ha_call_service · Hass MCP: Risk & Policy

WHAT IT DOES

What ha_call_service does on Hass

AI agents invoke ha_call_service to trigger actions in Hass. 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 ha_call_service needs a policy

Calling an arbitrary Home Assistant service can trigger a wide range of actions: turning devices on/off, running automations, unlocking doors, adjusting thermostats, etc. The effect depends entirely on the domain/service and payload provided. This is a general-purpose executor with a large blast radius — an AI agent could misuse it to trigger unintended physical or security-sensitive operations across the smart home.

From the tool's definition 'Call a Home Assistant service (domain/service) with data payload' — triggers arbitrary external operations in Home Assistant depending on arguments

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

ha_call_service Rate-limited

ha_call_service 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": {
    "ha_call_service": {
      "limits": [
        {
          "counter": "ha_call_service_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 Hass, so you can run agents against it safely.

FAQ

Questions about ha_call_service

What does the ha_call_service tool do? +

Call a Home Assistant service (domain/service) with data payload. It is categorised as a Execute tool in the Hass MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on ha_call_service? +

Register the Hass MCP server in PolicyLayer and add a rule for ha_call_service: 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 Hass. Nothing to install.

What risk level is ha_call_service? +

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

Can I rate-limit ha_call_service? +

Yes. Add a rate_limit block to the ha_call_service 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 ha_call_service completely? +

Set action: deny in the PolicyLayer policy for ha_call_service. 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 ha_call_service? +

ha_call_service is provided by the Hass MCP server (thewhykiki/hass-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.