> ## Documentation Index
> Fetch the complete documentation index at: https://docs.apowerb.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Orchestration

> The four execution patterns and how sub-agents compose.

`agent_type` selects how an agent executes.

| Type           | Behaviour                                                         |
| -------------- | ----------------------------------------------------------------- |
| `llm` / `base` | A single agent, executed directly                                 |
| `parallel`     | Sub-agents run concurrently, results are combined                 |
| `sequential`   | Sub-agents run one after another, each seeing the previous output |
| `loop`         | Iterative execution until a stop condition is met                 |

## Sub-agents

Composite agents reference children through `sub_agents`. Children are themselves
agents — they can carry their own tools, model and instruction, and can be composite
in turn, which is how hierarchies are built.

```json theme={null}
{
  "agent_name": "Research pipeline",
  "agent_type": "sequential",
  "sub_agents": ["collector_agent_id", "analyst_agent_id", "writer_agent_id"]
}
```

<Note>
  Under Google ADK, sub-agents of one invocation share an execution tree. When reading
  traces, do not assume one identifier per agent — correlate on the agent name as well.
</Note>


## Related topics

- [Agents](/concepts/agents.md)
- [Introduction](/index.md)