Reading Cua Driver 0.26.1 Computer Use Internals — Pinning the Window Target and Separating Action Facts from Postconditions

I recently wrote back-to-back code-reading tours of Hermes Agent and Open Interpreter Computer Use. Both delegate the actual OS operations to an external cua-driver, so the natural next question was what happens on that other side of the boundary. This article is that follow-up, pinned to commit b4e3caec of the trycua/cua monorepo and scoped to Cua Driver 0.26.1. It follows one request from MCP, CLI, or the in-process SDK through to the OS input API, and back out through the public ActionResult and verify_state. Along the way it walks through eight design forks: the shared native dispatch, WindowTargetGuard, element_token, ScreenCaptureKit on macOS, WGC / PrintWindow on Windows, X11 / Wayland on Linux, the closed action contract, and the three-layer permission story.