AI agents use update_function to create or update resources in Open WebUI MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Open WebUI MCP Server environment.
This tool modifies existing function definitions including their executable code. While categorized as Write (reversible modification), it carries high severity because updating function code could introduce malicious logic that executes in the platform context, affecting all users who invoke that function.
From the tool's definition Update a function's name or code
Documented attack patterns abuse exactly the kind of access update_function gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Open WebUI MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for update_function:
{
"version": "1",
"default": "deny",
"tools": {
"update_function": {
"limits": [
{
"counter": "update_function_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}update_function 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 a function's name or code. It is categorised as a Write tool in the Open WebUI MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Open WebUI MCP Server MCP server in PolicyLayer and add a rule for update_function: 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 Open WebUI MCP Server. Nothing to install.
update_function 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 update_function 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 update_function. 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.
update_function is provided by the Open WebUI MCP Server MCP server (troylar/open-webui-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Open WebUI MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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82 Open WebUI MCP Server tools catalogued and risk-classified — across an index of 43,000+ MCP servers.