write_variable · Jlink MCP: Risk & Policy

WHAT IT DOES

What write_variable does on Jlink

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

WHY IT NEEDS A POLICY

Why write_variable needs a policy

This tool creates or modifies data (global variable values) in a running microcontroller via JLink debugging interface. It is reversible since the previous value can be written back. While it could affect microcontroller behavior, it does not irreversibly delete data (Destructive), execute arbitrary code (Execute), or move money (Financial).

From the tool's definition Tool name is 'write_variable' and description translates to 'modify the value of global variables by variable name.' This directly modifies data in the microcontroller's memory.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

write_variable Rate-limited

write_variable 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": {
    "write_variable": {
      "limits": [
        {
          "counter": "write_variable_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 Jlink, so you can run agents against it safely.

FAQ

Questions about write_variable

What does the write_variable tool do? +

按变量名修改全局变量的值。. It is categorised as a Write tool in the Jlink MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on write_variable? +

Register the Jlink MCP server in PolicyLayer and add a rule for write_variable: 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 Jlink. Nothing to install.

What risk level is write_variable? +

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

Can I rate-limit write_variable? +

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

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

write_variable is provided by the Jlink MCP server (xun123456/jlink-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.