diff --git a/README.md b/README.md
index cfb3f30..56bbefb 100755
--- a/README.md
+++ b/README.md
@@ -203,9 +203,20 @@ In-game keys (set by the script's MangoHud block):
 
 | Key | Action |
 |---|---|
-| `Shift+F2` | Toggle logging on/off mid-game |
+| `Shift+F2` | Toggle logging on/off mid-game — **desktop only, see below** |
 | Your existing `toggle_hud` key | Hide the HUD overlay (logging keeps running) |
 
+> **`Shift+F2` does not work in SteamOS Game Mode.** There the logger is
+> mangoapp, not the in-game Vulkan layer, and mangoapp ignores the keybind.
+> Verified under nested gamescope: the keypress produced no log at all. To
+> start/stop a log by hand in Game Mode, use `mangohudctl` (ships with MangoHud),
+> which drives mangoapp over a message queue and needs no display or focus:
+>
+> ```bash
+> mangohudctl set log_session true    # start
+> mangohudctl set log_session false   # stop (also writes a _summary.csv)
+> ```
+
 CSV files appear in `~/Downloads/mango-logs/`. Drop them into
 <https://flightlessmango.com/> for graphs, or open in a spreadsheet.