set_engine · Voice Bridge MCP: Risk & Policy

WHAT IT DOES

What set_engine does on Voice Bridge

AI agents use set_engine to create or update resources in Voice Bridge — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Voice Bridge environment.

WHY IT NEEDS A POLICY

Why set_engine needs a policy

This tool creates or modifies settings (the default TTS engine selection) in a reversible manner. It does not read data, execute external code, destroy data, or involve financial transactions. The blast radius is minimal—a mistaken engine switch merely changes which voice synthesis backend is used, easily reverted by calling set_engine again with the correct engine name.

From the tool's definition Tool name 'set_engine' indicates it modifies configuration state by switching the default TTS engine. The description states the tool changes which engine is used, a reversible configuration update.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_engine Rate-limited

set_engine 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "set_engine": {
      "limits": [
        {
          "counter": "set_engine_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about set_engine

What does the set_engine tool do? +

Switch the default TTS engine. It is categorised as a Write tool in the Voice Bridge MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_engine? +

Register the Voice Bridge MCP server in PolicyLayer and add a rule for set_engine: 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 Voice Bridge. Nothing to install.

What risk level is set_engine? +

set_engine is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit set_engine? +

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

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

set_engine is provided by the Voice Bridge MCP server (Tomorrow-You/voice-bridge). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.