fill_by_selector · Visual Annotation MCP: Risk & Policy

WHAT IT DOES

What fill_by_selector does on Visual Annotation MCP

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

WHY IT NEEDS A POLICY

Why fill_by_selector needs a policy

This tool creates or modifies data (form inputs) on a web page in a reversible manner. It does not execute arbitrary code, delete data, or move money—it simply enters text into form fields.

From the tool's definition The tool name 'fill_by_selector' and description 'Fill a field selected by CSS selector' indicate it modifies form field values. In the context of a browser automation server, 'fill' means to enter or change data in input fields.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fill_by_selector Rate-limited

fill_by_selector 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": {
    "fill_by_selector": {
      "limits": [
        {
          "counter": "fill_by_selector_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 Visual Annotation MCP, so you can run agents against it safely.

FAQ

Questions about fill_by_selector

What does the fill_by_selector tool do? +

Fill a field selected by CSS selector (no inspect id needed). It is categorised as a Write tool in the Visual Annotation MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on fill_by_selector? +

Register the Visual Annotation MCP server in PolicyLayer and add a rule for fill_by_selector: 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 Visual Annotation MCP. Nothing to install.

What risk level is fill_by_selector? +

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

Can I rate-limit fill_by_selector? +

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

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

fill_by_selector is provided by the Visual Annotation MCP server (mstocker1/visual_annotation_mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.