diff --git a/docs/plan/hybrid.md b/docs/plan/hybrid.md
index 7898df6..0178b27 100644
--- a/docs/plan/hybrid.md
+++ b/docs/plan/hybrid.md
@@ -407,6 +407,47 @@ Four reasons it is the right one and not just the interesting one:
 abstraction, and it is where third-party drivers will show up first. This is what
 DriverKit was built for and the resemblance is not accidental.
 
+#### It is not the same shape as the GPU, and that was missed
+
+Written before the GPU eviction had been done, H4 says *"USB, same shape"*. It
+is not, and the difference is one this document's own rule already decides.
+
+The GPU could leave because **there was a working fallback underneath it**: the
+VBE framebuffer stays in the kernel, so a GPU driver that fails to start
+degrades to a working screen. That is one of the four reasons the GPU was
+chosen to go first, listed a few lines above.
+
+USB has no fallback, and it carries two things that are on the path to a shell:
+
+- **The root disk.** `USBDISK_DEV` dispatches through `bio.c` to
+  `usb_storage_rw`, and `usbstick.img` is a real boot medium built by `make`.
+- **The keyboard.** `usb_hid` feeds `input_report(INPUT_KBD, ...)`, which is how
+  anything without PS/2 is typed on — including the Chromebox this is meant to
+  boot.
+
+So evicting `xhci.c` wholesale breaks the rule three sections down:
+
+> **The kernel keeps a minimal driver for everything on the path to a shell, and
+> userspace owns the good one.**
+>
+> Break this and you get the circular boot failure where the compositor cannot
+> start because the display driver is a process that needs a compositor.
+
+The USB version of that sentence is *the root filesystem cannot mount because
+the storage driver is a process that lives on the root filesystem*, and it is
+the same failure wearing different clothes.
+
+There is a second difference, subtler and probably the harder one. `/dev/pci`
+hands over a **PCI function** — one device, one owner, nothing shared. A USB
+device is not that: it sits behind a controller that other devices also sit
+behind, so the unit of ownership and the unit of hardware are no longer the
+same thing. Handing a process "the xHCI controller" gives it the keyboard too;
+handing it "one USB device" is an interface `/dev/pci` does not have and that
+this plan has not designed.
+
+**H4 needs that interface designed before it needs code.** Recorded here rather
+than discovered halfway through 2,550 lines.
+
 ### The rule that keeps the boot path honest
 
 > **The kernel keeps a minimal driver for everything on the path to a shell, and