AI agents invoke subscribe_to_events to trigger actions in Mcp Freecad. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
Subscribing to events triggers an ongoing external operation that establishes a persistent listener/callback mechanism within FreeCAD. This goes beyond passive reading — it registers a handler that will react to future events, which is an active operation with side effects. It falls under Execute as it triggers external operations whose effects depend on arguments (which events to subscribe to).
From the tool's definition Subscribe to FreeCAD events
Documented attack patterns abuse exactly the kind of access subscribe_to_events gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Mcp Freecad, and nothing reaches the server without passing your rules. This is the rule we recommend for subscribe_to_events:
{
"version": "1",
"default": "deny",
"tools": {
"subscribe_to_events": {
"limits": [
{
"counter": "subscribe_to_events_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}subscribe_to_events stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
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Subscribe to FreeCAD events. It is categorised as a Execute tool in the Mcp Freecad MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Mcp Freecad MCP server in PolicyLayer and add a rule for subscribe_to_events: 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 Mcp Freecad. Nothing to install.
subscribe_to_events is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the subscribe_to_events 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 subscribe_to_events. 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.
subscribe_to_events is provided by the Mcp Freecad MCP server (jango-blockchained/mcp-freecad). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Mcp Freecad, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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