Sandbox Engine

What It Does

The Sandbox Engine provides isolated code execution environments for AI agents. When an agent calls hexr.sandbox.exec(), the code runs inside a Firecracker microVM — completely isolated from the host and other agents.

Security Model

Layer	Protection
Firecracker microVM	Hardware-level isolation (KVM)
No network	No outbound network by default
Read-only rootfs	Cannot modify the execution environment
Resource limits	CPU, memory, and time bounded
No persistent state	VM destroyed after execution

Execution Flow

Agent requests code execution

Your agent calls hexr.sandbox.exec(code, language="python"). The request routes through Envoy to the Gateway.

Gateway forwards to Sandbox Engine

The Gateway forwards the execute request along with the caller’s SPIFFE ID.

Firecracker microVM boots

The Sandbox Engine boots a Firecracker microVM in under 125ms with the requested runtime.

Code executes in isolation

The code runs inside the microVM with no network access and no host filesystem visibility.

Results captured and VM destroyed

stdout, stderr, and exit_code are captured. Execution telemetry is recorded. The microVM is immediately destroyed.

Agent → Gateway (via Envoy) → Sandbox Engine → Boot microVM (<125ms) → Execute → Result → Destroy VM

Supported Languages

Language	Runtime
Python 3.11	CPython with numpy, pandas, scipy
JavaScript	Node.js 20
Bash	GNU Bash 5

API

Method	Path	Description
`POST`	`/api/v1/exec`	Execute code
`GET`	`/health`	Health check

Request Body

{
  "code": "import pandas as pd\nprint(pd.DataFrame({'a': [1,2,3]}).describe())",
  "language": "python",
  "timeout": 30,
  "memory_mb": 256
}

Response Body

{
  "stdout": "         a\ncount  3.0\nmean   2.0\n...",
  "stderr": "",
  "exit_code": 0,
  "execution_time_ms": 234,
  "memory_used_mb": 45
}

Configuration

Environment Variable	Default	Description
`MAX_EXECUTION_TIME`	`30s`	Maximum execution time
`MAX_MEMORY_MB`	`512`	Maximum memory per VM
`LISTEN_ADDRESS`	`:8080`	HTTP listen address
`FIRECRACKER_BIN`	`/usr/bin/firecracker`	Firecracker binary path

Image

us-central1-docker.pkg.dev/hexr-cloud-prod/hexr-images/hexr-sandbox:v0.2.1

Platform Overview

Pod Containers

Cluster Services

Infrastructure

Sandbox Engine

What It Does

Security Model

Execution Flow

Supported Languages

API

Request Body

Response Body

Configuration

Image

​What It Does

​Security Model

​Execution Flow

​Supported Languages

​API

​Request Body

​Response Body

​Configuration

​Image

What It Does

Security Model

Execution Flow

Supported Languages

API

Request Body

Response Body

Configuration

Image