diff --git a/crates/punktfunk-host/src/main.rs b/crates/punktfunk-host/src/main.rs
index ff2d30f..043c432 100644
--- a/crates/punktfunk-host/src/main.rs
+++ b/crates/punktfunk-host/src/main.rs
@@ -27,7 +27,7 @@ mod vdisplay;
 #[cfg(target_os = "linux")]
 mod zerocopy;
 
-use anyhow::{bail, Result};
+use anyhow::{bail, Context, Result};
 use encode::Codec;
 use m0::{Options, Source};
 use std::path::PathBuf;
@@ -68,6 +68,15 @@ fn real_main() -> Result<()> {
         // Zero-copy FFI/GPU probe: init the EGL importer + CUDA context (no capture needed).
         #[cfg(target_os = "linux")]
         Some("zerocopy-probe") => zerocopy::probe(),
+        // Compositor readiness probe: exit 0 iff the (detected or PUNKTFUNK_COMPOSITOR-forced)
+        // compositor is up and able to create a virtual output *now*. A session-bringup
+        // script polls this to gate on real readiness instead of a blind `sleep`.
+        Some("probe-compositor") => {
+            let compositor = vdisplay::detect()?;
+            vdisplay::probe(compositor).with_context(|| format!("{compositor:?} not ready"))?;
+            println!("{compositor:?} ready");
+            Ok(())
+        }
         // M0 pipeline spike.
         Some("m0") => m0::run(parse_m0(&args[1..])?),
         // M3: native punktfunk/1 host (QUIC control plane + UDP data plane).
@@ -308,6 +317,7 @@ USAGE:
                                   + the management REST API
     punktfunk-host openapi            print the management API's OpenAPI document (codegen)
     punktfunk-host m3-host [OPTIONS]  native punktfunk/1 host (QUIC control plane + UDP data plane)
+    punktfunk-host probe-compositor   exit 0 iff the compositor is up + ready (session-bringup gate)
     punktfunk-host m0 [OPTIONS]       M0 capture→encode→file pipeline spike
 
 SERVE OPTIONS:
diff --git a/crates/punktfunk-host/src/vdisplay.rs b/crates/punktfunk-host/src/vdisplay.rs
index c771b02..b5a63cb 100644
--- a/crates/punktfunk-host/src/vdisplay.rs
+++ b/crates/punktfunk-host/src/vdisplay.rs
@@ -111,6 +111,30 @@ pub fn open(compositor: Compositor) -> Result<Box<dyn VirtualDisplay>> {
     }
 }
 
+/// Readiness probe for `compositor`: is it up and able to create a virtual output *right
+/// now*? A session-bringup script polls this (via `punktfunk-host probe-compositor`) to gate
+/// on actual readiness instead of racing the compositor with a blind sleep.
+///
+/// KWin gets a real check (the privileged `zkde_screencast` global must be advertised). The
+/// others are spawn/D-Bus/portal-based and have no equivalent pre-flight global, so a probe
+/// just confirms the backend opens — `Ok(())` means "go ahead and try `create`".
+pub fn probe(compositor: Compositor) -> Result<()> {
+    #[cfg(target_os = "linux")]
+    {
+        match compositor {
+            Compositor::Kwin => kwin::probe(),
+            // gamescope spawns its own nested session per `create`; Mutter is D-Bus on demand;
+            // wlroots creates the output on demand — nothing to pre-check beyond "Linux".
+            Compositor::Gamescope | Compositor::Mutter | Compositor::Wlroots => Ok(()),
+        }
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        let _ = compositor;
+        anyhow::bail!("virtual displays require Linux (Wayland compositor)")
+    }
+}
+
 /// Path of the file where the gamescope backend relays the nested session's `LIBEI_SOCKET`
 /// (gamescope's EIS server) for the input injector.
 #[cfg(target_os = "linux")]
diff --git a/crates/punktfunk-host/src/vdisplay/kwin.rs b/crates/punktfunk-host/src/vdisplay/kwin.rs
index 8d2a85e..931a153 100644
--- a/crates/punktfunk-host/src/vdisplay/kwin.rs
+++ b/crates/punktfunk-host/src/vdisplay/kwin.rs
@@ -223,6 +223,29 @@ fn virtual_output_thread(
     }
 }
 
+/// Readiness probe: connect to the KWin Wayland socket, roundtrip the registry, and confirm
+/// the privileged `zkde_screencast` global is actually advertised. This is exactly what
+/// [`run`] needs before it can create a virtual output, so a session-bringup script can poll
+/// this to gate on the compositor being *ready* (not merely the socket existing) instead of
+/// racing it with a blind sleep. `Ok(())` = ready; `Err` = not ready / no global yet.
+pub fn probe() -> Result<()> {
+    let conn = Connection::connect_to_env()
+        .context("connect to KWin Wayland (is WAYLAND_DISPLAY set to the KWin socket?)")?;
+    let mut queue = conn.new_event_queue();
+    let qh = queue.handle();
+    let _registry = conn.display().get_registry(&qh, ());
+    let mut state = State::default();
+    queue.roundtrip(&mut state).context("registry roundtrip")?;
+    if state.screencast.is_none() {
+        bail!(
+            "KWin is up but does not (yet) expose zkde_screencast_unstable_v1 — needs a real \
+             KDE session (or KWIN_WAYLAND_NO_PERMISSION_CHECKS=1), and KWin ≥ 6.5.6 for the \
+             headless virtual output"
+        );
+    }
+    Ok(())
+}
+
 fn run(
     width: u32,
     height: u32,
diff --git a/scripts/headless/run-headless-kde.sh b/scripts/headless/run-headless-kde.sh
index bba4443..04cbeab 100755
--- a/scripts/headless/run-headless-kde.sh
+++ b/scripts/headless/run-headless-kde.sh
@@ -1,12 +1,11 @@
 #!/usr/bin/env bash
 # Headless KDE Plasma session for the punktfunk host (no KMS scanout → kwin --virtual).
 #
-# Brings up the full desktop, not just the compositor. The env matters: without
+# Brings up the full desktop, not just the compositor, and waits for it to be *actually
+# ready* before starting the portals/plasma — no blind `sleep`. The env matters: without
 # XDG_MENU_PREFIX=plasma- the launcher resolves ${XDG_MENU_PREFIX}applications.menu →
 # "applications.menu", which doesn't exist on KDE installs (it ships
-# plasma-applications.menu) — plasmashell runs fine but the menu shows NO applications
-# and no System Settings entry. kded6/krunner/kglobalacceld are D-Bus-activated once
-# plasmashell starts in the right session env.
+# plasma-applications.menu) — plasmashell runs fine but the menu shows NO applications.
 #
 #   bash scripts/headless/run-headless-kde.sh [WxH]    # default 1920x1080
 #
@@ -29,18 +28,54 @@ export DESKTOP_SESSION=plasma
 export WAYLAND_DISPLAY=wayland-kde
 export KWIN_WAYLAND_NO_PERMISSION_CHECKS=1
 
-kwin_wayland --virtual --width "$W" --height "$H" --no-lockscreen \
-    --socket "$WAYLAND_DISPLAY" &
-KWIN_PID=$!
-sleep 2
+# The probe binary (gates readiness on KWin actually exposing zkde_screencast — not merely
+# the socket existing). Use a release build if present, else fall back to `cargo run`.
+ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+if [[ -x "$ROOT/target/release/punktfunk-host" ]]; then
+    PROBE=("$ROOT/target/release/punktfunk-host" probe-compositor)
+elif [[ -x "$ROOT/target/debug/punktfunk-host" ]]; then
+    PROBE=("$ROOT/target/debug/punktfunk-host" probe-compositor)
+else
+    PROBE=(cargo run -q --manifest-path "$ROOT/Cargo.toml" -p punktfunk-host -- probe-compositor)
+fi
 
-# The xdg-desktop-portal processes bind to the compositor that existed when THEY started;
-# after a kwin restart the stale instances point at a dead socket and RemoteDesktop/EIS
-# (mouse/keyboard injection) times out. Restart them against the fresh compositor.
-systemctl --user try-restart plasma-xdg-desktop-portal-kde.service xdg-desktop-portal.service 2>/dev/null || true
+# kwin to its own log (so its EGL/GPU-init errors are captured, not lost to the terminal).
+KWIN_LOG="${TMPDIR:-/tmp}/punktfunk-kwin.log"
+kwin_wayland --virtual --width "$W" --height "$H" --no-lockscreen \
+    --socket "$WAYLAND_DISPLAY" >"$KWIN_LOG" 2>&1 &
+KWIN_PID=$!
+
+# Active readiness wait: poll until KWin is up AND advertises the zkde_screencast global
+# (what the virtual-output backend needs), or fail fast with a useful message. kwin can also
+# exit immediately if EGL/GPU init fails — catch that.
+echo "waiting for KWin ($RES) to become ready…"
+DEADLINE=$(( SECONDS + 30 ))
+until "${PROBE[@]}" >/dev/null 2>&1; do
+    if ! kill -0 "$KWIN_PID" 2>/dev/null; then
+        echo "ERROR: kwin_wayland exited during startup — see $KWIN_LOG:" >&2
+        tail -n 20 "$KWIN_LOG" >&2 || true
+        exit 1
+    fi
+    if (( SECONDS >= DEADLINE )); then
+        echo "ERROR: KWin did not become ready within 30s. Last probe:" >&2
+        "${PROBE[@]}" >&2 || true
+        exit 1
+    fi
+    sleep 0.5
+done
+echo "KWin ready."
+
+# Only NOW restart the portals, and against the correct env: the xdg-desktop-portal chain
+# binds the compositor that existed when it started, so a stale portal points at a dead
+# socket and RemoteDesktop/EIS (input injection) times out. Import the session env into the
+# systemd/D-Bus activation environment FIRST (the missing piece — the Sway script does this;
+# without it the restarted portal can inherit an empty WAYLAND_DISPLAY), then restart.
+systemctl --user import-environment WAYLAND_DISPLAY XDG_CURRENT_DESKTOP DBUS_SESSION_BUS_ADDRESS XDG_RUNTIME_DIR 2>/dev/null || true
+dbus-update-activation-environment --systemd WAYLAND_DISPLAY XDG_CURRENT_DESKTOP DBUS_SESSION_BUS_ADDRESS 2>/dev/null || true
+systemctl --user try-restart plasma-xdg-desktop-portal-kde.service xdg-desktop-portal-kde.service xdg-desktop-portal.service 2>/dev/null || true
 
 kbuildsycoca6 >/dev/null 2>&1 || true # rebuild the menu cache under the correct env
 plasmashell &
 
-echo "headless KDE up on $WAYLAND_DISPLAY ($RES), kwin pid $KWIN_PID"
+echo "headless KDE up on $WAYLAND_DISPLAY ($RES), kwin pid $KWIN_PID (log: $KWIN_LOG)"
 wait "$KWIN_PID"