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-Number 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 writes data to a live PLC (Programmable Logic Controller) controlling industrial processes. While categorized as Write (reversible modification), the severity is critical because writing incorrect values to a PLC tag can cause physical harm, equipment damage, or safety incidents in industrial environments.
From the tool's definition 'write a single process tag to the CPU' — directly modifies PLC program variables on an industrial Siemens S7-1500/1200 controller
Documented attack patterns abuse exactly the kind of access PlcProgram-Write-Number 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-Number:
{
"version": "1",
"default": "deny",
"tools": {
"PlcProgram-Write-Number": {
"limits": [
{
"counter": "plcprogram-write-number_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}PlcProgram-Write-Number 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-Number: 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-Number 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-Number 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-Number. 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-Number 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.