Updates tags, based on the provided TagValueInput list. If a TagValueInput does not define a specific timestamp, the optional timestamp parameter will be used as a fallback. If the optional timestamp parameter is not set, the current time will be used instead. Sample timestamp:
AI agents use write-tag-values to create or update resources in WinCC Unified MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your WinCC Unified MCP Server environment.
This tool modifies tag values in a SIEMENS WinCC Unified SCADA system, which are industrial control system parameters. While the modification is reversible (tags can be updated again), the blast radius is high because incorrect tag value updates in a SCADA system could disrupt industrial processes, equipment, or safety-critical operations.
From the tool's definition Tool name 'write-tag-values' and description 'Updates tags' indicates creation/modification of data. The description explicitly states it 'Updates tags, based on the provided TagValueInput list,' confirming reversible data modification.
Documented attack patterns abuse exactly the kind of access write-tag-values gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and WinCC Unified MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for write-tag-values:
{
"version": "1",
"default": "deny",
"tools": {
"write-tag-values": {
"limits": [
{
"counter": "write-tag-values_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}write-tag-values 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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Updates tags, based on the provided TagValueInput list. If a TagValueInput does not define a specific timestamp, the optional timestamp parameter will be used as a fallback. If the optional timestamp parameter is not set, the current time will be used instead. Sample timestamp:. It is categorised as a Write tool in the WinCC Unified MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the WinCC Unified MCP Server MCP server in PolicyLayer and add a rule for write-tag-values: 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 WinCC Unified MCP Server. Nothing to install.
write-tag-values 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 write-tag-values 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 write-tag-values. 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.
write-tag-values is provided by the WinCC Unified MCP Server MCP server (vogler75/winccua-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from WinCC Unified MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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9 WinCC Unified MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.