Use the PlcProgram.Write method to write a single process tag to the CPU. To call the PlcProgram.Write method, you need the
AI agents use PlcProgram-Write-Boolean to create or update resources in ThinkPLC-MCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your ThinkPLC-MCP environment.
This tool creates or modifies data on a SIEMENS PLC controller. Writing process tags to a CPU can alter process variables, setpoints, or control logic parameters, potentially affecting physical processes controlled by the PLC. However, it is reversible (a subsequent write can restore previous values), so it is categorized as Write rather than Destructive or Execute.
From the tool's definition Tool description states 'Use the PlcProgram.Write method to write a single process tag to the CPU.' The method explicitly writes data to a PLC controller's memory, which modifies the state of an industrial control system.
Documented attack patterns abuse exactly the kind of access PlcProgram-Write-Boolean gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and ThinkPLC-MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for PlcProgram-Write-Boolean:
{
"version": "1",
"default": "deny",
"tools": {
"PlcProgram-Write-Boolean": {
"limits": [
{
"counter": "plcprogram-write-boolean_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}PlcProgram-Write-Boolean 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.
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Use the PlcProgram.Write method to write a single process tag to the CPU. To call the PlcProgram.Write method, you need the. It is categorised as a Write tool in the ThinkPLC-MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the ThinkPLC- MCP server in PolicyLayer and add a rule for PlcProgram-Write-Boolean: 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 ThinkPLC-MCP. Nothing to install.
PlcProgram-Write-Boolean is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the PlcProgram-Write-Boolean 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.
Set action: deny in the PolicyLayer policy for PlcProgram-Write-Boolean. 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.
PlcProgram-Write-Boolean is provided by the ThinkPLC- MCP server (mrwan84/thinkplc-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from ThinkPLC-MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
23 ThinkPLC-MCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.