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mujoco-mcp

by Lecoeurdelest
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Python

mujoco_load_model

Load a MuJoCo model into a new simulation session using a sample, file path, or raw MJCF XML. Get a session ID plus joint, actuator, and sensor names for stepping and controlling the simulation.

Instructions

Load a MuJoCo model into a new simulation session. Provide exactly ONE of sample | file_path | xml_string.

Returns JSON: {session_id, source, nq, nv, nu, nsensor, nkey, timestep,
joints: [names], actuators: [names], sensors: [names], keyframes: [names]}.
The name arrays define the element order of qpos/qvel/ctrl vectors used by every other tool.
Next: mujoco_step to advance physics, mujoco_get_state to read, mujoco_model_info for ranges/types.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
sampleNoBundled sample scene: pendulum, cartpole, arm2, box_drop
file_pathNoPath to an MJCF .xml (or compiled .mjb) on the machine running this server, e.g. a mujoco_menagerie scene.xml
session_idNoExplicit session id (letters, digits, '_', '-', '.'); auto-generated when omitted
xml_stringNoRaw MJCF XML document

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
resultYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4.5/5.0
Behavior4/5

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

The annotations are uninformative; the description carries the burden. It discloses the new-session side effect, the exclusive-input requirement, the full JSON return shape, and the crucial fact that the returned name arrays define qpos/qvel/ctrl ordering for all other tools. It does not discuss failure/validation behavior or repeated-call consequences, but the provided behavioral context is strong.

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

Conciseness5/5

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

Every sentence earns its place: core action, input constraint, return contract, ordering implication, and next-step routing. It is front-loaded with the core purpose and keeps the detail compact.

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

Completeness5/5

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

The description is complete for an agent to call the tool successfully: it states what to provide, what comes back, how to interpret the returned arrays, and which tools to use next. The richer return details also compensate for any ambiguity in the schema-only view.

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

Parameters4/5

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

Schema coverage is 100%, so the schema already documents all four parameters. The description adds the essential constraint "exactly ONE of sample | file_path | xml_string," which is not captured by the optional/required schema fields. It also explains the downstream meaning of the returned names in terms of vector ordering.

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

Purpose5/5

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

States a clear verb and resource: "Load a MuJoCo model into a new simulation session." The phrase "new simulation session" distinguishes this from state-restoration and inspection siblings, and the explicit source options add operational precision.

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

Usage Guidelines4/5

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

The description gives clear contextual guidance: use it to create a new session, and exactly one of sample, file_path, or xml_string must be supplied. It also routes the agent to subsequent tools. It does not explicitly say when to prefer this over mujoco_restore_snapshot, but the "new simulation session" wording covers the main distinction.

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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