> ## Documentation Index
> Fetch the complete documentation index at: https://bedrockdynamics.studio/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom Models

> Load URDF/SDF models and browse Gazebo Fuel

Substrate supports loading robot and environment models from URDF and SDF files, browsing the Gazebo Fuel model repository, and spawning models directly into a running simulation.

## Loading from File

Open any `.urdf` or `.sdf` file from the file explorer. Substrate parses the model description -- links, joints, visual meshes, and collision geometry -- and renders it in the 3D Viewer panel.

<CardGroup cols={2}>
  <Card title="URDF" icon="robot">
    Universal Robot Description Format. The standard for ROS-based robot descriptions. Substrate renders visual meshes, joint axes, and link frames from the URDF tree.
  </Card>

  <Card title="SDF" icon="globe">
    Simulation Description Format. Used by Gazebo for both robot models and world files. Substrate supports SDF models, lights, and physics properties.
  </Card>
</CardGroup>

Supported mesh formats within model descriptions include STL, DAE (Collada), and OBJ. Mesh file paths are resolved relative to the model file's location.

## Gazebo Fuel Browser

The **Model Browser** panel provides access to the Gazebo Fuel model library, which contains thousands of community-contributed and officially maintained 3D models.

<Steps>
  <Step title="Open the Model Browser">
    Open the Model Browser panel from the left sidebar, or use the Command Palette and search for **"Model Browser"**.
  </Step>

  <Step title="Browse local or remote models">
    By default, the panel displays models from your local Fuel cache. Toggle **Remote** to search the Gazebo Fuel API directly. Type a search query to filter results -- search is debounced so results update as you type.
  </Step>

  <Step title="Spawn into simulation">
    Click any model in the browser to spawn it into the running Gazebo simulation. The model appears at the world origin and is immediately visible in the 3D Viewer.
  </Step>
</Steps>

<Tip>
  Spawned models appear at the world origin. Use the transform gizmo to reposition them -- select the model in the 3D Viewer, then drag the translation handles to move it to the desired location.
</Tip>

## Local Cache

Models downloaded from Gazebo Fuel are cached on disk at `~/.gz/fuel/`. The Model Browser scans this cache directory automatically and displays cached models without requiring a network connection.

This means that models you have previously used in Gazebo -- whether through Substrate or the command line -- are immediately available in the browser.

<AccordionGroup>
  <Accordion title="Cache structure">
    The Fuel cache follows Gazebo's standard directory layout:

    ```
    ~/.gz/fuel/
    └── fuel.gazebosim.org/
        └── <owner>/
            └── models/
                └── <model-name>/
                    └── <version>/
                        ├── model.sdf
                        └── meshes/
    ```

    Each model version is stored independently, and Substrate reads the latest available version by default.
  </Accordion>

  <Accordion title="Clearing the cache">
    To free disk space, you can safely delete directories within `~/.gz/fuel/`. The Model Browser will update its listing on the next scan. Models will be re-downloaded from Fuel if you need them again.
  </Accordion>
</AccordionGroup>

<Note>
  Spawning a model requires an active Gazebo simulation. If no simulation is running, the Model Browser displays models for preview only. Launch a simulation from the SITL Launcher panel to enable spawning.
</Note>