call_ui_event · HC3 MCP Server MCP: Risk & Policy

WHAT IT DOES

What call_ui_event does on HC3 MCP Server

AI agents invoke call_ui_event to trigger actions in HC3 MCP Server. 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.

WHY IT NEEDS A POLICY

Why call_ui_event needs a policy

This tool executes external operations (UI event triggers) whose consequences are argument-dependent. While not inherently destructive, it can invoke side effects through plugin interface elements controlling physical devices or system settings in a smart home.

From the tool's definition Tool description states it 'Trigger[s] UI events on plugin interface elements (buttons, sliders, switches, etc.)' - this invokes actions on smart home interface components whose effects depend on which specific UI elements are triggered.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

call_ui_event Rate-limited

call_ui_event 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "call_ui_event": {
      "limits": [
        {
          "counter": "call_ui_event_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches HC3 MCP Server, so you can run agents against it safely.

FAQ

Questions about call_ui_event

What does the call_ui_event tool do? +

Trigger UI events on plugin interface elements (buttons, sliders, switches, etc.). It is categorised as a Execute tool in the HC3 MCP Server MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on call_ui_event? +

Register the HC3 MCP Server MCP server in PolicyLayer and add a rule for call_ui_event: 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 HC3 MCP Server. Nothing to install.

What risk level is call_ui_event? +

call_ui_event is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit call_ui_event? +

Yes. Add a rate_limit block to the call_ui_event 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.

How do I block call_ui_event completely? +

Set action: deny in the PolicyLayer policy for call_ui_event. 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.

What MCP server provides call_ui_event? +

call_ui_event is provided by the HC3 MCP Server MCP server (jangabrielsson/hc3_mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.