set_variable_value · MCP JS Debugger MCP: Risk & Policy

WHAT IT DOES

What set_variable_value does on MCP JS Debugger

AI agents use set_variable_value to create or update resources in MCP JS Debugger — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCP JS Debugger environment.

WHY IT NEEDS A POLICY

Why set_variable_value needs a policy

This tool writes/mutates program state during a debugging session by changing variable values in a running process. While technically reversible in theory, modifying runtime variables can cause unpredictable application behavior, corrupt state, or lead to security issues.

From the tool's definition Modifies the value of a variable in a specific call frame

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_variable_value Rate-limited

set_variable_value 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "set_variable_value": {
      "limits": [
        {
          "counter": "set_variable_value_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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 set_variable_value

What does the set_variable_value tool do? +

Modifies the value of a variable in a specific call frame. It is categorised as a Write tool in the MCP JS Debugger MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_variable_value? +

Register the MCP JS Debugger MCP server in PolicyLayer and add a rule for set_variable_value: 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 set_variable_value? +

set_variable_value is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit set_variable_value? +

Yes. Add a rate_limit block to the set_variable_value 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 set_variable_value completely? +

Set action: deny in the PolicyLayer policy for set_variable_value. 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 set_variable_value? +

set_variable_value 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.