create_material_expression
Add an expression node to a material's graph (e.g., TextureSample, Multiply, Constant3Vector).
This record as markdown: /tools/jjpxdev-unreal-mcp/create-material-expression.md
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.
| Parameter | Type | Required | Description |
|---|---|---|---|
x | number | — | X position in graph |
y | number | — | Y position in graph |
material_path | string | Yes | Material asset path |
expression_class | string | Yes | Expression class (e.g., MaterialExpressionTextureSample, MaterialExpressionMultiply, MaterialExpressionConstant3Vector, MaterialExpressionVectorParameter) |
Parameters from the server's own tool schema.
Why create_material_expression is rated Medium
An AI agent can call create_material_expression faster than any human can review: one bad instruction and it creates or modifies resources in Unreal by the hundred, each call as confident as the last.
Attacks that exploit this kind of access
The rule that runs create_material_expression safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Unreal, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For create_material_expression, this is the rule to start with:
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.
The button opens the PolicyLayer dashboard: create your workspace, connect Unreal, apply this rule, and every create_material_expression call is checked against it from then on.
Questions about create_material_expression
Add an expression node to a material's graph (e.g., TextureSample, Multiply, Constant3Vector). 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.
create_material_expression accepts 4 parameters: x, y, material_path, expression_class. Required: material_path, expression_class. The full parameter table on this page comes from the server's own tool schema.
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.
create_material_expression 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 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.
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.
create_material_expression is provided by the Unreal MCP server (@jjpxdev/unreal-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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