Agent sandboxes / Modal alternative

A Modal sandbox alternative for a reusable agent environment

Modal can snapshot an entire sandbox root filesystem. For an agent returning across tasks, compare snapshot retention, resource-meter semantics, and the work needed to reconnect an environment over its lifetime.

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

When Modal fits

Modal fits teams already using its Python infrastructure, Functions, volumes, and GPU ecosystem. Its sandbox API can share those building blocks; GPU sandbox support depends on the selected runtime.

When to consider Sandbox0

Sandbox0 is designed around returning to the same persistent sandbox identity, with online RootFS growth and compute pauses. It can suit agents whose accumulated system changes and outputs form one durable environment.

Capabilities

Modal and Sandbox0: what persists?

Feature comparison with documentation and verification status
Environment and runtime lifetimeModalA sandbox can run up to 24 hours. Use a snapshot and subsequent sandbox for a longer environment workflow.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 filesModalFilesystem snapshots persist files for later sandbox creation.View evidence ↗Sandbox0The writable RootFS survives successful checkpoints, including software, system configuration, dependencies, and user data.Read documentation →
Whole RootFSModalRoot filesystem snapshots; mounted 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 stateModalSnapshot/restore workflow; memory snapshots have separate Alpha constraints.View evidence ↗Sandbox0Pause compute between tasks; resume on available capacity. Default resume starts fresh processes from the saved filesystem.Read documentation →
Memory and process stateModalAlpha memory snapshots capture memory and filesystem; they expire after 7 days.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.

Modal documents fixed 512 GiB root filesystems for the VM runtime and does not support increasing that VM disk. Match the runtime-specific filesystem limit to the agent data lifecycle.

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 →

Keep the snapshot identifier and its retention policy

Modal filesystem snapshots include the root filesystem and can seed new sandboxes; mounted volumes are managed separately. Filesystem snapshots default to a 30-day TTL, with custom or indefinite retention available. Your controller needs to retain the correct snapshot and expiry policy.

Source 1 ↗

Account for the resource request and actual use

Modal sandbox billing uses the higher of requested and actual resources. CPU is priced per physical core, with two vCPUs per core. Compare the sandbox-specific rate card rather than applying the cheaper general Function meters to an agent sandbox.

Source 1 ↗Source 2 ↗

Make multi-session execution explicit

Modal documents a sandbox timeout up to 24 hours and recommends restoring a filesystem snapshot for longer use. Sandbox0 exposes configurable sandbox TTL policies, with pause/resume tied to the same identity. Filesystem continuity does not by itself keep an in-flight process alive.

Source 1 ↗Source 2 ↗

Memory snapshot chains retain the original deadline

Alpha memory snapshots expire after seven days; snapshots of restored instances inherit the original deadline. Open TCP connections close, and snapshotting currently terminates the source sandbox. Running exec commands must finish first, and background processes launched through exec are not properly restored. Plan recovery around these limitations.

Source 1 ↗

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

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

One Modal physical core equals two vCPUs. The estimate uses requested resources without bursting and deducts Starter’s $30 recurring compute credit. Storage and regional uplifts are separate.

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: $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 rate source ↗Sandbox0 rates ↗

Modal

Pricing reviewed 2026-10-19

Sandbox CPU
$0.00003942 per physical-core-second
One physical core equals two vCPUs; minimum request is 0.125 cores.
Rate source ↗
Sandbox memory
$0.00000667 per GiB-second
Rate source ↗
Team plan
$250/month plus compute
$100 monthly compute credit is listed separately; Starter lists $30 monthly credit.
Rate source ↗

CPU and memory are billed by the second under the sandbox resource policy. GPU, retained volume storage, egress, and plan entitlements can add costs; the general Functions rate is a different product meter.

Check current Modal 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

  1. Inventory Modal filesystem snapshots, their TTLs, and mounted volumes as separate state sources.
  2. Replace App/Image/Sandbox orchestration with a Sandbox0 template and persistent sandbox identity; port secrets and network restrictions.
  3. Measure the same agent task under the actual CPU allocation and check pause/resume with growing files rather than comparing nominal core prices alone.

Common questions

Is Sandbox0 a drop-in Modal 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/sandbox-snapshots ↗modal.com/docs/guide/sandbox-resources ↗modal.com/pricing ↗modal.com/docs/guide/sandboxes ↗/docs/sandbox/pause-resume ↗Sandbox0 RootFS capacity →

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