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pi-sbx-llamacpp

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Run Pi coding agent isolated in a Docker Sandbox microVM with a local llama-server as the inference backend

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# Pi Coding Agent + Docker Sandbox + llama-server: Setup Guide

This guide runs the Pi coding agent inside an isolated Docker Sandbox (sbx) microVM, with inference served by a local llama-server running in router mode on the host machine. The microVM provides hypervisor-level isolation. Pi cannot access host files outside the mounted workspace, cannot reach the host keychain or SSH keys, and cannot make network requests beyond what is explicitly permitted. The models run on the host GPU at full speed and are loaded dynamically on demand.

Official documentation links:
- sbx: https://docs.docker.com/ai/sandboxes/get-started/
- Pi coding agent: https://pi.dev / https://github.com/earendil-works/pi
- llama.cpp: https://github.com/ggml-org/llama.cpp

---

## Architecture

```
┌──────────────────────────────────────────────────────────────────────┐
│ HOST (macOS / Linux)                                                 │
│                                                                      │
│  llama-server (Router Mode)                                          │
│  127.0.0.1:8001 ───► Dynamic Child Instances (Ports 556xx...)        │
│                                                                      │
│  sbx host-side proxy                                                 │
│  • allowedDomains enforcement                                        │
│  • forwards allowed traffic to host services                         │
│                                                                      │
│  ┌──────────────────────────────────────────────────────────────┐    │
│  │ microVM (Linux, isolated)                                    │    │
│  │                                                              │    │
│  │  Docker Engine                                               │    │
│  │  ┌────────────────────────────────────────────────────────┐  │    │
│  │  │ Pi coding agent container                              │  │    │
│  │  └────────────────────────────────────────────────────────┘  │    │
│  └──────────────────────────────────────────────────────────────┘    │
└──────────────────────────────────────────────────────────────────────┘
```

**How Pi reaches llama-server across the VM boundary:**

Inside the microVM, `localhost` refers to the VM itself. `host.docker.internal` resolves to your host machine via the sbx gateway. Requests to it travel through the sbx proxy that runs on the host machine. Only destinations listed in `allowedDomains` are forwarded, everything else is blocked.

---

## Step 1 — Install sbx

sbx is Docker's standalone CLI for running AI agents in isolated microVMs. It does not require Docker Desktop.

**macOS**

```bash
brew install docker/tap/sbx
sbx login
```

**Linux (Ubuntu/Debian)**

```bash
curl -fsSL https://get.docker.com | sudo REPO_ONLY=1 sh
sudo apt-get install docker-sbx
sudo usermod -aG kvm $USER
newgrp kvm
sbx login
```

The `kvm` group membership is required on Linux because sbx uses KVM for hardware-accelerated microVM isolation.

On first login, sbx prompts you to choose a default network policy. Select **Balanced** — it allows common development traffic while blocking everything else by default.

For other Linux distributions or manual binary installation, see: https://docs.docker.com/ai/sandboxes/get-started/

---

## Step 2 — Install llama.cpp

**macOS**

```bash
brew install llama.cpp
```

**Linux**

Homebrew also works on Linux:

```bash
brew install llama.cpp
```

Alternatively, build from source:

```bash
git clone https://github.com/ggml-org/llama.cpp
cd llama.cpp
cmake -B build
cmake --build build --config Release
sudo cmake --install build
```

On Linux with an Nvidia GPU, add `-DGGML_CUDA=ON` to the first cmake command to enable CUDA acceleration. For AMD GPUs use `-DGGML_ROCM=ON`.

For all installation options see: https://github.com/ggml-org/llama.cpp

---

## Step 3 — Download the Models with llama-cli
 
This guide uses two specific Gemma models: the 12B and 26B variants. Because `llama-server` runs in dynamic router mode, it will automatically download missing models on the first API call. However, we prefer to explicitly pre-download them upfront for clarity and to avoid any download delays or timeout issues during active use in Pi.
 
Use `llama-cli` with `-st` and `-p` to pass a dummy prompt so the CLI downloads the model and exits immediately:
 
```bash
# Download Gemma 4 12B
llama-cli -hf unsloth/gemma-4-12B-it-qat-GGUF:Q4_K_XL -st -p "hi" -n 1
 
# Download Gemma 4 26B
llama-cli -hf unsloth/gemma-4-26B-A4B-it-qat-GGUF:Q4_K_XL -st -p "hi" -n 1
```
---

## Step 4 — Start llama-server on the host (Router Mode)

Instead of targeting a single model file, start `llama-server` in router mode. This exposes a unified endpoint that spins up dedicated sub-processes and loads/unloads models from memory automatically based on requests.

```bash
llama-server \
    --parallel 1 \
    --cache-ram 0 \
    --ctx-size 64000 \
    --no-mmproj \
    --reasoning on \
    --temp 1.0 \
    --top-p 0.95 \
    --top-k 64 \
    --models-max 1 \
    --sleep-idle-seconds 240 \
    --port 8001
```

---

## Step 5 — Create the kit directory

An sbx kit is a directory containing a `spec.yaml` and an optional `files/` subtree. Files placed under `files/home/` are injected into the agent's home directory inside the microVM at sandbox creation time, before any install commands run.

A kit can be placed anywhere on disk and referenced via the `--kit` flag when running sbx. This guide places it at `~/.config/docker-sbx/pi-llamacpp`.

```bash
mkdir -p ~/.config/docker-sbx/pi-llamacpp/files/home/.pi/agent
```

Full directory layout:

```
~/.config/docker-sbx/pi-llamacpp/
├── spec.yaml
└── files/
    └── home/
        └── .pi/
            └── agent/
                ├── models.json
                └── settings.json
```
> **Alternative:** Instead of creating the files manually in the steps below, you can clone the [pi-sbx-llamacpp](https://github.com/cuolm/pi-sbx-llamacpp) repo to your preferred projects location and copy the kit files into place:
> ```bash
> git clone https://github.com/cuolm/pi-sbx-llamacpp.git ~/Projects/pi-sbx-llamacpp
> cp -r ~/Projects/pi-sbx-llamacpp/kit ~/.config/docker-sbx/pi-llamacpp
> ```
> Then skip to Step 9.

---

## Step 6 — Create spec.yaml

```yaml
schemaVersion: "1"
kind: agent
name: pi

agent:
  image: "docker/sandbox-templates:shell"
  entrypoint:
    run: [pi]

network:
  allowedDomains:
    - "registry.npmjs.org"
    - "host.docker.internal:8001"
    - "localhost:8001"

commands:
  install:
    - command: "npm install -g --ignore-scripts @earendil-works/pi-coding-agent"
```

Both `host.docker.internal:8001` and `localhost:8001` must be included in `allowedDomains` to authorize and route proxy traffic to the host. `registry.npmjs.org` is needed for the npm install during sandbox creation.

---

## Step 7 — Create models.json

Pi reads `~/.pi/agent/models.json` inside the microVM to discover custom model providers. Without this file it only knows about its built-in providers such as Claude.

Place this at `~/.config/docker-sbx/pi-llamacpp/files/home/.pi/agent/models.json`:

```json
{
  "providers": {
    "llamacpp": {
      "baseUrl": "http://host.docker.internal:8001/v1",
      "api": "openai-completions",
      "apiKey": "local-llama",
      "models": [
        {
          "id": "unsloth/gemma-4-12B-it-qat-GGUF:Q4_K_XL",
          "name": "unsloth/gemma-4-12B-it-qat-GGUF:Q4_K_XL",
          "input": ["text"]
        },
        {
          "id": "unsloth/gemma-4-26B-A4B-it-qat-GGUF:Q4_K_XL",
          "name": "unsloth/gemma-4-26B-A4B-it-qat-GGUF:Q4_K_XL",
          "input": ["text"]
        }
      ]
    }
  }
}
```

The `"id"` string must match the actual name of the downloaded model that `llama-server` in router mode exposes. For simplicity, we also set the `"name"` string to the same. The `baseUrl` uses `host.docker.internal` for the reason explained in the Architecture section.
You can verify the models available on the running llama-server with:
 
```bash
curl http://127.0.0.1:8001/v1/models | jq
```
 
You should see both Gemma models listed in the response.

---

## Step 8 — Create settings.json

Pi creates `~/.pi/agent/settings.json` on first launch. It does not set `defaultProvider` or `defaultModel`, so without this file in the kit Pi will prompt you to select a provider and model interactively on every session.

By injecting `settings.json` via the kit's `files/` directory, Pi picks up the defaults before it runs for the first time and never asks.

Place this at `~/.config/docker-sbx/pi-llamacpp/files/home/.pi/agent/settings.json`:

```json
{
  "defaultProvider": "llamacpp",
  "defaultModel": "unsloth/gemma-4-12B-it-qat-GGUF:Q4_K_XL"
}
```

---

## Step 9 — Run the sandbox

From the project directory you want Pi to work in:

```bash
sbx run --kit ~/.config/docker-sbx/pi-llamacpp pi
```

On first run sbx creates the microVM, injects `files/home/` into `~/` inside the VM, and runs the npm install. This takes about 30-60 seconds. On subsequent runs it reattaches to the existing sandbox instantly.

> **Important:** The `files/` injection only happens at sandbox creation time. If you modify `models.json` or `settings.json` after the sandbox already exists, destroy it and recreate it:
>
> ```bash
> sbx rm <sandbox-name>
> sbx run --kit ~/.config/docker-sbx/pi-llamacpp pi
> ```

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