set_expression

Set a facial morph (blendshape) on a skinned model — e.g. smile, frown, blink. Call with NO morph to list the model

SERVERSbox SOURCEsbox-mcp-server

Medium RISK CLASS

Category Write

Parameters 00 required

Recommended Rate-limitedsee the rule below

Registry record Grade F, identity unverified Pull the record →

This record as markdown: /tools/sbox/set-expression.md

WHAT IT DOES

What set_expression does on Sbox

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

RISK

Why set_expression is rated Medium

This tool modifies game object properties (facial expressions on skinned models) in a reversible manner. It creates or updates blendshape values but does not execute arbitrary code, delete data permanently, or move money. The blast radius is limited to visual state changes on a specific model within the game world.

From the tool's definition Tool description states 'Set a facial morph (blendshape) on a skinned model' — this is a modification operation that alters the state of a game object's animation/appearance properties. The term 'Set' indicates a write/update action rather than retrieval.

Attacks that exploit this kind of access

RECOMMENDED RULE

The rule that runs set_expression safely

PolicyLayer is an MCP gateway: it sits between your AI agents and Sbox, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For set_expression, this is the rule to start with:

set_expression Rate-limited

set_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": {
    "set_expression": {
      "limits": [
        {
          "counter": "set_expression_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL → Instant setup, no code required.

The button opens the PolicyLayer dashboard: create your workspace, connect Sbox, apply this rule, and every set_expression call is checked against it from then on.

FAQ

Questions about set_expression

What does the set_expression tool do? +

Set a facial morph (blendshape) on a skinned model — e.g. smile, frown, blink. Call with NO morph to list the model. It is categorised as a Write tool in the Sbox 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_expression? +

Register the Sbox MCP server in PolicyLayer and add a rule for set_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 Sbox. Nothing to install.

What risk level is set_expression? +

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

Can I rate-limit set_expression? +

Yes. Add a rate_limit block to the set_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 set_expression completely? +

Set action: deny in the PolicyLayer policy for set_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 set_expression? +

set_expression is provided by the Sbox MCP server (sbox-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

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