Spawn a new primitive entity (box, sphere, plane, etc.)
AI agents use spawn_entity to create or update resources in Threlte — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Threlte environment.
Spawning new entities is a reversible modification operation (entities can be deleted via destroy_entity or duplicate_entity tools on the same server), making it Write rather than Destructive or Execute. The severity is medium because uncontrolled entity spawning could degrade scene performance, consume resources, or clutter the visualization, but the effect is limited to the 3D scene and reversible.
From the tool's definition The tool description states it will 'Spawn a new primitive entity', which creates new data objects in the Three.js/Threlte scene. This is a create operation that adds new elements to the scene state.
Documented attack patterns abuse exactly the kind of access spawn_entity gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Threlte, and nothing reaches the server without passing your rules. This is the rule we recommend for spawn_entity:
{
"version": "1",
"default": "deny",
"tools": {
"spawn_entity": {
"limits": [
{
"counter": "spawn_entity_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}spawn_entity 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.
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Spawn a new primitive entity (box, sphere, plane, etc.). It is categorised as a Write tool in the Threlte MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Threlte MCP server in PolicyLayer and add a rule for spawn_entity: 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 Threlte. Nothing to install.
spawn_entity 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 spawn_entity 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 spawn_entity. 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.
spawn_entity is provided by the Threlte MCP server (serifeusstudio/threlte-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Threlte, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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30 Threlte tools catalogued and risk-classified — across an index of 43,000+ MCP servers.