evaluate_script
Evaluate a JavaScript function inside the currently selected page. Returns the response as JSON so returned values have to JSON-serializable.
This record as markdown: /tools/1600822305-js-reverse-mcp/evaluate-script.md
What evaluate_script does on Js Reverse
AI agents invoke evaluate_script to trigger actions in Js Reverse. 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.
Why evaluate_script is rated High
This tool executes arbitrary JavaScript in the context of a live browser page. An AI agent could use it to exfiltrate data, manipulate DOM, make network requests, access cookies/credentials, or perform any action possible in a browser context. This is essentially unrestricted code execution with critical blast radius.
From the tool's definition "Evaluate a JavaScript function inside the currently selected page"
Attacks that exploit this kind of access
The rule that runs evaluate_script safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Js Reverse, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For evaluate_script, this is the rule to start with:
evaluate_script stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Js Reverse, apply this rule, and every evaluate_script call is checked against it from then on.
Questions about evaluate_script
Evaluate a JavaScript function inside the currently selected page. Returns the response as JSON so returned values have to JSON-serializable. It is categorised as a Execute tool in the Js Reverse MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Js Reverse MCP server in PolicyLayer and add a rule for evaluate_script: 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 Js Reverse. Nothing to install.
evaluate_script 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 evaluate_script 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 evaluate_script. 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.
evaluate_script is provided by the Js Reverse MCP server (1600822305/js-reverse-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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