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

# Multi-container

> Multi-container documentation for Harbor.

Multi-container environments enable additional services to your environment to represent databases, APIs, MCPs, etc.

## Specification

In most cases, use **`environment/docker-compose.yaml`** to specify the multi-container environment.

<Note>
  Harbor itself is agnostic to which infra-as-code spec you use, which means it is possible to support multi-container environments using something other than `docker-compose.yaml`. We have not implemented support for anything else natively, but users are free to do so by implementing a [custom environment](/core-concepts/sandboxes/custom-sandboxes).
</Note>

## Reserved services and default compose

Harbor reserves the `main` service name for the agent container. All other services are treated as sidecars. Harbor merges your `environment/docker-compose.yaml` on top of a base compose file, so you do not need to define the `main` service build or keepalive command yourself.

For tasks with a `Dockerfile`, the base compose is [`docker-compose-build.yaml`](https://github.com/harbor-framework/harbor/blob/main/src/harbor/environments/docker/docker-compose-build.yaml):

```yaml title="src/harbor/environments/docker/docker-compose-build.yaml" theme={"system"}
services:
  main:
    build:
      context: ${CONTEXT_DIR}
    pull_policy: build
    command: [ "sh", "-c", "sleep infinity" ]
```

For tasks that use a prebuilt image instead of building locally, Harbor uses [`docker-compose-prebuilt.yaml`](https://github.com/harbor-framework/harbor/blob/main/src/harbor/environments/docker/docker-compose-prebuilt.yaml):

```yaml title="src/harbor/environments/docker/docker-compose-prebuilt.yaml" theme={"system"}
services:
  main:
    image: ${PREBUILT_IMAGE_NAME}
    command: [ "sh", "-c", "sleep infinity" ]
```

## Example Layout

```bash theme={"system"}
my-task/
├── instruction.md
├── task.toml
└── environment/
    ├── Dockerfile              # agent (main) service
    ├── docker-compose.yaml     # sidecars + depends_on
    └── mcp-server/             # optional service build context
        ├── Dockerfile
        └── server.py
```

Harbor treats the agent service as **`main`**. In compose you typically only add overrides such as `depends_on` and healthchecks; Harbor wires the rest.

```yaml title="environment/docker-compose.yaml" theme={"system"}
services:
  main:
    depends_on:
      mcp-server:
        condition: service_healthy

  mcp-server:
    build:
      context: ./mcp-server
    expose:
      - "8000"
    healthcheck:
      test: ["CMD", "python", "-c", "import socket; s=socket.create_connection(('localhost',8000),timeout=2); s.close()"]
      interval: 2s
      timeout: 5s
      retries: 15
      start_period: 5s
```

Sidecars share a Docker network with `main`; reach them by **service name** (e.g. `http://mcp-server:8000`).

## Support

| Support                              | Environments                                                                                                                                                       |
| ------------------------------------ | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| Native Compose                       | `docker`, `podman`, `ec2`, `islo`, `vercel`                                                                                                                        |
| DinD Docker Compose                  | `blaxel`, `beam`, `daytona`, `gke`, `hyperbrowser`, `langsmith`, `modal`, `novita`, `tensorlake`                                                                   |
| Other multi-container specifications | Custom environments only                                                                                                                                           |
| Not supported                        | `e2b`, `runloop`, `ack`, `openshift`, `apple-container`, `singularity`, `cwsandbox`, `wandb`, `use-computer`, `cua-cloud`, `opensandbox`, `skypilot`, `hf-sandbox` |