diff --git a/design/steam-controller-deck-support.md b/design/steam-controller-deck-support.md
index cea6a0a..378c825 100644
--- a/design/steam-controller-deck-support.md
+++ b/design/steam-controller-deck-support.md
@@ -732,3 +732,29 @@ configfs) or a kernel USB bus driver — a much larger, less portable lift, and
 1. A **live SDL/non-Steam game** on a Linux host actually consuming the virtual Deck's grips/trackpads
    (the path that *does* work) — needs a real Deck/SC client + a Steam-Input-disabled consumer.
 2. The **Moonlight paddle regression** from the M3 xpad-map change (stock paddle client → host).
+
+### Gadget PoC — interface 2 is PROVEN on the Deck (2026-06-29)
+
+SteamOS ships every primitive (`CONFIG_USB_DUMMY_HCD=m`, `CONFIG_USB_RAW_GADGET=m`,
+`CONFIG_USB_CONFIGFS_F_HID=y`), so the gadget path is testable on the Deck itself with no
+module-building. A pure-shell **configfs gadget** (`deck_gadget_up.sh`) stood up a real 3-interface
+USB Deck on a `dummy_hcd` loopback UDC — keyboard = interface 0, mouse = 1, **controller = interface
+2** (`STEAMDECK_RDESC`), `28DE:1205`. Result:
+
+- It enumerates as a real USB device (`lsusb: 28de:1205 Valve Software Steam Deck Controller`) and
+  `hid-steam` binds **all three** interfaces — the controller on `bInterfaceNumber=02`.
+- **Steam promoted it:** `Local Device Found … Interface: 2 … !! Steam controller device opened for
+  index 14 … Steam Controller reserving XInput slot 1`. *This is the proof: a device on interface 2
+  IS opened + XInput-reserved by Steam, where the interface-`-1` UHID device was filtered out.*
+- It then failed at the next step — `f_hid` can't serve HID **feature reports** (`hid-steam:
+  steam_send_report: error -32 (ae 16 01)` → serial `XXXXXXXXXX`; Steam: `couldn't get controller
+  details … GetControllerInfo failed … Disconnecting zombie controller`). No gamepad evdev was
+  created either, for the same reason (hid-steam can't complete Deck init without the feature/output
+  channel).
+
+**Conclusion: the wall is fully characterised and climbable.** Interface 2 is necessary *and*
+sufficient for Steam to open + XInput-reserve the Deck; the only remaining piece is serving the
+HID feature/output reports, which `f_hid` can't but **`raw_gadget` can** (userspace handles every
+control transfer, exactly like the UHID path). Next: a `raw_gadget` userspace emulator of the
+3-interface Deck (controller on interface 2) that answers the serial/attribute/settings feature
+reports + streams the 64-byte state report — then re-test hid-steam gamepad evdev + Steam promotion.