set_material_instance_scalar · Unreal MCP: Risk & Policy

WHAT IT DOES

What set_material_instance_scalar does on Unreal

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

WHY IT NEEDS A POLICY

Why set_material_instance_scalar needs a policy

The tool modifies material instance parameters, which changes asset properties in the Unreal Engine project. This is a write operation that creates or modifies data reversibly - material parameter changes can be undone through the editor's undo system or by setting different values. It does not execute arbitrary code, delete assets, or cause irreversible changes.

From the tool's definition set_material_instance_scalar - sets a scalar parameter on a material instance, modifying material properties

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_material_instance_scalar Rate-limited

set_material_instance_scalar 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": {
    "set_material_instance_scalar": {
      "limits": [
        {
          "counter": "set_material_instance_scalar_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Unreal, so you can run agents against it safely.

FAQ

Questions about set_material_instance_scalar

What does the set_material_instance_scalar tool do? +

Set a scalar parameter on a material instance. It is categorised as a Write tool in the Unreal MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_material_instance_scalar? +

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

What risk level is set_material_instance_scalar? +

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

Can I rate-limit set_material_instance_scalar? +

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

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

set_material_instance_scalar is provided by the Unreal MCP server (sam-david/unreal-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.