Write content to a file inside the sandbox. Creates parent directories as needed.
AI agents use sandbox_fs_write to create or update resources in Microsandbox — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Microsandbox environment.
This tool creates or modifies files within a sandbox environment. While it performs irreversible overwrites, it operates within an isolated sandbox context rather than the host system, limiting blast radius. It is categorized as Write rather than Destructive because the primary purpose is creation/modification (not deletion), and the sandboxed scope reduces severity compared to host filesystem writes.
From the tool's definition Tool name 'sandbox_fs_write' and description 'Write content to a file inside the sandbox' clearly indicate file creation/modification capability. The phrase 'Creates parent directories as needed' confirms it modifies the sandbox filesystem.
Documented attack patterns abuse exactly the kind of access sandbox_fs_write gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Microsandbox, and nothing reaches the server without passing your rules. This is the rule we recommend for sandbox_fs_write:
{
"version": "1",
"default": "deny",
"tools": {
"sandbox_fs_write": {
"limits": [
{
"counter": "sandbox_fs_write_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}sandbox_fs_write stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.
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Write content to a file inside the sandbox. Creates parent directories as needed. It is categorised as a Write tool in the Microsandbox MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Microsandbox MCP server in PolicyLayer and add a rule for sandbox_fs_write: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Microsandbox. Nothing to install.
sandbox_fs_write is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the sandbox_fs_write rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for sandbox_fs_write. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
sandbox_fs_write is provided by the Microsandbox MCP server (superradcompany/microsandbox-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Microsandbox, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
19 Microsandbox tools catalogued and risk-classified — across an index of 43,000+ MCP servers.