evaluate-expression · Debugger MCP Server MCP: Risk & Policy

WHAT IT DOES

What evaluate-expression does on Debugger MCP Server

AI agents invoke evaluate-expression to trigger actions in Debugger MCP Server. 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

This tool executes arbitrary JavaScript code within a debugging call frame context. An AI agent could misuse this to run malicious scripts, access sensitive application state, exfiltrate data, or cause unintended side effects in the running application. Executing arbitrary code in a live browser/application context is a high-severity Execute-category action.

From the tool's definition 'Evaluate JavaScript expressions in the context of a call frame'

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 Debugger MCP Server, so you can run agents against it safely.

FAQ

Questions about evaluate-expression

What does the evaluate-expression tool do? +

Evaluate JavaScript expressions in the context of a call frame. It is categorised as a Execute tool in the Debugger MCP Server 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 Debugger MCP Server 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 Debugger MCP Server. 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 Debugger MCP Server MCP server (phoenixrr2113/debugger-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.