openpilot is a full-stack autonomous driver assistance system that runs on embedded Linux hardware (comma four/three) and controls 300+ car models. It is a safety-critical, real-time system written primarily in Python and C/C++, with a custom messaging layer (cereal/msgq) and a Cap'n Proto-based schema for all inter-process communication. The architecture is a collection of independent processes (controlsd, plannerd, radard, modeld, locationd, calibrationd, dmonitoringd, etc.) communicating via a pub/sub message queue, managed by a central process manager. The system ingests CAN bus data, camera streams (road, wide-road, driver), IMU/GPS sensor data, and runs neural network models (ONNX/tinygrad) for perception and planning. The codebase demonstrates strong engineering discipline: a comprehensive process replay regression testing framework that replays logged routes through individual processes and compares outputs bit-for-bit; a safety model implemented in the panda firmware (separate submodule) with ISO 26262 guidelines; real-time scheduling and CPU affinity management across processes; and a sophisticated build system using SCons with Cython compilation. The UI is built on raylib (with a Qt-based Cabana tool for CAN analysis), and there's a MetaDrive simulator bridge for development. Key architectural patterns include: a generic car interface abstraction (opendbc) that allows supporting 300+ car models through fingerprinting and parameter configuration; a model-driven control pipeline where neural network outputs (desired curvature, lead vehicle trajectories) feed into lateral controllers (PID, torque-based, curvature-based, angle-based) and longitudinal MPC planners; an EKF-based sensor fusion system (locationd) combining GPS, IMU, and vehicle odometry; and a driver monitoring system using a separate neural network for distraction detection. The system uses a parameter store (params) backed by a filesystem for persistent configuration. The repository includes extensive tooling: replay infrastructure for regression testing, PlotJuggler/Cabana integration for data visualization, automated car porting tools with fuzzy fingerprinting, release automation with stripped builds, and a comprehensive CI/CD pipeline with GitHub Actions. The codebase is actively maintained with daily commits, a structured release process (release-mici/release-tizi branches), and active community contribution through Discord and GitHub issues.