Run a JavaScript snippet in a temporary disposable container with optional npm dependencies, then automatically clean up. The code must be valid ESModules (import/export syntax). Ideal for simple one-shot executions without maintaining a sandbox or managing cleanup manually. When reading and writ...
AI agents invoke run_js_ephemeral to trigger actions in Node Code Sandbox MCP. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
This tool executes code in a Node.js environment with the ability to install npm packages and retrieve results. While it runs in an isolated container with automatic cleanup, the core action is code execution—JavaScript snippets run with side effects determined by the code content. An AI agent could execute malicious JavaScript, install malicious packages, or perform unauthorized operations.
From the tool's definition Tool description states: 'Run a JavaScript snippet in a temporary disposable container' and 'valid ESModules (import/export syntax)'.
Attacks that exploit this kind of access
Run a JavaScript snippet in a temporary disposable container with optional npm dependencies, then automatically clean up. The code must be valid ESModules (import/export syntax). Ideal for simple one-shot executions without maintaining a sandbox or managing cleanup manually. When reading and writing from the Node.js processes, you always need to read from and write to the. It is categorised as a Execute tool in the Node Code Sandbox MCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Node Code Sandbox MCP server in PolicyLayer and add a rule for run_js_ephemeral: 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 Node Code Sandbox MCP. Nothing to install.
run_js_ephemeral is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the run_js_ephemeral 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 run_js_ephemeral. 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.
run_js_ephemeral is provided by the Node Code Sandbox MCP server (mozicim/node-code-sandbox-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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