set_material_instance_scalar
Set a scalar parameter on a material instance.
This record as markdown: /tools/jjpxdev-unreal-mcp/set-material-instance-scalar.md
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.
| Parameter | Type | Required | Description |
|---|---|---|---|
value | number | Yes | Parameter value |
instance_path | string | Yes | Material instance asset path |
parameter_name | string | Yes | Scalar parameter name |
Parameters from the server's own tool schema.
Why set_material_instance_scalar is rated Medium
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
The rule that runs set_material_instance_scalar 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 set_material_instance_scalar, this is the rule to start with:
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.
The button opens the PolicyLayer dashboard: create your workspace, connect Unreal, apply this rule, and every set_material_instance_scalar call is checked against it from then on.
Questions about set_material_instance_scalar
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.
set_material_instance_scalar accepts 3 parameters: value, instance_path, parameter_name. Required: value, instance_path, parameter_name. 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 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.
set_material_instance_scalar 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 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.
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.
set_material_instance_scalar 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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