> For the complete documentation index, see [llms.txt](https://docs.hello-robot.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hello-robot.com/stretch4-body-repo/readme.md).

# Introduction

The `stretch4_body` repository contains the core Python software stack that allows developers to interact with the hardware of Stretch 4 robots. The repository for Stretch 3 and below can be found in the [stretch\_body](https://github.com/hello-robot/stretch_body) repo. This repo provides a robust, soft real-time capable framework for managing low-level motor communication, subsystem coordination, autonomous behaviors, and a high-level API for user applications. This repository is intended to be imported by other code that needs access to these features.

This package can be installed by:

```
python3 -m pip install -U hello-robot-stretch4-body
```

## Architecture Block Diagram

At its heart, the architecture is built around a Client-Server model. A dedicated `RobotServer` runs as a background daemon managing the physical hardware at 100Hz, executing safety monitoring, self-collision detection, and hardware command multiplexing. Developers build their applications using the `RobotClient`, which asynchronously communicates with the server over ZeroMQ. This decouples user scripts from strict hardware timing constraints and allows for safe, concurrent control of the robot.

```mermaid
graph TD
    ClientCode["User Application RobotClient"]
    Server["Robot Server 100Hz Loop"]

    Subsystems["Hardware Subsystems"]
    Arm
    Lift
    Omnibase
    PowerPeriph
    EndOfArm

    Behaviors["Behaviors"]
    Sentries["Sentries Safety Monitors"]
    SafeMotions["Safe Motions Collision Avoidance"]
    Routines["Routines Autonomous Actions"]

    Workers["Background Workers"]
    LineSensorLoop["Line Sensor Loop"]
    CollisionLoop["Self Collision Loop"]
    EOALoop["End Of Arm Loop"]

    ClientCode -->|ZeroMQ Commands and Status| Server

    Server --> Behaviors
    Behaviors --> Sentries
    Behaviors --> SafeMotions
    Behaviors --> Routines

    Server --> Subsystems
    Subsystems --> Arm
    Subsystems --> Lift
    Subsystems --> Omnibase
    Subsystems --> PowerPeriph
    Subsystems --> EndOfArm

    Server --> Workers
    Workers --> LineSensorLoop
    Workers --> CollisionLoop
    Workers --> EOALoop
```

## Technical Primers

For an in-depth understanding of how specific parts of the system are designed, refer to the following technical primers:

| Primer                                                                                  | Description                                                                                                 |
| --------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------- |
| [Core Architecture](/stretch4-body-repo/core-framework/primer_core.md)                  | Maps out the foundational classes, IPC communication, and file organization of the core library.            |
| [Robot Parameters](/stretch4-body-repo/core-framework/primer_robot_params.md)           | Explains the multi-layered parameter system (default vs user) and dynamic runtime generation.               |
| [Robot Client API](/stretch4-body-repo/core-framework/primer_robot_client.md)           | A guide to using the RobotClient API for reading status and commanding motion asynchronously.               |
| [Hardware Subsystems](/stretch4-body-repo/subsystems-and-hardware/primer_subsystems.md) | Overview of the primary hardware abstractions (Arm, Lift, Base) and how they are instantiated.              |
| [End-Of-Arm EOA](/stretch4-body-repo/subsystems-and-hardware/primer_end_of_arm.md)      | Details the dynamically instantiated, multi-process architecture for interchangeable tool attachments.      |
| [Line Sensors](/stretch4-body-repo/subsystems-and-hardware/primer_line_sensor.md)       | Details the operation and background processing for the downward-facing Pixart line sensors.                |
| [Server Behaviors](/stretch4-body-repo/behaviors-and-safety/primer_behaviors.md)        | Explains the plugin architecture for Sentries, Safe Motions, and Routines within the 100Hz server loop.     |
| [Self-Collision](/stretch4-body-repo/behaviors-and-safety/primer_self_collision.md)     | Details the MuJoCo-based collision checking system, its background loop, and configuration parameters.      |
| [Gamepad Teleop](/stretch4-body-repo/teleoperation/primer_gamepad_teleop.md)            | Explains how different control schemes can be mapped onto a standard gamepad controller + how to extend it. |
| [Cameras](/stretch4-body-repo/subsystems-and-hardware/primer_cameras.md)                | A guide to the cameras on Stretch 4's head and wrist, with an overview of the CLIs and API.                 |

## Installation

1. `pip3 install -e .`
2. `stretch_body_server --launch`

*Note: The C++ shared libraries for `transport` and `SCSerial` will compile automatically via Meson during the `pip install`.*

If you want to install the object detection dependencies:

```bash
pip3 install -e .[object_detection]
```

### Troubleshooting Editable Installs

If you make a C++ syntax error or typo in the source files and attempt to run a command while in editable mode (e.g., launching `stretch_body_server`), you may encounter an obscure Python exception instead of the actual C++ compiler error message:

```
subprocess.CalledProcessError: Command '['ninja']' returned non-zero exit status 1.
```

Because `meson-python` editable builds run quietly in the background on import, it drops the standard output of the C++ compiler natively, hiding your C++ syntax error. To see the actual compiler output and locate the line where C++ failed, prepend your command with the verbose flag:

```bash
MESONPY_EDITABLE_VERBOSE=1 stretch_body_server --launch
```


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