write_tag · Ignition MCP: Risk & Policy

WHAT IT DOES

What write_tag does on Ignition

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

WHY IT NEEDS A POLICY

Why write_tag needs a policy

This tool creates or modifies data (tag values in a SCADA system) reversibly. While the description is empty, the name 'write_tag' combined with context (Ignition SCADA platform where tags represent real-time data and control points) clearly indicates Write capability.

From the tool's definition Tool named 'write_tag' on an Ignition SCADA server. 'Write' in the name indicates modification of tag values. SCADA systems control industrial/critical infrastructure; writing tags can alter system state, sensor readings, or control outputs.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

write_tag Rate-limited

write_tag 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_tag": {
      "limits": [
        {
          "counter": "write_tag_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 Ignition, so you can run agents against it safely.

FAQ

Questions about write_tag

What does the write_tag tool do? +

write_tag. It is categorised as a Write tool in the Ignition 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_tag? +

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

What risk level is write_tag? +

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

Can I rate-limit write_tag? +

Yes. Add a rate_limit block to the write_tag 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_tag completely? +

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

write_tag is provided by the Ignition MCP server (nodeblue-ai/ignition-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.