Do not give a server a write-capable API key just to find out whether it is safe. Evaluate it first in a shadow environment: use test data, keep powerful credentials outside its reach, and make read-only access an enforced permission rather than a request. A sandbox helps only when its actual filesystem, network, credential, and tool boundaries are constrained.
What a shadow evaluation is—and what it can establish
A shadow evaluation lets you examine a server’s behavior without allowing it to make consequential changes. Treat code, plugins, uploaded files, and external content as potentially hostile. Chromium’s sandbox design guidance says threat models should assume sandboxed code is malicious once it has received external input: Chromium sandbox design.
This is a way to limit exposure, not a guarantee that a service is secure. The available documentation describes general controls and product capabilities; it does not independently audit any particular free server or configuration. “Sandbox” is not enough information to judge the boundary: inspect what the service can read, change, and contact.
Run the evaluation without a write key
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Keep consequential credentials outside the boundary
Start with no write-capable key in the server, its agent environment, mounted files, logs, or tool configuration. The Unified Harness Protocol says provider credentials should not be placed where agent tools can read them. If a credential is essential to a meaningful test, use one that is temporary, limited to a single session, and independently revocable: Unified Harness Protocol security guidance.
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Use test data and a controlled workspace
Evaluate against test data in a separate environment, and expose only the files the test needs. Prefer an isolated clone or a read-only mount where available; verify the effective mode rather than relying on a label. Docker documents that a direct workspace mount is read-write, so edits made in the container are visible in the host working tree: Docker sandbox security. Its documentation also describes separate controls for mounts and network access: Docker sandbox networking.
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Make read-only access a real permission
If the task is meant to be read-only, enforce that at the filesystem or account level—for example, with a read-only mount or a user lacking write permission. The Unified Harness Protocol explicitly rejects relying on an instruction not to write; the server should make the write fail. This distinction matters even when a task prompt or interface says “read only.”
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Default to no outbound network access
Deny egress initially, then allow only destinations required for the evaluation. Check whether the server can reach an API endpoint with credentials attached or send data elsewhere. Cloudflare’s sandbox overview explains that the application decides which APIs and data code receives and whether it can reach the public internet: Cloudflare Sandbox documentation. Docker documents policy-controlled outbound TCP, but a policy’s existence does not tell you which destinations your particular setup permits.
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Minimize tools, plugins, and integrations
Give the task only the tools it needs, and keep untrusted-input work separate from harnesses holding privileged tools. A plugin brings its author into the trust boundary. A local MCP process may also run outside the sandbox, depending on configuration, so verify where it runs and what it can access.
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Check stored data and session access
Find out who can access sessions and artifacts, how long they persist, and what deletion actually removes or makes unreachable. If data is shared, check whether access is read-only and can be revoked. Do not treat the end of a session as proof that its files, logs, or outputs have disappeared.
Compare servers by their effective boundaries
When choosing between services, compare documented and testable controls—not a general claim that a product is secure or sandboxed.
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| Boundary | What to verify |
|---|---|
| Credentials | Can the execution process read the raw secret? Is credential injection brokered? Can a test credential be revoked independently? |
| Files | Is the host workspace unavailable, read-only, a private clone, or directly mounted read-write? Which artifacts remain after a session? |
| Network | Is egress off by default? Are permitted destinations narrow and inspectable? Can requests be brokered with secrets attached? |
| Tools and plugins | Can you restrict the task to necessary tools? Do plugins and local MCP servers run within the same isolation boundary? |
| Sessions and tenants | Are sessions and artifacts scoped to the right owner? Are access and deletion isolated between users? |
| Operations | Are duration limits, upload-size limits, rate limits, logs, and revocation documented and testable? |
When to consider write access
Consider it only after reviewing the effective permissions and the evaluation’s results. If write access is justified, grant the minimum required scope for the shortest useful duration, preserve an independent way to revoke it, and protect production changes with review and branch controls. A successful test in a constrained environment does not establish that a different configuration—or a free server with unknown controls—has the same protections.
Product availability also matters: Cloudflare’s overview says its sandbox feature is available on a Workers Paid plan. That documentation does not establish the described sandbox feature as free.
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