fill · WebControl MCP: Risk & Policy

WHAT IT DOES

What fill does on WebControl

AI agents invoke fill to trigger actions in WebControl. 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 fill needs a policy

Filling a form field is a browser action that triggers external operations — it modifies the state of a live web page, potentially submitting data or triggering dynamic UI changes. This falls under Execute as it interacts with external systems through browser automation. The blast radius is medium because it can submit sensitive data or trigger workflows, but on its own it doesn't delete or move money.

From the tool's definition Fill a form field by its ref ID with the given value. Returns updated page content.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fill Rate-limited

fill 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": {
    "fill": {
      "limits": [
        {
          "counter": "fill_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 WebControl, so you can run agents against it safely.

FAQ

Questions about fill

What does the fill tool do? +

Fill a form field by its ref ID with the given value. Returns updated page content. It is categorised as a Execute tool in the WebControl MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on fill? +

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

What risk level is fill? +

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

Can I rate-limit fill? +

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

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

fill is provided by the WebControl MCP server (matansht/webcontrol). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.