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Roblox MCP

Roblox MCP

by sdhzcxnb0
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Local

create_instance

Creates a new Roblox instance such as a Part or Script with optional properties and parent to build or modify the Studio scene.

Instructions

Create a new instance. class_name is a Roblox class such as 'Part', 'Script', 'Model'. properties is an optional JSON object like {"Size": "Vector3(4,1,2)", "Anchored": true}.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "class_name": "<class_name>"
    },
    "name": "create_instance"
  }
}

Input Schema

NameRequiredDescriptionDefault
nameNo
parentNoWorkspace
class_nameYes
propertiesNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

C2.9/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations at all, the description carries the full burden: it does not disclose whether the instance is persisted immediately, whether the created object (or its path) is returned, what happens on an invalid class name, or whether this requires an active Studio session. The only behavioral value added is the string-encoded property format, which is helpful but not the safety/mutation profile this tool needs.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two tight sentences with the action front-loaded and the parameter details following in the order an agent would use them. The property example earns its space; nothing is redundant.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a mutation tool with no annotations and no output schema, the description should say what the caller gets back and how to reference the new instance afterward, plus any session prerequisites. Neither is present, so an agent can invoke it but cannot plan its next step from the definition.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, so the description must compensate, and it does so well for two of four parameters: class_name is illustrated with concrete Roblox classes, and properties is shown as a JSON object with a typed value example ("Vector3(4,1,2)"). It says nothing about name or parent — notably that parent defaults to Workspace and what path syntax it accepts — leaving half the parameters undocumented in both schema and description.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description gives a specific verb and resource ('Create a new instance') and clarifies that the domain is Roblox objects, so the agent knows this builds a class instance rather than a file or script. It does not explicitly distinguish itself from the close sibling insert_model, which is the main ambiguity for a Roblox-savvy agent.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

There is no guidance on when to use this versus insert_model, set_property, or write_script, and no stated prerequisites such as the target parent needing to exist or the studio session needing to be connected. The agent must infer the usage context from the tool name alone.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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