Agent testing / HUD alternative

HUD.ai alternative

HUD is an evaluation and RL-environment platform that manages task templates, runs and traces on several execution backends. Its model gateway and environment runtime have separate billing rules. Compare that evaluation workflow with the persistent Linux computer for a production agent.

Start building Compare capabilities ↓

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

When HUD fits

HUD fits teams creating and grading agent tasks, running benchmarks and inspecting traces with a managed deployment/scheduling layer. Its backend choice can retain an existing evaluation workflow while changing where each rollout executes.

When to consider Sandbox0

Consider Sandbox0 when the agent needs the same durable writable RootFS across tasks, including system packages and growing files. You supply or retain evaluation tasks, grading, scheduling and model access; a runtime alone does not reproduce the HUD platform.

Capabilities

HUD and Sandbox0: what persists?

Feature comparison with documentation and verification status
Capability	HUD	Sandbox0
Environment and runtime lifetime	HUDHosted environment runs cap at four hours on Pay as you go/Pro and 24 hours on Enterprise; these are bounded evaluations, not an indefinite computer.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 files	HUDStored builds and traces do not establish a durable general-purpose disk returned to by every rollout.View evidence ↗	Sandbox0The writable RootFS survives successful checkpoints, including software, system configuration, dependencies, and user data.Read documentation →
Whole RootFS	HUDThe hosted Modal path starts fresh sandboxes; a whole writable RootFS persistence contract across rollouts is not established.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 state	HUDSelecting a rollout backend is not an arbitrary-machine pause/restore contract.View evidence ↗	Sandbox0Pause compute between tasks; resume on available capacity. Default resume starts fresh processes from the saved filesystem.Read documentation →
Memory and process state	HUDArbitrary process-memory checkpoint/restore is not established.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.

HUD’s build/source files and per-rollout environments do not establish automatic expansion of a persistent full Linux RootFS across an agent’s production tasks. Sandbox0 retains the broader system installation and accumulated files; evaluation data and traces remain separate concerns.

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.

HUD’s Files tab for an environment is uploaded build source, not the live filesystem of a paused run. The reviewed public docs do not establish the same complete paused-RootFS read/write interface as Sandbox0.

See paused file operations →

An environment build is separate from each running instance

HUD separates environment definitions, builds and task templates. The environment page’s Files tab contains source uploaded for a build, explicitly not a trace’s live workspace. Its hosted Modal path starts each rollout in a fresh isolated sandbox; the SDK also offers local, Docker and remote backends. That reproducibility model differs from returning to one accumulated Linux RootFS.

Source 1 ↗Source 2 ↗

Compute backend changes billing and limits

EC2 sizes have a five-minute minimum and then minute billing. Hosted Modal compute is billed by the second for requested CPU, memory and GPUs. Current numeric resource rates are shown in the account’s Environment pricing view, not established by the public Pro fee. Standard plans cap an environment at four hours, Enterprise at 24 hours; a task can set a shorter timeout.

Source 1 ↗Source 2 ↗

Own provider keys still have a platform fee

The billing guide adds 5% to gateway model token costs. With your own key, the provider bills tokens and HUD charges 5% of what the call would have cost through HUD. A blocking spend check can refuse later model calls, but an environment already running continues and is billed when it stops. Do not treat a failed balance check as immediate zero runtime cost.

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.

HUD

Pricing reviewed 2026-10-18

Pay as you go: No subscription; prepaid usage credits; Environment runtime and model usage draw on the team balance.; Rate source ↗
Pro: $50/month, adding $50 credits; Higher limits; work can start down to a $50 negative balance, subject to blocking spend checks.; Rate source ↗
Gateway model usage: Provider model price plus 5%; With your own provider key, HUD still charges 5% of the equivalent call cost and the provider bills tokens.; Rate source ↗
Environment compute: Backend/resource-dependent rate; EC2: five-minute minimum then by minute. Hosted Modal: per-second requested resources. Confirm actual rates in account Environment pricing.; Rate source ↗

The Pro amount becomes credits; do not add it twice to the same consumed balance. HUD’s hosted Modal option does not require your own Modal account, while the SDK’s direct ModalRuntime does. Standard resource sizes and duration limits differ, so this is not an equal-throughput VM quote. Running environments can continue to accrue cost after later model calls are refused.

Check current HUD 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 HUD

Record tasksets, templates, verifiers, traces, backend overrides, model-provider routes and effective resource rates.
Port compatible evaluation/agent code to Sandbox0 or retain HUD around a supported execution integration; do not assume an existing backend adapter already supports Sandbox0.
Validate grading and trace requirements, runtime deadlines, RootFS retention/growth, paused files and full model-fee plus compute costs.

Common questions

Is Sandbox0 a drop-in HUD replacement?

Evaluate API compatibility and product scope. These products serve different core workflows, so a change may require a different architecture.

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 ↗docs.hud.ai/platform/environments.md ↗docs.hud.ai/v6/reference/runtime.md ↗docs.hud.ai/platform/billing.md ↗docs.hud.ai/platform/limits.md ↗Sandbox0 RootFS capacity →

Related comparisons

Archal vs Sandbox0 →Arga Labs vs Sandbox0 →AutoComputer Research / Computer-use Services vs Sandbox0 →Chronicle Labs vs Sandbox0 →Halluminate vs Sandbox0 →Limrun vs Sandbox0 →