write_device · Kc Modbus MCP: Risk & Policy

WHAT IT DOES

What write_device does on Kc Modbus

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

WHY IT NEEDS A POLICY

Why write_device needs a policy

This tool creates or modifies data on industrial control systems (PLC registers) reversibly. While the action is reversible (unlike Destructive), it directly alters device state in a production environment, presenting high risk if an AI agent writes incorrect values to critical registers (e.g., valve positions, motor speeds, setpoints).

From the tool's definition Tool name 'write_device' and description 'Write a value to a named register on a device' explicitly indicate the tool modifies device state by writing to PLC registers via Modbus TCP.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

write_device Rate-limited

write_device 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_device": {
      "limits": [
        {
          "counter": "write_device_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 Kc Modbus, so you can run agents against it safely.

FAQ

Questions about write_device

What does the write_device tool do? +

Write a value to a named register on a device. It is categorised as a Write tool in the Kc Modbus 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_device? +

Register the Kc Modbus MCP server in PolicyLayer and add a rule for write_device: 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 Kc Modbus. Nothing to install.

What risk level is write_device? +

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

Can I rate-limit write_device? +

Yes. Add a rate_limit block to the write_device 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_device completely? +

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

write_device is provided by the Kc Modbus MCP server (kerberosclaw/kc_modbus_mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.