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

# Execution architecture

> An agent product combines two kinds of machinery: a Harness that runs the agent loop, and a platform that keeps everything around it durable and governed.

An agent product combines two kinds of machinery:

1. The [**Harness**](/concepts/harnesses-and-models) runs the agent loop: it prepares model context, calls a model, invokes tools, and decides what to do next.
2. The **platform** is everything that has to outlive that loop: durable state, governance, and a record of what happened.

Checkfu owns the second layer and treats the first as replaceable.

```mermaid theme={"theme":{"light":"github-light","dark":"github-dark"}}
%%{init: {"theme":"neutral","themeVariables":{"fontFamily":"ui-sans-serif, system-ui, sans-serif","primaryColor":"#F5F5F4","primaryBorderColor":"#A8A29E","primaryTextColor":"#282828","lineColor":"#78716C","secondaryColor":"#FAFAF9","tertiaryColor":"#FFFFFF"}}}%%
flowchart TD
  A[Your product] --> B
  subgraph B [Checkfu]
    direction LR
    C[Control plane] -->|admits & governs| D[Data plane]
  end
  D --> E[Harness]
  E --> F[Model provider]
```

## The boundary

Your product calls Checkfu's HTTP surface. Checkfu resolves the Agent's published definition, provisions a sandbox, and launches its configured Harness. Model calls, capabilities, credentials, and Memory cross platform-owned boundaries so policy and accounting remain consistent across harnesses.

| Layer                 | Responsibility                                                                                        |
| --------------------- | ----------------------------------------------------------------------------------------------------- |
| Your product          | User experience, business logic, and [Principal](/concepts/tenancy-and-governance) identity assertion |
| Checkfu control plane | Definitions, [Sessions](/concepts/sessions-and-runs), policy, audit, lifecycle, and usage             |
| Checkfu data plane    | Sandboxed [Runs](/concepts/sessions-and-runs) and platform-owned runtime boundaries                   |
| Harness               | The agent loop program                                                                                |
| Model provider        | Inference through the Workspace's configured credentials and routing policy                           |

## Capabilities are searched; data is mounted

Checkfu keeps capability descriptions out of the model context until they are needed. Tools, skills, and Memory recall live in searchable catalogs. Data such as repositories, files, and Memory directories mounts into the sandbox filesystem, where it does not consume model context merely by existing.

So a Workspace can carry a large catalog and a large repository without either one filling the model's context window before the first turn.

## Ports, not permanent dependencies

Each infrastructure plane sits behind a replaceable port. Local development can use in-memory stores, a local sandbox, and a mock model; production can use hosted adapters or customer-side infrastructure. The same domain behavior is expected from each adapter.

## Next steps

<CardGroup cols={2}>
  <Card title="Sessions and Runs" icon="timeline" href="/concepts/sessions-and-runs">
    See how the control plane turns an Agent launch into an event-sourced Session and its sandboxed Runs.
  </Card>

  <Card title="API reference" icon="code" href="/reference/overview">
    Explore the HTTP surface your product calls to resolve, launch, and observe agents.
  </Card>
</CardGroup>