browser_fill · Claude Flow MCP: Risk & Policy

WHAT IT DOES

What browser_fill does on Claude Flow

AI agents invoke browser_fill to trigger actions in Claude Flow. 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 browser_fill needs a policy

This tool performs browser automation by clearing and filling input elements in a live browser session. It can interact with login forms, manipulate DOM inputs, and drive cookie-authenticated sessions. This constitutes execution of external browser operations whose effects depend on arguments — for example, filling credentials into login forms, submitting data, or manipulating SPA state.

From the tool's definition 'Clear and fill an input element' combined with 'real browser automation', 'login flows with cookie reuse', and 'JS-heavy SPA scraping'

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

browser_fill Rate-limited

browser_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": {
    "browser_fill": {
      "limits": [
        {
          "counter": "browser_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 Claude Flow, so you can run agents against it safely.

FAQ

Questions about browser_fill

What does the browser_fill tool do? +

Clear and fill an input element Use when native WebFetch is wrong because you need real browser automation — JS-heavy SPA scraping, login flows with cookie reuse, replay against DOM-drifted versions, AIDefence PII gating before content reaches Claude. For static HTML pages, native WebFetch is faster and free. It is categorised as a Execute tool in the Claude Flow MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on browser_fill? +

Register the Claude Flow MCP server in PolicyLayer and add a rule for browser_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 Claude Flow. Nothing to install.

What risk level is browser_fill? +

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

Can I rate-limit browser_fill? +

Yes. Add a rate_limit block to the browser_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 browser_fill completely? +

Set action: deny in the PolicyLayer policy for browser_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 browser_fill? +

browser_fill is provided by the Claude Flow MCP server (claude-flow). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.