tap_id · App Screen MCP: Risk & Policy

WHAT IT DOES

What tap_id does on App Screen

AI agents invoke tap_id to trigger actions in App Screen. 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 tap_id needs a policy

Tapping a UI element triggers an interaction in the simulator that can cause side effects depending on what is tapped (e.g., submitting forms, navigating, deleting data). This is an Execute-level action as it performs a browser/UI action whose effects depend on the target element. Severity is medium because the blast radius is bounded to the simulator context, though misuse could trigger significant in-app actions.

From the tool's definition "Find a UI element by its accessibility identifier and tap its center" — performs a tap action on a UI element in an iOS Simulator

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

tap_id Rate-limited

tap_id 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": {
    "tap_id": {
      "limits": [
        {
          "counter": "tap_id_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 App Screen, so you can run agents against it safely.

FAQ

Questions about tap_id

What does the tap_id tool do? +

Find a UI element by its accessibility identifier and tap its center. It is categorised as a Execute tool in the App Screen MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on tap_id? +

Register the App Screen MCP server in PolicyLayer and add a rule for tap_id: 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 App Screen. Nothing to install.

What risk level is tap_id? +

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

Can I rate-limit tap_id? +

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

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

tap_id is provided by the App Screen MCP server (xmuweili/app-screen-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.