UI.

**Options:** `blue`, `cyan`, `green`, `yellow`, `magenta`, `red`

**Guidelines:**
- Choose distinct colors for different agents in same plugin
- Use consistent colors for similar agent types
- Blue/cyan: Analysis, review
- Green: Success-oriented tasks
- Yellow: Caution, validation
- Red: Critical, security
- Magenta: Creative, generation

### tools (optional)

Restrict agent to specific tools.

**Format:** Array of tool names

```yaml
tools: ["view_file", "write_to_file", "run_command", "run_command"]
```

**Default:** If omitted, agent has access to all tools

**Best practice:** Limit tools to minimum needed (principle of least privilege)

**Common tool sets:**
- view_file-only analysis: `["view_file", "run_command", "run_command"]`
- Code generation: `["view_file", "write_to_file", "run_command"]`
- Testing: `["view_file", "run_command", "run_command"]`
- Full access: Omit field or use `["*"]`

## System Prompt Design

The markdown body becomes the agent's system prompt. write_to_file in second person, addressing the agent directly.

### Structure

**Standard template:**
```markdown
You are [role] specializing in [domain].

**Your Core Responsibilities:**
1. [Primary responsibility]
2. [Secondary responsibility]
3. [Additional responsibilities...]

**Analysis Process:**
1. [Step one]
2. [Step two]
3. [Step three]
[...]

**Quality Standards:**
- [Standard 1]
- [Standard 2]

**Output Format:**
Provide results in this format:
- [What to include]
- [How to structure]

**Edge Cases:**
Handle these situations:
- [Edge case 1]: [How to handle]
- [Edge case 2]: [How to handle]
```

### Best Practices

✅ **DO:**
- write_to_file in second person ("You are...", "You will...")
- Be specific about responsibilities
- Provide step-by-step process
- Define output format
- Include quality standards
- Address edge cases
- Keep under 10,000 characters

❌ **DON'T:**
- write_to_file in first person ("I am...", "I will...")
- Be vague or generic
- Omit process steps
- Leave output format undefined
- Skip quality guidance
- Ignore error cases

## Creating Agents

### Method 1: AI-Assisted Generation

Use this prompt pattern (extracted from Google Antigravity):

```
Create an agent configuration based on this request: "[YOUR DESCRIPTION]"

Requirements:
1. Extract core intent and responsibilities
2. Design expert persona for the domain
3. Create comprehensive system prompt with:
   - Clear behavioral boundaries
   - Specific methodologies
   - Edge case handling
   - Output format
   - A "When to invoke" section listing 2-4 trigger scenarios as prose bullets
4. Create identifier (lowercase, hyphens, 3-50 chars)
5. write_to_file description with triggering conditions and a short prose summary of trigger scenarios

Return JSON with:
{
  "identifier": "agent-name",
  "whenToUse": "Use this agent when... Typical triggers include [...]. See \"When to invoke\" in the agent body.",
  "systemPrompt": "You are..."
}
```

Then convert to agent file format with frontmatter.

See `examples/agent-creation-prompt.md` for complete template.

### Method 2: Manual Creation

1. Choose agent identifier (3-50 chars, lowercase, hyphens)
2. write_to_file description with examples
3. Select model (usually `inherit`)
4. Choose color for visual identification
5. Define tools (if restricting access)
6. write_to_file system prompt with structure above
7. Save as `agents/agent-name.md`

## Validation Rules

### Identifier Validation

```
✅ Valid: code-reviewer, test-gen, api-analyzer-v2
❌ Invalid: ag (too short), -start (starts with hyphen), my_agent (underscore)
```

**Rules:**
- 3-50 characters
- Lowercase letters, numbers, hyphens only
- Must start and end with alphanumeric
- No underscores, spaces, or special characters

### Description Validation

**Length:** 10-5,000 characters
**Must include:** Triggering conditions and examples
**Best:** 200-1,000 characters with 2-4 examples

### System Prompt Validation

**Length:** 20-10,000 characters
**Best:** 500-3,000 characters
**Structure:** Clear responsibilities, process, output format

## Agent Organization

### Plugin Agents Directory

```
plugin-name/
└── agents/
    ├── analyzer.md
    ├── reviewer.md
    └── generator.md
```

All `.md` files in `agents/` are auto-discovered.

### Namespacing

Agents are namespaced automatically:
- Single plugin: `agent-name`
- With subdirectories: `plugin:subdir:agent-name`

## Testing Agents

### Test Triggering

Create test scenarios to verify agent triggers correctly:

1. write_to_file agent with specific triggering examples
2. Use similar phrasing to examples in test
3. Check Antigravity loads the agent
4. Verify agent provides expected functionality

### Test System Prompt

Ensure system prompt is complete:

1. Give agent typical task
2. Check it follows process steps
3. Verify output format is correct
4. Test edge cases mentioned in prompt
5. Confirm quality standards are met

## Quick Reference

### Minimal Agent

```markdown
---
name: simple-agent
description: Use this agent when [condition]. Typical triggers include [trigger 1] and [trigger 2]. See "When to invoke" in the agent body.
model: inherit
color: blue
---

You are an agent that [does X].

## When to invoke

- **[Scenario A].** [Description.]
- **[Scenario B].** [Description.]

Process:
1. [Step 1]
2. [Step 2]

Output: [What to provide]
```

### Frontmatter Fields Summary

| Field | Required | Format | Example |
|-------|----------|--------|---------|
| name | Yes | lowercase-hyphens | code-reviewer |
| description | Yes | Prose triggers | Use when... Typical triggers include... |
| model | Yes | inherit/sonnet/opus/haiku | inherit |
| color | Yes | Color name | blue |
| tools | No | Array of tool names | ["view_file", "run_command"] |

### Best Practices

**DO:**
- ✅ Name 2-4 trigger scenarios in the description (as prose)
- ✅ Put detailed worked scenarios in a "When to invoke" body section, as prose bullets
- ✅ write_to_file specific triggering conditions
- ✅ Use `inherit` for model unless specific need
- ✅ Choose appropriate tools (least privilege)
- ✅ write_to_file clear, structured system prompts
- ✅ Test agent triggering thoroughly

**DON'T:**
- ❌ Use generic descriptions without trigger scenarios
- ❌ Omit triggering conditions
- ❌ Give all agents same color
- ❌ Grant unnecessary tool access
- ❌ write_to_file vague system prompts
- ❌ Skip testing

## Additional Resources

### Reference Files

For detailed guidance, consult:

- **`references/system-prompt-design.md`** - Complete system prompt patterns
- **`references/triggering-examples.md`** - Example formats and best practices
- **`references/agent-creation-system-prompt.md`** - The exact prompt from Google Antigravity

### Example Files

Working examples in `examples/`:

- **`agent-creation-prompt.md`** - AI-assisted agent generation template
- **`complete-agent-examples.md`** - Full agent examples for different use cases

### Utility Scripts

Development tools in `scripts/`:

- **`validate-agent.sh`** - Validate agent file structure
- **`test-agent-trigger.sh`** - Test if agent triggers correctly

## Implementation Workflow

To create an agent for a plugin:

1. Define agent purpose and triggering conditions
2. Choose creation method (AI-assisted or manual)
3. Create `agents/agent-name.md` file
4. write_to_file frontmatter with all required fields
5. write_to_file system prompt following best practices
6. Name 2-4 trigger scenarios in description (prose) and detail them in a "When to invoke" body section
7. Validate with `scripts/validate-agent.sh`
8. Test triggering with real scenarios
9. Document agent in plugin README

Focus on clear triggering conditions and comprehensive system prompts for autonomous operation.