Agent sandboxes / Modal (VM Sandboxes) alternative

Comparing Modal VM sandboxes with a growing agent RootFS

Modal VM sandboxes provide their own Linux kernel for workloads such as Docker, FUSE, and nested cgroups. Evaluate that compatibility requirement first, then compare disk growth and the cost of retaining the agent environment.

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Reviewed 2026-10-19 · Published by the Sandbox0 team

When Modal (VM Sandboxes) fits

Choose Modal VM sandboxes when the workload requires their Linux-kernel capabilities. Modal documents Docker and FUSE support in the VM runtime; GPU sandboxes use its gVisor runtime instead.

When to consider Sandbox0

Consider Sandbox0 when the workload fits gVisor and needs a durable writable RootFS that expands online. Test tooling that depends on kernel behavior before moving an existing VM-oriented agent.

Capabilities

Modal (VM Sandboxes) and Sandbox0: what persists?

Feature comparison with documentation and verification status
Environment and runtime lifetimeModal (VM Sandboxes)The sandbox timeout is at most 24 hours; longer workflows require an explicit saved-state lifecycle.View evidence ↗Sandbox0ttl and hard_ttl default to 0 (disabled). Configure idle pause and deletion policies for the same persistent sandbox identity; an idle running sandbox still accrues compute charges.Read documentation →
Persistent filesModal (VM Sandboxes)Filesystem snapshots can create a later VM sandbox.View evidence ↗Sandbox0The writable RootFS survives successful checkpoints, including software, system configuration, dependencies, and user data.Read documentation →
Whole RootFSModal (VM Sandboxes)Root filesystem captured, including VM Docker state; separate volumes are excluded.View evidence ↗Sandbox0Persistence extends beyond /workspace to paths such as /usr/local, /etc, and /home. Runtime tmpfs and virtual filesystems are excluded.Read documentation →
Release compute, keep stateModal (VM Sandboxes)Snapshot-driven restoration with distinct filesystem and Alpha memory paths.View evidence ↗Sandbox0Pause compute between tasks; resume on available capacity. Default resume starts fresh processes from the saved filesystem.Read documentation →
Memory and process stateModal (VM Sandboxes)Alpha; verify VM-specific restrictions, including mounted volumes.View evidence ↗Sandbox0Experimental memory checkpoints can preserve compatible process state. Use memory: true on both pause and resume on a compatible deployment. A filesystem checkpoint alone does not preserve running processes.Read documentation →

Online RootFS growth

On growth-enabled images, Sandbox0 automatically expands the writable RootFS as free space runs low, within a configured sparse logical ceiling up to 64 TiB. Agent processes keep running. Growth is enabled in the qualified Cloud region; confirm activation for other regions. Existing images and snapshots retain their captured storage geometry. The ceiling is logical capacity, not preallocated host disk.

The Modal VM root filesystem is fixed at 512 GiB. Snapshotting preserves it but does not provide an automatically expanding disk.

Files while compute is paused

Sandbox0's Files API reads persistent files without resuming compute. Filesystem-only pauses also permit edits; retained-memory pauses are read-only and reject mutations with 409. SSH, SFTP, file watching, and runtime-only mounts require running compute.

See paused file operations →

A private Linux kernel changes compatibility

Modal lets a sandbox select the VM runtime. That is useful for Docker, FUSE, and nested cgroup workflows. Sandbox0 uses gVisor, so compare required syscalls and device behavior with a real workload rather than treating the isolation models as interchangeable.

Source 1 ↗Source 2 ↗

Plan the lifecycle of large Docker state

Modal includes /var/lib/docker in filesystem snapshots of VM sandboxes. Its VM root disk is fixed at 512 GiB. In Sandbox0, keep Docker or tool data inside persistent RootFS paths when it must survive; a configured ephemeral mount deliberately excludes its contents from checkpoints.

Source 1 ↗Source 2 ↗Source 3 ↗

Test process snapshots independently

Modal Alpha memory snapshots expire after seven days; descendants inherit the original deadline. VM sandboxes with memory snapshotting enabled cannot mount Volumes. Snapshotting terminates the source and closes TCP connections; running exec commands must finish first, and exec-launched background processes are not properly restored. Test process continuity separately from filesystem recovery. Sandbox0 memory restoration is also experimental.

Source 1 ↗Source 2 ↗

Pricing

Compare billing models, then price your workload

CPU allocation, memory, billing minimums, disk, plan fees, and supported workload differ. These meters are not equal-performance machine quotes. Confirm region, resource units, and included allowances before estimating a total.

Same size. Lower compute cost.

2 vCPU · 4 GiB RAM · 730 running hours / month

Save $99.91 / month

The same CPU and memory allocation, with runtime aggregated across sessions. Monthly allowances and credits below are applied to this workload.

Modal (VM Sandboxes)

Starter · $30 monthly compute credit

$143.71

$0.23796/hour before monthly allowances

Sandbox0

Pay as you go · no monthly plan fee

$43.80

$0.06/hour · 69% lower in this example

Uses Modal’s Sandbox meters, with one physical core for two vCPUs, and deducts Starter’s $30 monthly compute credit. Actual resource usage above the request and regional uplifts can change the bill.

Compute and stated plan fees only. Storage, network, taxes and one-time credits are excluded; hardware and throughput can differ.

Calculation and billing assumptions

Modal (VM Sandboxes): $173.7108 compute after 0 included vCPU-hours and 0 included GiB-hours, plus $0.00 plan fee, minus $30.00 applied monthly credit.

Sandbox0: 4 GiB × 730 hours × its published compute rate. CPU uses the Cloud allocation shown on the pricing page.

Includes compute and the selected plan. Excludes storage, network, taxes, one-time grants and negotiated discounts. Assumes this workload can use the full listed monthly allowances. CPU and memory counts do not guarantee equal hardware or throughput.

Checked Modal (VM Sandboxes) rate source ↗Sandbox0 rates ↗

Modal (VM Sandboxes)

Pricing reviewed 2026-10-19

Sandbox CPU
$0.00003942 per physical-core-second
A physical core is two vCPUs; use the sandbox rate rather than the Functions rate.
Rate source ↗
Sandbox memory
$0.00000667 per GiB-second
Rate source ↗
Starter subscription
$0/month plus compute
$30/month compute credit; paid plans change concurrency and other entitlements.
Rate source ↗

These are public Sandbox meters, not a fixed-price VM shape. Check runtime-specific provisioning, GPU availability, plan fees, retained storage, and egress before estimating a total.

Check current Modal (VM Sandboxes) pricing ↗

Sandbox0

$0.015/GiB-hour compute (billed by the second at approximately $0.000004167/GiB-second), plus $0.02/GiB-month of retained incremental RootFS and memory checkpoint data. Shared base images and platform recovery retention are excluded; paused sandboxes accrue storage charges only.

Compute / GiB-hour
$0.015
Retained data / GiB-month
$0.02
Paused compute
$0.00
Understand storage billing →

Before migrating from Modal (VM Sandboxes)

  1. List kernel-dependent tools, Docker daemon settings, FUSE mounts, nested cgroups, and persistent container data.
  2. Run the required tools on a Sandbox0 test environment and choose persistent versus ephemeral paths deliberately.
  3. Move data through a filesystem export or reproducible template; validate memory checkpoints and network reconnection as separate requirements.

Common questions

Is Sandbox0 a drop-in Modal (VM Sandboxes) replacement?

Evaluate API compatibility and product scope. A shared sandbox category does not make SDKs, lifecycle semantics, or limits interchangeable.

Does keeping files also keep a running agent?

Filesystem state and volatile process state are separate. Sandbox0 requires explicit compatible memory checkpoints to preserve running processes; its default filesystem resume starts fresh processes.

Sources and review scope

The Boat team's Battleships directory is the discovery source. References below describe the product and plan discussed on this page. Check current availability, regional terms and billing units before migrating. Where the cited material does not establish a capability or price, the comparison identifies that uncertainty.

Battleships research record ↗modal.com/docs/guide/sandboxes ↗/docs/sandbox ↗modal.com/docs/guide/sandbox-resources ↗/docs/sandbox/template/configuration ↗modal.com/docs/guide/sandbox-snapshots ↗/docs/sandbox/pause-resume ↗modal.com/pricing ↗Sandbox0 RootFS capacity →

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