ui_set_value · UI Bridge MCP: Risk & Policy

WHAT IT DOES

What ui_set_value does on UI Bridge MCP

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

WHY IT NEEDS A POLICY

Why ui_set_value needs a policy

This tool creates or modifies data reversibly by changing input element values in the UI. While it modifies state, the change is reversible (a value can be set again to a different state) and does not delete data or execute arbitrary code, distinguishing it from Destructive or Execute categories.

From the tool's definition Tool name 'ui_set_value' combined with description 'Set the value of an input element directly in the runner' indicates direct modification of UI input state. The word 'set' explicitly describes a write operation that modifies data in the runner's UI.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

ui_set_value Rate-limited

ui_set_value 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": {
    "ui_set_value": {
      "limits": [
        {
          "counter": "ui_set_value_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 UI Bridge MCP, so you can run agents against it safely.

FAQ

Questions about ui_set_value

What does the ui_set_value tool do? +

Set the value of an input element directly in the runner. It is categorised as a Write tool in the UI Bridge 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 ui_set_value? +

Register the UI Bridge MCP server in PolicyLayer and add a rule for ui_set_value: 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 UI Bridge MCP. Nothing to install.

What risk level is ui_set_value? +

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

Can I rate-limit ui_set_value? +

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

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

ui_set_value is provided by the UI Bridge MCP server (qontinui/ui-bridge-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.