evaluate_expression · MCP JS Debugger MCP: Risk & Policy

WHAT IT DOES

What evaluate_expression does on MCP JS Debugger

AI agents invoke evaluate_expression to trigger actions in MCP JS Debugger. 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 IT NEEDS A POLICY

Why evaluate_expression needs a policy

Evaluating arbitrary JavaScript expressions in a live debugging context can execute any code — read files, modify variables, call functions, trigger network requests, or cause side effects — making this an Execute-level tool with high severity since a misused expression could affect the running application or environment in unpredictable ways.

From the tool's definition Evaluates a JavaScript expression in the context of a specific call frame or global context

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

evaluate_expression Rate-limited

evaluate_expression 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "evaluate_expression": {
      "limits": [
        {
          "counter": "evaluate_expression_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches MCP JS Debugger, so you can run agents against it safely.

FAQ

Questions about evaluate_expression

What does the evaluate_expression tool do? +

Evaluates a JavaScript expression in the context of a specific call frame or global context. It is categorised as a Execute tool in the MCP JS Debugger MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on evaluate_expression? +

Register the MCP JS Debugger MCP server in PolicyLayer and add a rule for evaluate_expression: 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 MCP JS Debugger. Nothing to install.

What risk level is evaluate_expression? +

evaluate_expression is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit evaluate_expression? +

Yes. Add a rate_limit block to the evaluate_expression 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.

How do I block evaluate_expression completely? +

Set action: deny in the PolicyLayer policy for evaluate_expression. 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.

What MCP server provides evaluate_expression? +

evaluate_expression is provided by the MCP JS Debugger MCP server (johngrimes/mcp-js-debugger). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.