Robot applications, comprising independent components that mutually publish/subscribe messages, are built on inter-process communication (IPC) middleware such as Robot Operating System 2 (ROS 2). In large-scale ROS 2 systems like autonomous driving platforms, true zero-copy communication -- eliminating serialization and deserialization -- is crucial for efficiency and real-time performance. However, existing true zero-copy middleware solutions lack widespread adoption as they fail to meet three essential requirements: 1) Support for all ROS 2 message types including unsized ones; 2) Minimal modifications to existing application code; 3) Selective implementation of zero-copy communication between specific nodes while maintaining conventional communication mechanisms for other inter-node communications including inter-host node communications. This first requirement is critical, as production-grade ROS 2 projects like Autoware rely heavily on unsized message types throughout their codebase to handle diverse use cases (e.g., various sensors), and depend on the broader ROS 2 ecosystem, where unsized message types are pervasive in libraries. The remaining requirements facilitate seamless integration with existing projects. While IceOryx middleware, a practical true zero-copy solution, meets all but the first requirement, other studies achieving the first requirement fail to satisfy the remaining criteria. This paper presents Agnocast, a true zero-copy IPC framework applicable to ROS 2 C++ on Linux that fulfills all these requirements. Our evaluation demonstrates that Agnocast maintains constant IPC overhead regardless of message size, even for unsized message types. In Autoware PointCloud Preprocessing, Agnocast achieves a 16% improvement in average response time and a 25% improvement in worst-case response time.