Update existing userscripts
AI agents use extension_tool_userscripts_update to create or update resources in WebMCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your WebMCP environment.
This tool modifies existing userscripts, which are JavaScript code that executes in the browser with potentially broad access to web page content and APIs. While updates are theoretically reversible (unlike deletion), they alter persistent executable code that could affect browser behavior, security posture, and user data access.
From the tool's definition Tool name 'extension_tool_userscripts_update' and description 'Update existing userscripts' indicate modification of existing data/scripts. Userscripts are persistent code executed in the browser environment.
Documented attack patterns abuse exactly the kind of access extension_tool_userscripts_update gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and WebMCP, and nothing reaches the server without passing your rules. This is the rule we recommend for extension_tool_userscripts_update:
{
"version": "1",
"default": "deny",
"tools": {
"extension_tool_userscripts_update": {
"limits": [
{
"counter": "extension_tool_userscripts_update_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}extension_tool_userscripts_update 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.
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Update existing userscripts. It is categorised as a Write tool in the WebMCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Web MCP server in PolicyLayer and add a rule for extension_tool_userscripts_update: 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 WebMCP. Nothing to install.
extension_tool_userscripts_update is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the extension_tool_userscripts_update 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.
Set action: deny in the PolicyLayer policy for extension_tool_userscripts_update. 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.
extension_tool_userscripts_update is provided by the Web MCP server (miguelspizza/webmcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from WebMCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
13 WebMCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.