AI agents use write_coil to create or update resources in Modbus — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Modbus environment.
Modbus coils control physical industrial equipment and processes. Writing to coils can change the state of connected devices (pumps, motors, valves, etc.). While not destructive in the data-deletion sense, misuse could cause unintended physical actions. The empty description lowers confidence slightly, but the name and context are sufficiently clear.
From the tool's definition Tool name is 'write_coil' and is part of the Modbus protocol toolset. Modbus coils are binary output registers that control physical devices. The 'write_' prefix indicates a modification operation.
Documented attack patterns abuse exactly the kind of access write_coil gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Modbus, and nothing reaches the server without passing your rules. This is the rule we recommend for write_coil:
{
"version": "1",
"default": "deny",
"tools": {
"write_coil": {
"limits": [
{
"counter": "write_coil_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}write_coil 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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write_coil. It is categorised as a Write tool in the Modbus MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Modbus MCP server in PolicyLayer and add a rule for write_coil: 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 Modbus. Nothing to install.
write_coil 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_coil 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_coil. 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_coil is provided by the Modbus MCP server (kukapay/modbus-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Modbus, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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6 Modbus tools catalogued and risk-classified — across an index of 43,000+ MCP servers.