Agent sandboxes / Runtime (withruntime.com) alternative

A WithRuntime sandbox alternative for a growing durable environment

Runtime, at withruntime.com, already offers persistent Linux microVMs and memory-preserving pause. For a returning agent, distinguish local continuity from off-server recovery and compare its active-CPU economics with Sandbox0’s durable, automatically growing RootFS.

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Reviewed 2026-10-18 · Published by the Sandbox0 team

When Runtime (withruntime.com) fits

Runtime fits teams that want full guest-kernel tooling, active CPU billing, memory pause, forks, and integrated agent infrastructure. Its runtime has no time limit unless configured and can pause automatically when idle.

When to consider Sandbox0

Consider Sandbox0 when the durable writable RootFS should live separately from compute and expand online while the agent runs. Supported paused file operations can inspect persistent state without waking compute; process retention uses explicit compatible experimental memory mode.

Capabilities

Runtime (withruntime.com) and Sandbox0: what persists?

Feature comparison with documentation and verification status
Environment and runtime lifetimeRuntime (withruntime.com)No time limit unless configured; idle and credit policies still apply.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 filesRuntime (withruntime.com)Stop retains disk until deletion; durability across server loss is a separate storage contract.View evidence ↗Sandbox0The writable RootFS survives successful checkpoints, including software, system configuration, dependencies, and user data.Read documentation →
Whole RootFSRuntime (withruntime.com)The sandbox has its own writable Linux filesystem, retained through pause and stop.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 stateRuntime (withruntime.com)Pause preserves execution state; stop/restart is disk-only.View evidence ↗Sandbox0Pause compute between tasks; resume on available capacity. Default resume starts fresh processes from the saved filesystem.Read documentation →
Memory and process stateRuntime (withruntime.com)Pause retains memory and processes, subject to paid retention and credit rules.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.

Runtime documents its own writable disk and fixed-size volumes, with account and sandbox capacity limits. Its reviewed storage and environment guides do not establish automatic mounted-RootFS expansion driven by free space.

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.

Runtime’s documented idle-pause workflow can wake on a file call. Sandbox0’s supported Files API leaves compute paused; retained-memory pauses allow reads and reject mutations.

See paused file operations →

A kept local disk is different from an off-server recovery point

Runtime’s storage guide locates a sandbox’s own disk on its server and recommends volumes for data that cannot be rebuilt. Volumes have backups, and retained snapshots gain off-server protection after compression, copying, and verification. Check the durability state rather than equating ready-for-local-use with off-server recovery.

Source 1 ↗

Pause keeps processes; stop starts from disk

Runtime pause retains memory and processes, while stop keeps disk and later restart discards memory. Its environment guide documents optional execution deadlines and idle pause. Account credit and retained-state policies remain separate from the running-time setting.

Source 1 ↗Source 2 ↗

Active CPU still has a floor, and memory is reserved

Runtime bills active CPU above a default 50-millicore floor and charges allocated memory while compute runs. Saved pause/snapshot state uses a decimal-GB storage meter, while volumes use a different capacity-based meter. Compare measured CPU use, memory allocation, and retention rather than an unsupported universal savings percentage.

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.

Runtime (withruntime.com)

Pricing reviewed 2026-10-18

Active CPU
$0.025 per active vCPU-hour
Default minimum CPU floor is 50 millicores; reserved-CPU mode has different utilization behavior.
Rate source ↗
Reserved running memory
$0.0075 per GiB-hour
Rate source ↗
Paused state / snapshots / custom images
$0.08 per decimal GB per 30-day month
Saved state is billed on uniquely stored blocks, not a universal allocated-RAM meter.
Rate source ↗
Volume capacity
About $0.11 per GiB per 30-day month
Based on full created volume size; backup storage has another meter.
Rate source ↗
Outbound traffic above allowance
$0.02 per GB
The current page includes 100 GiB outbound per month.
Rate source ↗

No plan fee, seat fee, or compute minimum duration is listed. Retained memory, snapshots, images, volumes, and backups have separate storage terms. Do not treat local sandbox retention as a guarantee against host loss; check backedUp and durability.state for the intended saved artifact.

Check current Runtime (withruntime.com) 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

A 4 GiB agent, 100 running hours

$6.00 compute

4 GiB × 100 hours, billed by the second. Keeping 20 GiB of billable incremental data for a full billing month adds $0.40. CPU is derived from memory by the platform; choose a tier that fits your workload.

Understand storage billing →

Before migrating from Runtime (withruntime.com)

  1. Inventory sandbox disks, volumes, backups, kept snapshots, durability states, memory retention, and credit policies before removing a host-local source.
  2. Export or verify protected copies, then port images, commands, Docker-dependent tooling, credentials, service exposure, and idle behavior.
  3. Validate installed-package state, RootFS growth, paused files, process-restoration requirements, and active CPU plus retained-storage economics.

Common questions

Is Sandbox0 a drop-in Runtime (withruntime.com) 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 ↗withruntime.com/docs/storage.md ↗withruntime.com/docs/sandbox-environment.md ↗withruntime.com/how-to/pause-and-resume-a-sandbox.md ↗withruntime.com/pricing ↗Sandbox0 RootFS capacity →

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