visual_compare · React Native MCP: Risk & Policy

WHAT IT DOES

What visual_compare does on React Native

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

WHY IT NEEDS A POLICY

Why visual_compare needs a policy

The primary operation is a visual regression check (Read/Execute), but the tool can also 'update baseline', which writes/overwrites the baseline PNG file. Since updating a baseline is a reversible write operation (overwriting a file), Write is the most severe applicable category. Misuse could silently update baselines to incorrect states, masking UI regressions.

From the tool's definition 'capture current screen... pixel-diff against baseline PNG... or to update baseline'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

visual_compare Rate-limited

visual_compare 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": {
    "visual_compare": {
      "limits": [
        {
          "counter": "visual_compare_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 React Native, so you can run agents against it safely.

FAQ

Questions about visual_compare

What does the visual_compare tool do? +

Visual regression: capture current screen (or crop by selector), pixel-diff against baseline PNG; use to verify UI didn’t change or to update baseline. It is categorised as a Write tool in the React Native MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on visual_compare? +

Register the React Native MCP server in PolicyLayer and add a rule for visual_compare: 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 React Native. Nothing to install.

What risk level is visual_compare? +

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

Can I rate-limit visual_compare? +

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

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

visual_compare is provided by the React Native MCP server (@ohah/react-native-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.