create_material_expression · Unreal MCP: Risk & Policy

WHAT IT DOES

What create_material_expression does on Unreal

AI agents use create_material_expression 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 create_material_expression needs a policy

This tool creates new nodes within a material asset, which is a reversible write operation. It modifies data (material definitions) but does not delete, execute arbitrary code, move funds, or have destructive side effects. The impact is scoped to material asset structure.

From the tool's definition The tool 'create_material_expression' adds (creates) an expression node to a material, as stated in the description 'Add an expression node to a material'. This is a create/write operation that modifies material assets.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

create_material_expression Rate-limited

create_material_expression 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": {
    "create_material_expression": {
      "limits": [
        {
          "counter": "create_material_expression_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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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 create_material_expression

What does the create_material_expression tool do? +

Add an expression node to a material. 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 create_material_expression? +

Register the Unreal MCP server in PolicyLayer and add a rule for create_material_expression: 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 create_material_expression? +

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

Can I rate-limit create_material_expression? +

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

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

create_material_expression 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.