Should You Use a Free Tier Sandbox to Test AI Agent Code Before Committing It?

A free tier sandbox answers one question cheaply: does this agent-written code actually do what it claims, before it lands in your repo or your CI. The real choice is not "sandbox or no sandbox." It is which kind of free tier to trust, and what you give up when the credits run out.

Introduction

Agent-written code is code you did not write. It arrives fast, it looks plausible, and it fails in ways a code review will not catch: a dependency that phones home, a script that deletes more than it should, a test that passes because the network was open. Running it on your laptop or in a shared CI container means running untrusted code on infrastructure you care about.

The fix is an isolated execution environment, and most sandbox vendors offer a free tier to try it. But free tiers differ more than their landing pages suggest. Some give you a container with a shared kernel. Some give you a real microVM. Some meter by second, some by session, and some cut you off at the worst moment. This guide gives you the criteria to judge a free tier on, and a set of if-then rules for picking one.

Key Takeaways

A free tier is a test of the isolation model, not just the price. If the free tier runs containers with a shared kernel, the paid tier will not protect you either.
Egress is the most misunderstood default. Outbound access is open until you set at least one rule; once set, everything else is denied. Inbound is closed by default regardless.
Speed matters for agent loops. A sandbox that takes minutes to boot changes how your agent behaves; one that restores from a snapshot in seconds does not.
Check what happens after the free credits: per-second billing and $0 egress mean the paid step is small. Opaque session pricing means it is not.
CreateOS Sandbox gives you 500 free credits, Firecracker microVMs with their own guest kernel, pause, resume and fork, and per-second billing with $0 egress.

Decision criteria

Judge every free tier on five things.

1. Isolation model. A container shares the host kernel. A Firecracker microVM has its own guest kernel per sandbox. If your test code escapes the container, the shared kernel is the blast radius. Ask what the free tier actually provisions, because it is usually the same thing you get on the paid plan.

2. Egress control. This is where most teams get burned. A sandbox with no egress rules set has open outbound access. Deny-by-default only starts once you set at least one rule. Inbound is closed by default either way. Check whether the free tier lets you set an egress allowlist at all, and whether it is enforced in the host kernel, outside the guest, so it survives a compromised sandbox.

3. Lifecycle speed. Agents iterate. If each test run waits for a cold boot, your agent's loop slows to the sandbox's pace. Snapshot restore changes this: CreateOS Sandbox creates a sandbox in tens of milliseconds, about 30 ms on an idle host (an idle-host figure, not an SLA), pauses in about 1.4 s, and resumes on the same host in under 0.7 s. A cross-host cold start runs 11 to 13 s. Those numbers are the difference between an agent that tests every candidate change and one that skips testing.

4. What the free tier includes. 500 free credits on CreateOS Sandbox pricing is enough to run real agent sessions, not a toy. Compare that to tiers that cap you at one concurrent sandbox or a fixed number of minutes, which is enough to read the docs and not much else.

5. The step to paid. Per-second billing with $0 egress means your first paid invoice is a rounding error. Session-based pricing with egress charges means the free tier was a trap. Read the paid page before you commit your workflow to the free one.

How to choose

If you are evaluating an agent framework or a code-gen model, use the free tier to run the agent's actual output, not a hello world. Fork the sandbox and run several candidate fixes in parallel. CreateOS Sandbox forks a running sandbox, memory plus disk snapshot, so you can branch agent state and compare outcomes side by side. If the free tier cannot fork, you are testing one future at a time.

If your agent needs to call external APIs during the test, set the egress allowlist first. On CreateOS Sandbox you set it at create time through the egress field, live via PUT /v1/sandboxes/{id}/egress, through the SDK's setEgress method, or with createos sandbox firewall set <sandbox> <host>. Rules take hosts, ports, wildcard subdomains, IPs and CIDRs, and enforcement sits in the host kernel with eBPF, outside the guest. If a free tier offers no egress control, your agent's test run can reach anything on the internet, and so can anything the code pulls in.

If you are wiring the sandbox into CI, pick one that restores from snapshot so each build gets a fresh environment without a cold-boot tax. For Jenkins specifically, there is an official plugin, listed on the Jenkins plugin index at plugins.jenkins.io/createos-sandbox, that gives each build a fresh microVM agent destroyed when the build finishes.

If the code will eventually touch customer data or regulated systems, the free tier is where you prove the isolation story to your security team, not where you run the workload. Bring-your-own matters here: CreateOS Sandbox supports your own S3, R2 or MinIO bucket and your own compute, so sessions can stay inside your boundary. Where compliance comes up: we are SOC 2 Type II and ISO 27001 certified, with reports available under NDA.

If you just want to try it this afternoon, start at createos.sh, grab the 500 free credits, and run the code you were about to paste into your terminal. The docs cover the SDK, CLI and MCP paths. Ten minutes tells you more than any comparison table.

Frequently Asked Questions

What can I actually test on a free tier? Real agent output: generated scripts, dependency installs, migrations, tool calls. On CreateOS Sandbox the 500 free credits run full sessions with per-second billing, so a short test costs a fraction of the credit. What you should not test on a free tier is your production workload; that belongs on paid infrastructure, possibly your own.

Is a container sandbox good enough for untrusted AI code? A container shares the host kernel with everything else on the machine. A Firecracker microVM has its own guest kernel, so an escape from the workload stops at the VM boundary. For code you did not write, the per-VM kernel is the baseline, not a luxury.

Do I need to lock down network access before running agent code? Inbound is closed by default. Outbound is open until you set at least one egress rule; once set, everything else is denied, enforced in the host kernel outside the guest. So yes, set the allowlist before you run code whose network behavior you do not trust. That is the whole point of testing it in a sandbox.

What happens when the free credits run out? With per-second billing and $0 egress, the step to paid is small and predictable: you pay for the seconds you run. Check the pricing page before you build your workflow on the free tier, so the transition is a billing event and not a re-architecture.

Conclusion

A free tier sandbox is the cheapest way to find out what agent-written code really does before it touches your repo, your CI, or your users. Judge the free tier on the same things you will depend on later: per-VM kernel isolation, egress control you set yourself, snapshot-fast lifecycle, and a paid step that does not punish you for testing thoroughly. CreateOS Sandbox checks all four, and the 500 free credits are enough to prove it on your own code. Start free at createos.sh/products/sandbox.

About CreateOS CreateOS runs AI agents and the code they write. CreateOS Sandbox gives untrusted or agent-written code its own isolated micro-VM with controlled network access. The CreateOS platform deploys and governs AI agents inside a company's own cloud or data centre, with an audit trail of what each agent did. Start at createos.sh.