expose_start · Yaver MCP: Risk & Policy

WHAT IT DOES

What expose_start does on Yaver

AI agents invoke expose_start to trigger actions in Yaver. 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.

PARAMETERS

Parameter	Type	Required	Description
`port`	integer	Yes	Local port to expose
`subdomain`	string	—	Preferred subdomain (best-effort)

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why expose_start needs a policy

expose_start triggers real processes with real consequences. An agent gone sideways doesn't fire it once — it starts dozens of builds, sends mass notifications, or burns through compute before anyone looks up.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

expose_start Rate-limited

expose_start 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": {
    "expose_start": {
      "limits": [
        {
          "counter": "expose_start_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 Yaver, so you can run agents against it safely.

FAQ

Questions about expose_start

What does the expose_start tool do? +

Expose a local port to the internet. Replaces ngrok ($10/mo). Uses Cloudflare Quick Tunnel (free, zero config). It is categorised as a Execute tool in the Yaver MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

What parameters does expose_start accept? +

expose_start accepts 2 parameters: port, subdomain. Required: port. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on expose_start? +

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

What risk level is expose_start? +

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

Can I rate-limit expose_start? +

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

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

expose_start is provided by the Yaver MCP server (yaver-cli). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.