feat: client-selectable compositor (protocol → host → client → C ABI → mgmt → web)

A client can now request which compositor backend the host drives its virtual
output on (gamescope/KWin/Mutter/wlroots). The host honors the request if that
backend is available, else falls back to auto-detect and reports the resolved
choice back — wire-compatible both directions (no ABI bump).

Protocol (punktfunk-core):
- New CompositorPref (config.rs): Auto|Kwin|Wlroots|Mutter|Gamescope with
  u8/name mappings. Appended as one optional byte to Hello (client preference)
  and Welcome (host's resolved choice). Both decoders already tolerate trailing
  bytes, so old↔new interop is preserved — ABI_VERSION stays 2. Round-trip +
  back-compat (truncated-message) tests.
- C ABI: punktfunk_connect_ex(compositor) + PUNKTFUNK_COMPOSITOR_* constants;
  punktfunk_connect delegates with AUTO, so the existing symbol is unchanged.
  NativeClient::connect / worker_main thread the preference through.

Host:
- vdisplay::available() enumerates usable backends via cheap, side-effect-free
  probes (KWin zkde global, gamescope binary+version, GNOME/Sway env), plus
  Compositor id/label/as_pref/from_pref/all helpers.
- m3 handshake resolves the preference to a concrete backend during the
  handshake (pick_compositor pure + resolved logging), reports it in Welcome,
  and threads it into virtual_stream (replacing the unconditional detect()).
- mgmt GET /v1/compositors lists every backend with availability + the
  auto-detected default (OpenAPI regenerated).

Client:
- punktfunk-client-rs --compositor NAME; logs the host's resolved choice from
  the Welcome ("session offer … compositor=…").

Web console:
- Host page gains a Compositors card (availability + default badges) via the
  codegen'd useListCompositors hook; en/de strings added.

Also fixes a pre-existing, env-dependent test-isolation bug:
mgmt::tests::paired_clients_list_and_unpair seeded the real
~/.config/punktfunk/paired.json (AppState::new loads it), so a real
GameStream-paired client leaked into body[0] on a dev box — now cleared first.

Live-validated against headless KWin: --compositor kwin honored, --compositor
mutter falls back to kwin (available=[kwin, gamescope]), resolved choice
round-trips to the client. Tests: +6 (wire/back-compat, resolution precedence,
endpoint); workspace green, clippy/fmt clean, C ABI harness PASS at abi_version=2,
web typecheck + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-client-rs/src/main.rs

@@ -17,15 +17,19 @@
 //! Host→client datagrams (Opus audio, rumble) are counted and reported with the stream
 //! stats — decode/playback is the platform clients' job.
 //!
+//! `--compositor NAME` requests a host compositor backend (`auto`|`kwin`|`wlroots`|`mutter`|
+//! `gamescope`); the host honors it if available, else auto-detects and reports the resolved
+//! choice in its Welcome (logged as `session offer … compositor=…`).
+//!
 //! Usage: `punktfunk-client-rs [--connect HOST:PORT] [--mode WxHxFPS] [--out FILE] [--input-test]
-//!         [--pin HEX]` (M4 adds VAAPI decode + wgpu present on this same skeleton.)
+//!         [--pin HEX] [--compositor NAME]` (M4 adds VAAPI decode + wgpu present on this skeleton.)
 
 use anyhow::{anyhow, Context, Result};
 use punktfunk_core::config::Role;
 use punktfunk_core::input::{InputEvent, InputKind};
 use punktfunk_core::quic::{endpoint, io, Hello, Reconfigure, Reconfigured, Start, Welcome};
 use punktfunk_core::transport::UdpTransport;
-use punktfunk_core::{Mode, PunktfunkError, Session};
+use punktfunk_core::{CompositorPref, Mode, PunktfunkError, Session};
 use std::io::Write;
 
 struct Args {
@@ -40,6 +44,8 @@ struct Args {
     pair: Option<String>,
     /// `--name LABEL` — how the host labels this client when pairing.
     name: String,
+    /// `--compositor NAME` — request a host compositor backend (auto|kwin|wlroots|mutter|gamescope).
+    compositor: CompositorPref,
 }
 
 fn parse_mode(m: &str) -> Option<Mode> {
@@ -115,6 +121,17 @@ fn parse_args() -> Args {
             }
         }
     };
+    // A present-but-unrecognized --compositor must abort rather than silently auto-detect.
+    let compositor = match get("--compositor") {
+        None => CompositorPref::Auto,
+        Some(s) => match CompositorPref::from_name(s) {
+            Some(c) => c,
+            None => {
+                eprintln!("--compositor must be one of: auto, kwin, wlroots, mutter, gamescope");
+                std::process::exit(2);
+            }
+        },
+    };
     Args {
         connect: get("--connect").unwrap_or("127.0.0.1:9777").to_string(),
         mode,
@@ -124,6 +141,7 @@ fn parse_args() -> Args {
         remode,
         pair: get("--pair").map(String::from),
         name: get("--name").unwrap_or("punktfunk-client-rs").to_string(),
+        compositor,
     }
 }
 
@@ -207,6 +225,7 @@ async fn session(args: Args) -> Result<()> {
         &Hello {
             abi_version: punktfunk_core::ABI_VERSION,
             mode: args.mode,
+            compositor: args.compositor,
         }
         .encode(),
     )
@@ -218,6 +237,7 @@ async fn session(args: Args) -> Result<()> {
         fec = ?welcome.fec,
         encrypt = welcome.encrypt,
         frames = welcome.frames,
+        compositor = welcome.compositor.as_str(),
         "session offer"
     );
 

crates/punktfunk-core/src/abi.rs

@@ -477,8 +477,23 @@ unsafe fn opt_cstr<'a>(p: *const std::os::raw::c_char) -> std::result::Result<Op
         .map_err(|_| ())
 }
 
+/// Compositor preference for [`punktfunk_connect_ex`] (`compositor` arg). `AUTO` lets the host
+/// pick (auto-detect from its running desktop); a concrete value is honored only if that backend
+/// is available on the host right now, else the host falls back to auto-detect. The resolved
+/// choice is reported back over the protocol (see `punktfunk/1` `Welcome`).
+pub const PUNKTFUNK_COMPOSITOR_AUTO: u32 = 0;
+/// KWin / KDE Plasma.
+pub const PUNKTFUNK_COMPOSITOR_KWIN: u32 = 1;
+/// wlroots (Sway / Hyprland).
+pub const PUNKTFUNK_COMPOSITOR_WLROOTS: u32 = 2;
+/// Mutter / GNOME.
+pub const PUNKTFUNK_COMPOSITOR_MUTTER: u32 = 3;
+/// gamescope (spawned nested).
+pub const PUNKTFUNK_COMPOSITOR_GAMESCOPE: u32 = 4;
+
 /// Connect to a `punktfunk/1` host and start a session at `width`x`height`@`refresh_hz`.
-/// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure.
+/// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure. Equivalent to
+/// [`punktfunk_connect_ex`] with `compositor = PUNKTFUNK_COMPOSITOR_AUTO`.
 ///
 /// Trust: `pin_sha256` (NULL or 32 bytes) is the expected SHA-256 fingerprint of the host's
 /// certificate — a mismatching host is rejected. NULL = trust on first use; persist the
@@ -507,6 +522,45 @@ pub unsafe extern "C" fn punktfunk_connect(
     client_cert_pem: *const std::os::raw::c_char,
     client_key_pem: *const std::os::raw::c_char,
     timeout_ms: u32,
+) -> *mut PunktfunkConnection {
+    unsafe {
+        punktfunk_connect_ex(
+            host,
+            port,
+            width,
+            height,
+            refresh_hz,
+            PUNKTFUNK_COMPOSITOR_AUTO,
+            pin_sha256,
+            observed_sha256_out,
+            client_cert_pem,
+            client_key_pem,
+            timeout_ms,
+        )
+    }
+}
+
+/// Like [`punktfunk_connect`], but requests a specific `compositor` backend on the host (one of
+/// the `PUNKTFUNK_COMPOSITOR_*` values). `PUNKTFUNK_COMPOSITOR_AUTO` (or any unrecognized value)
+/// lets the host decide; a concrete value is honored only if available, else the host falls back
+/// to auto-detect. The resolved choice is logged host-side and returned over the protocol.
+///
+/// # Safety
+/// Same as [`punktfunk_connect`].
+#[cfg(feature = "quic")]
+#[no_mangle]
+pub unsafe extern "C" fn punktfunk_connect_ex(
+    host: *const std::os::raw::c_char,
+    port: u16,
+    width: u32,
+    height: u32,
+    refresh_hz: u32,
+    compositor: u32,
+    pin_sha256: *const u8,
+    observed_sha256_out: *mut u8,
+    client_cert_pem: *const std::os::raw::c_char,
+    client_key_pem: *const std::os::raw::c_char,
+    timeout_ms: u32,
 ) -> *mut PunktfunkConnection {
     let r = std::panic::catch_unwind(AssertUnwindSafe(|| {
         if host.is_null() {
@@ -521,6 +575,7 @@ pub unsafe extern "C" fn punktfunk_connect(
             height,
             refresh_hz,
         };
+        let pref = crate::config::CompositorPref::from_u8(compositor as u8);
         let pin = if pin_sha256.is_null() {
             None
         } else {
@@ -539,6 +594,7 @@ pub unsafe extern "C" fn punktfunk_connect(
             host,
             port,
             mode,
+            pref,
             pin,
             identity,
             std::time::Duration::from_millis(timeout_ms as u64),

crates/punktfunk-core/src/client.rs

@@ -11,7 +11,7 @@
 //! invariant) plus a blocking data-plane pump; frames cross to the embedder over a bounded
 //! channel. All methods are safe to call from any single embedder thread.
 
-use crate::config::{Mode, Role};
+use crate::config::{CompositorPref, Mode, Role};
 use crate::error::{PunktfunkError, Result};
 use crate::input::InputEvent;
 use crate::quic::{endpoint, io, Hello, Reconfigure, Reconfigured, Start, Welcome};
@@ -76,6 +76,7 @@ impl NativeClient {
         host: &str,
         port: u16,
         mode: Mode,
+        compositor: CompositorPref,
         pin: Option<[u8; 32]>,
         identity: Option<(String, String)>,
         timeout: Duration,
@@ -110,6 +111,7 @@ impl NativeClient {
                     host,
                     port,
                     mode,
+                    compositor,
                     pin,
                     identity,
                     frame_tx,
@@ -309,6 +311,7 @@ struct WorkerArgs {
     host: String,
     port: u16,
     mode: Mode,
+    compositor: CompositorPref,
     pin: Option<[u8; 32]>,
     identity: Option<(String, String)>,
     frame_tx: SyncSender<Frame>,
@@ -328,6 +331,7 @@ async fn worker_main(args: WorkerArgs) {
         host,
         port,
         mode,
+        compositor,
         pin,
         identity,
         frame_tx,
@@ -374,11 +378,18 @@ async fn worker_main(args: WorkerArgs) {
             &Hello {
                 abi_version: crate::ABI_VERSION,
                 mode,
+                compositor,
             }
             .encode(),
         )
         .await?;
         let welcome = Welcome::decode(&io::read_msg(&mut recv).await?)?;
+        if welcome.compositor != CompositorPref::Auto {
+            tracing::info!(
+                compositor = welcome.compositor.as_str(),
+                "host resolved compositor"
+            );
+        }
 
         // Reserve our data-plane port, then start the host.
         let probe = std::net::UdpSocket::bind("0.0.0.0:0")?;

crates/punktfunk-core/src/config.rs

@@ -58,6 +58,78 @@ pub struct Mode {
     pub refresh_hz: u32,
 }
 
+/// Which compositor backend a client would like the host to drive for its virtual output.
+///
+/// Sent in [`Hello`](crate::quic::Hello) as a *preference* and echoed back — resolved to the
+/// backend actually chosen — in [`Welcome`](crate::quic::Welcome). `Auto` (the default) lets the
+/// host decide (auto-detect from the running desktop). A concrete preference is honored only if
+/// that backend is available on the host right now; otherwise the host falls back to auto-detect
+/// and reports the real choice in `Welcome`. The wire form is a single byte (`0 = Auto`,
+/// `1..=4` concrete), appended to `Hello`/`Welcome` — older peers simply omit/ignore it.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
+pub enum CompositorPref {
+    /// Let the host pick (auto-detect from the running desktop / its configured default).
+    #[default]
+    Auto,
+    /// KWin / KDE Plasma.
+    Kwin,
+    /// wlroots (Sway / Hyprland).
+    Wlroots,
+    /// Mutter / GNOME.
+    Mutter,
+    /// gamescope (spawned nested — available wherever the binary is installed).
+    Gamescope,
+}
+
+impl CompositorPref {
+    /// Wire byte. `0 = Auto`, `1 = Kwin`, `2 = Wlroots`, `3 = Mutter`, `4 = Gamescope`.
+    pub fn to_u8(self) -> u8 {
+        match self {
+            CompositorPref::Auto => 0,
+            CompositorPref::Kwin => 1,
+            CompositorPref::Wlroots => 2,
+            CompositorPref::Mutter => 3,
+            CompositorPref::Gamescope => 4,
+        }
+    }
+
+    /// Inverse of [`to_u8`](Self::to_u8). An unknown byte decodes to `Auto` — forward-compatible:
+    /// a future concrete value a peer doesn't recognize degrades to "let the host decide".
+    pub fn from_u8(v: u8) -> Self {
+        match v {
+            1 => CompositorPref::Kwin,
+            2 => CompositorPref::Wlroots,
+            3 => CompositorPref::Mutter,
+            4 => CompositorPref::Gamescope,
+            _ => CompositorPref::Auto,
+        }
+    }
+
+    /// Parse a CLI/config name (case-insensitive, with the usual desktop aliases). `None` for an
+    /// unrecognized name, so callers can error rather than silently defaulting to `Auto`.
+    pub fn from_name(s: &str) -> Option<Self> {
+        Some(match s.trim().to_ascii_lowercase().as_str() {
+            "auto" | "detect" | "default" => CompositorPref::Auto,
+            "kwin" | "kde" | "plasma" => CompositorPref::Kwin,
+            "wlroots" | "sway" | "hyprland" | "wlr" => CompositorPref::Wlroots,
+            "mutter" | "gnome" => CompositorPref::Mutter,
+            "gamescope" => CompositorPref::Gamescope,
+            _ => return None,
+        })
+    }
+
+    /// Canonical lowercase identifier (`"auto"`, `"kwin"`, `"wlroots"`, `"mutter"`, `"gamescope"`).
+    pub fn as_str(self) -> &'static str {
+        match self {
+            CompositorPref::Auto => "auto",
+            CompositorPref::Kwin => "kwin",
+            CompositorPref::Wlroots => "wlroots",
+            CompositorPref::Mutter => "mutter",
+            CompositorPref::Gamescope => "gamescope",
+        }
+    }
+}
+
 /// Per-block FEC parameters. Recovery count is derived from `fec_percent` exactly as
 /// GameStream does: `m = ceil(k * fec_percent / 100)`.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]

crates/punktfunk-core/src/lib.rs

@@ -39,7 +39,7 @@ pub mod session;
 pub mod stats;
 pub mod transport;
 
-pub use config::{Config, FecConfig, FecScheme, Mode, ProtocolPhase, Role};
+pub use config::{CompositorPref, Config, FecConfig, FecScheme, Mode, ProtocolPhase, Role};
 pub use error::{PunktfunkError, PunktfunkStatus, Result};
 pub use session::{Frame, Session};
 pub use stats::Stats;

crates/punktfunk-core/src/quic.rs

@@ -22,7 +22,7 @@
 //! reported back for persisting). The data plane adds AES-GCM on top.
 //! All integers little-endian; every message is `u16 length || payload`.
 
-use crate::config::{Config, FecConfig, FecScheme, Mode, ProtocolPhase, Role};
+use crate::config::{CompositorPref, Config, FecConfig, FecScheme, Mode, ProtocolPhase, Role};
 use crate::error::{PunktfunkError, Result};
 
 /// Protocol magic + version, first bytes of the positional handshake (Hello/Welcome/Start).
@@ -40,6 +40,11 @@ pub const CTL_MAGIC: &[u8; 4] = b"PKFc";
 pub struct Hello {
     pub abi_version: u32,
     pub mode: Mode,
+    /// Which compositor the client would like the host to drive (`Auto` = host decides). The
+    /// host honors it only if that backend is available, else falls back and reports the real
+    /// choice in [`Welcome::compositor`]. Appended to the wire form — omitted by older clients
+    /// (decodes to `Auto`).
+    pub compositor: CompositorPref,
 }
 
 /// `host → client`: the complete session offer.
@@ -56,6 +61,10 @@ pub struct Welcome {
     pub salt: [u8; 4],
     /// Seed/testing: how many frames the host will send (0 = unbounded).
     pub frames: u32,
+    /// The compositor the host actually resolved for this session (the client's
+    /// [`Hello::compositor`] preference if available, else the host's auto-detected choice).
+    /// Appended to the wire form — `Auto` when an older host omitted it (i.e. "unknown").
+    pub compositor: CompositorPref,
 }
 
 /// `client → host`: data plane is bound, begin streaming.
@@ -350,12 +359,13 @@ pub mod pake {
 
 impl Hello {
     pub fn encode(&self) -> Vec<u8> {
-        let mut b = Vec::with_capacity(20);
+        let mut b = Vec::with_capacity(21);
         b.extend_from_slice(MAGIC);
         b.extend_from_slice(&self.abi_version.to_le_bytes());
         b.extend_from_slice(&self.mode.width.to_le_bytes());
         b.extend_from_slice(&self.mode.height.to_le_bytes());
         b.extend_from_slice(&self.mode.refresh_hz.to_le_bytes());
+        b.push(self.compositor.to_u8()); // appended at offset 20 — older hosts read [0..20] and skip it
         b
     }
 
@@ -371,6 +381,11 @@ impl Hello {
                 height: u32at(12),
                 refresh_hz: u32at(16),
             },
+            // Optional trailing byte — an older client that omits it requests `Auto`.
+            compositor: b
+                .get(20)
+                .map(|&v| CompositorPref::from_u8(v))
+                .unwrap_or_default(),
         })
     }
 }
@@ -395,13 +410,14 @@ impl Welcome {
         b.extend_from_slice(&self.key);
         b.extend_from_slice(&self.salt);
         b.extend_from_slice(&self.frames.to_le_bytes());
+        b.push(self.compositor.to_u8()); // appended at offset 53 — older clients read [0..53] and skip it
         b
     }
 
     pub fn decode(b: &[u8]) -> Result<Welcome> {
         // Layout (LE): magic[0..4] abi[4..8] port[8..10] w[10..14] h[14..18] hz[18..22]
         // scheme[22] pct[23] max_data[24..26] shard[26..28] encrypt[28] key[29..45]
-        // salt[45..49] frames[49..53].
+        // salt[45..49] frames[49..53] compositor[53] (optional trailing byte).
         if b.len() < 53 || &b[0..4] != MAGIC {
             return Err(PunktfunkError::InvalidArg("bad Welcome"));
         }
@@ -433,6 +449,12 @@ impl Welcome {
             key,
             salt,
             frames: u32at(49),
+            // Optional trailing byte — an older host that omits it leaves the resolved
+            // compositor unknown (`Auto`).
+            compositor: b
+                .get(53)
+                .map(|&v| CompositorPref::from_u8(v))
+                .unwrap_or_default(),
         })
     }
 
@@ -931,6 +953,7 @@ mod tests {
             key: [7u8; 16],
             salt: [1, 2, 3, 4],
             frames: 600,
+            compositor: CompositorPref::Gamescope,
         };
         assert_eq!(Welcome::decode(&w.encode()).unwrap(), w);
     }
@@ -944,6 +967,7 @@ mod tests {
                 height: 720,
                 refresh_hz: 120,
             },
+            compositor: CompositorPref::Kwin,
         };
         assert_eq!(Hello::decode(&h.encode()).unwrap(), h);
         let s = Start {
@@ -952,6 +976,74 @@ mod tests {
         assert_eq!(Start::decode(&s.encode()).unwrap(), s);
     }
 
+    #[test]
+    fn compositor_pref_wire_and_names() {
+        for p in [
+            CompositorPref::Auto,
+            CompositorPref::Kwin,
+            CompositorPref::Wlroots,
+            CompositorPref::Mutter,
+            CompositorPref::Gamescope,
+        ] {
+            assert_eq!(CompositorPref::from_u8(p.to_u8()), p);
+            assert_eq!(CompositorPref::from_name(p.as_str()), Some(p));
+        }
+        // Aliases + unknowns.
+        assert_eq!(CompositorPref::from_name("KDE"), Some(CompositorPref::Kwin));
+        assert_eq!(
+            CompositorPref::from_name("sway"),
+            Some(CompositorPref::Wlroots)
+        );
+        assert_eq!(CompositorPref::from_name("nope"), None);
+        // Unknown wire byte degrades to Auto (forward-compatible).
+        assert_eq!(CompositorPref::from_u8(200), CompositorPref::Auto);
+    }
+
+    #[test]
+    fn hello_welcome_compositor_back_compat() {
+        // A new client/host appends one byte; the field is optional on decode, so a legacy
+        // peer's shorter message still decodes (compositor = Auto), and a legacy peer reading a
+        // new message ignores the trailing byte. Simulate both directions by truncation.
+        let h = Hello {
+            abi_version: 2,
+            mode: Mode {
+                width: 1920,
+                height: 1080,
+                refresh_hz: 60,
+            },
+            compositor: CompositorPref::Mutter,
+        };
+        let enc = h.encode();
+        assert_eq!(enc.len(), 21);
+        // Legacy (20-byte) Hello → Auto, mode intact.
+        let legacy = Hello::decode(&enc[..20]).unwrap();
+        assert_eq!(legacy.compositor, CompositorPref::Auto);
+        assert_eq!(legacy.mode, h.mode);
+
+        let w = Welcome {
+            abi_version: 2,
+            udp_port: 7000,
+            mode: h.mode,
+            fec: FecConfig {
+                scheme: FecScheme::Gf16,
+                fec_percent: 20,
+                max_data_per_block: 4096,
+            },
+            shard_payload: 1200,
+            encrypt: true,
+            key: [3u8; 16],
+            salt: [9, 8, 7, 6],
+            frames: 0,
+            compositor: CompositorPref::Kwin,
+        };
+        let wenc = w.encode();
+        assert_eq!(wenc.len(), 54);
+        let legacy_w = Welcome::decode(&wenc[..53]).unwrap();
+        assert_eq!(legacy_w.compositor, CompositorPref::Auto);
+        assert_eq!(legacy_w.frames, 0);
+        assert_eq!(legacy_w.key, w.key);
+    }
+
     #[test]
     fn reconfigure_roundtrip() {
         let rq = Reconfigure {
@@ -992,6 +1084,7 @@ mod tests {
                     height: 720,
                     refresh_hz: 60,
                 },
+                compositor: CompositorPref::Auto,
             }
             .encode();
             assert!(PairRequest::decode(&h).is_err(), "abi {abi} parsed as pair");

crates/punktfunk-host/src/m3.rs

@@ -23,7 +23,7 @@
 //! with GameStream pairing) and logs the SHA-256 fingerprint clients pin.
 
 use anyhow::{anyhow, Context, Result};
-use punktfunk_core::config::{FecConfig, FecScheme, Role};
+use punktfunk_core::config::{CompositorPref, FecConfig, FecScheme, Role};
 use punktfunk_core::input::{InputEvent, InputKind};
 use punktfunk_core::packet::{FLAG_PIC, FLAG_SOF};
 use punktfunk_core::quic::{
@@ -395,6 +395,21 @@ async fn serve_session(
         )
         .context("client-requested mode")?;
 
+        // Resolve the client's compositor preference to a concrete backend *now*, so the Welcome
+        // can report what we'll actually drive. Only the Virtual source has a compositor; the
+        // synthetic source has no virtual output. Blocking probes → spawn_blocking.
+        let compositor = match source {
+            M3Source::Virtual => {
+                let pref = hello.compositor;
+                Some(
+                    tokio::task::spawn_blocking(move || resolve_compositor(pref))
+                        .await
+                        .context("resolve compositor task")??,
+                )
+            }
+            M3Source::Synthetic => None,
+        };
+
         // Reserve a UDP port for the data plane (bind, read it back, rebind in UdpTransport).
         let probe = std::net::UdpSocket::bind("0.0.0.0:0")?;
         let udp_port = probe.local_addr()?.port();
@@ -420,19 +435,30 @@ async fn serve_session(
                 M3Source::Synthetic => frames,
                 M3Source::Virtual => 0, // unbounded — client streams until we close
             },
+            // Report the resolved backend back to the client (Auto for the synthetic source).
+            compositor: compositor
+                .map(|c| c.as_pref())
+                .unwrap_or(CompositorPref::Auto),
         };
         io::write_msg(&mut send, &welcome.encode()).await?;
 
         let start = Start::decode(&io::read_msg(&mut recv).await?)
             .map_err(|e| anyhow!("Start decode: {e:?}"))?;
-        Ok::<_, anyhow::Error>((hello, welcome, udp_port, start))
+        Ok::<_, anyhow::Error>((hello, welcome, udp_port, start, compositor))
     };
-    let (hello, welcome, udp_port, start) = tokio::time::timeout(HANDSHAKE_TIMEOUT, handshake)
-        .await
-        .map_err(|_| anyhow!("handshake timed out after {HANDSHAKE_TIMEOUT:?}"))??;
+    let (hello, welcome, udp_port, start, compositor) =
+        tokio::time::timeout(HANDSHAKE_TIMEOUT, handshake)
+            .await
+            .map_err(|_| anyhow!("handshake timed out after {HANDSHAKE_TIMEOUT:?}"))??;
     let (mut ctrl_send, mut ctrl_recv) = (send, recv);
     let client_udp = std::net::SocketAddr::new(peer.ip(), start.client_udp_port);
-    tracing::info!(%client_udp, udp_port, mode = ?hello.mode, "handshake complete — streaming");
+    tracing::info!(
+        %client_udp,
+        udp_port,
+        mode = ?hello.mode,
+        compositor = compositor.map(|c| c.id()).unwrap_or("none"),
+        "handshake complete — streaming"
+    );
 
     // Control task: the handshake stream stays open for mid-stream renegotiation. A
     // validated Reconfigure is acked, then handed to the data-plane thread, which rebuilds
@@ -541,7 +567,16 @@ async fn serve_session(
             match source {
                 M3Source::Synthetic => synthetic_stream(&mut session, frames, &stop_stream),
                 M3Source::Virtual => {
-                    virtual_stream(&mut session, mode, seconds, &stop_stream, &reconfig_rx)
+                    let compositor = compositor
+                        .expect("the Virtual source resolves a compositor during the handshake");
+                    virtual_stream(
+                        &mut session,
+                        mode,
+                        seconds,
+                        &stop_stream,
+                        &reconfig_rx,
+                        compositor,
+                    )
                 }
             }
         })
@@ -805,6 +840,56 @@ fn synthetic_stream(session: &mut Session, frames: u32, stop: &AtomicBool) -> Re
     Ok(())
 }
 
+/// Pure selection: choose the backend to drive from the client's `pref`, the set `available`
+/// right now, and the auto-`detected` default. A concrete preference wins only if it's available;
+/// otherwise (and for `Auto`) fall back to the detected default. `None` only when nothing is
+/// available *and* nothing was detected — the caller turns that into a handshake error.
+fn pick_compositor(
+    pref: CompositorPref,
+    available: &[crate::vdisplay::Compositor],
+    detected: Option<crate::vdisplay::Compositor>,
+) -> Option<crate::vdisplay::Compositor> {
+    if let Some(want) = crate::vdisplay::Compositor::from_pref(pref) {
+        if available.contains(&want) {
+            return Some(want);
+        }
+    }
+    detected
+}
+
+/// Resolve the client's compositor preference to a concrete backend (the I/O shell around
+/// [`pick_compositor`]): enumerate what's available, auto-detect the default, pick, and log
+/// whether the explicit request was honored or fell back. Runs blocking probes — call off the
+/// async reactor (`spawn_blocking`).
+fn resolve_compositor(pref: CompositorPref) -> Result<crate::vdisplay::Compositor> {
+    use crate::vdisplay::Compositor;
+    let available = crate::vdisplay::available();
+    let detected = crate::vdisplay::detect().ok();
+    let chosen = pick_compositor(pref, &available, detected).ok_or_else(|| {
+        anyhow!("no usable compositor (set PUNKTFUNK_COMPOSITOR or run inside a supported desktop)")
+    })?;
+    let avail_ids: Vec<&str> = available.iter().map(|c| c.id()).collect();
+    match Compositor::from_pref(pref) {
+        Some(want) if want == chosen => {
+            tracing::info!(
+                compositor = chosen.id(),
+                "honoring client compositor request"
+            )
+        }
+        Some(want) => tracing::warn!(
+            requested = want.id(),
+            chosen = chosen.id(),
+            available = ?avail_ids,
+            "client-requested compositor unavailable — falling back to auto-detect"
+        ),
+        None => tracing::info!(
+            compositor = chosen.id(),
+            "auto-detected compositor (client: auto)"
+        ),
+    }
+    Ok(chosen)
+}
+
 /// Real capture→encode→punktfunk/1: a native virtual output at the client's mode, NVENC AUs
 /// stamped with the capture wall clock (the client derives per-frame pipeline latency).
 ///
@@ -818,9 +903,13 @@ fn virtual_stream(
     seconds: u32,
     stop: &AtomicBool,
     reconfig: &std::sync::mpsc::Receiver<punktfunk_core::Mode>,
+    compositor: crate::vdisplay::Compositor,
 ) -> Result<()> {
-    let compositor = crate::vdisplay::detect().context("detect compositor")?;
-    tracing::info!(?compositor, ?mode, "punktfunk/1 virtual display");
+    tracing::info!(
+        compositor = compositor.id(),
+        ?mode,
+        "punktfunk/1 virtual display"
+    );
     let mut vd = crate::vdisplay::open(compositor)?;
     let (mut capturer, mut enc, mut frame, mut interval) =
         build_pipeline_with_retry(&mut vd, mode)?;
@@ -1000,6 +1089,36 @@ fn build_pipeline(
 mod tests {
     use super::*;
 
+    #[test]
+    fn compositor_resolution_precedence() {
+        use crate::vdisplay::Compositor::*;
+        // A concrete, available preference is honored.
+        assert_eq!(
+            pick_compositor(CompositorPref::Gamescope, &[Kwin, Gamescope], Some(Kwin)),
+            Some(Gamescope)
+        );
+        // A concrete but UNavailable preference falls back to the detected default.
+        assert_eq!(
+            pick_compositor(CompositorPref::Mutter, &[Kwin, Gamescope], Some(Kwin)),
+            Some(Kwin)
+        );
+        // Auto always uses the detected default.
+        assert_eq!(
+            pick_compositor(CompositorPref::Auto, &[Kwin, Gamescope], Some(Kwin)),
+            Some(Kwin)
+        );
+        // Unavailable preference + nothing detected → None (caller errors the handshake).
+        assert_eq!(
+            pick_compositor(CompositorPref::Mutter, &[Gamescope], None),
+            None
+        );
+        // Available preference still wins even when nothing was auto-detected.
+        assert_eq!(
+            pick_compositor(CompositorPref::Gamescope, &[Gamescope], None),
+            Some(Gamescope)
+        );
+    }
+
     #[test]
     fn permanent_errors_short_circuit_retry() {
         // Permanent: config / version / missing-tool — retrying within a session can't fix these.
@@ -1287,7 +1406,16 @@ mod tests {
 
         // 2: anonymous session on a pairing-required host → rejected (connect fails).
         assert!(
-            NativeClient::connect("127.0.0.1", 19778, mode, None, None, timeout).is_err(),
+            NativeClient::connect(
+                "127.0.0.1",
+                19778,
+                mode,
+                CompositorPref::Auto,
+                None,
+                None,
+                timeout
+            )
+            .is_err(),
             "anonymous session must be rejected"
         );
 
@@ -1305,6 +1433,7 @@ mod tests {
             "127.0.0.1",
             19778,
             mode,
+            CompositorPref::Auto,
             Some(host_fp),
             Some((cert.clone(), key.clone())),
             timeout,

crates/punktfunk-host/src/mgmt.rs

@@ -120,6 +120,7 @@ fn api_router_parts() -> (Router<Arc<MgmtState>>, utoipa::openapi::OpenApi) {
             OpenApiRouter::new()
                 .routes(routes!(get_health))
                 .routes(routes!(get_host_info))
+                .routes(routes!(list_compositors))
                 .routes(routes!(get_status))
                 .routes(routes!(list_paired_clients))
                 .routes(routes!(unpair_client))
@@ -446,6 +447,52 @@ async fn get_host_info(State(st): State<Arc<MgmtState>>) -> Json<HostInfo> {
     })
 }
 
+/// A compositor backend the host can drive a virtual output on, and whether it's usable now.
+#[derive(Serialize, ToSchema)]
+struct AvailableCompositor {
+    /// Stable identifier (`"kwin"` | `"wlroots"` | `"mutter"` | `"gamescope"`) — pass this to a
+    /// client's `--compositor` flag.
+    id: String,
+    /// Human-readable label for UIs.
+    label: String,
+    /// Usable on this host right now: the live session's own compositor, or gamescope wherever
+    /// its binary is installed.
+    available: bool,
+    /// True for the backend an `Auto` (unspecified) request resolves to right now.
+    default: bool,
+}
+
+/// Available compositor backends
+///
+/// Lists every backend the host knows how to drive, flags which are usable right now, and marks
+/// the one an unspecified (`Auto`) client request resolves to. Clients pass an `id` to their
+/// `--compositor` flag (or `PUNKTFUNK_COMPOSITOR_*` over the C ABI) to request it.
+#[utoipa::path(
+    get,
+    path = "/compositors",
+    tag = "host",
+    operation_id = "listCompositors",
+    responses(
+        (status = OK, description = "Compositor backends with availability + the auto-detected default", body = [AvailableCompositor]),
+        (status = UNAUTHORIZED, description = "Missing or invalid bearer token", body = ApiError),
+    )
+)]
+async fn list_compositors() -> Json<Vec<AvailableCompositor>> {
+    let available = crate::vdisplay::available();
+    let default = crate::vdisplay::detect().ok();
+    Json(
+        crate::vdisplay::Compositor::all()
+            .into_iter()
+            .map(|c| AvailableCompositor {
+                id: c.id().into(),
+                label: c.label().into(),
+                available: available.contains(&c),
+                default: default == Some(c),
+            })
+            .collect(),
+    )
+}
+
 /// Live host status
 #[utoipa::path(
     get,
@@ -771,6 +818,24 @@ mod tests {
         assert_eq!(body["codecs"], serde_json::json!(["h264", "h265", "av1"]));
     }
 
+    #[tokio::test]
+    async fn compositors_lists_all_backends_with_flags() {
+        let app = test_app(test_state(), None);
+        let (status, body) = send(&app, get_req("/api/v1/compositors")).await;
+        assert_eq!(status, StatusCode::OK);
+        let arr = body.as_array().expect("array");
+        // Every backend the host knows, in stable order.
+        let ids: Vec<&str> = arr.iter().map(|c| c["id"].as_str().unwrap()).collect();
+        assert_eq!(ids, ["kwin", "gamescope", "mutter", "wlroots"]);
+        for c in arr {
+            assert!(c["available"].is_boolean());
+            assert!(c["default"].is_boolean());
+            assert!(c["label"].as_str().is_some_and(|s| !s.is_empty()));
+        }
+        // At most one backend is the auto-detect default (none, if the test env has no desktop).
+        assert!(arr.iter().filter(|c| c["default"] == true).count() <= 1);
+    }
+
     #[tokio::test]
     async fn status_reflects_runtime_state() {
         let state = test_state();
@@ -808,7 +873,14 @@ mod tests {
             x509_parser::pem::parse_x509_pem(state.identity.cert_pem.as_bytes()).unwrap();
         let der = pem.contents.clone();
         let fingerprint = hex::encode(Sha256::digest(&der));
-        state.paired.lock().unwrap().push(der);
+        // Isolate from any real paired store on the dev box: AppState::new loads
+        // ~/.config/punktfunk/paired.json, so clear it before seeding our stand-in — otherwise
+        // a real GameStream-paired client lands at body[0] and this assertion sees its hash.
+        {
+            let mut p = state.paired.lock().unwrap();
+            p.clear();
+            p.push(der);
+        }
 
         let (status, body) = send(&app, get_req("/api/v1/clients")).await;
         assert_eq!(status, StatusCode::OK);

crates/punktfunk-host/src/vdisplay.rs

@@ -59,6 +59,91 @@ pub enum Compositor {
     Gamescope,
 }
 
+impl Compositor {
+    /// Stable lowercase id used on the wire / management API (matches
+    /// [`punktfunk_core::CompositorPref::as_str`]).
+    pub fn id(self) -> &'static str {
+        match self {
+            Compositor::Kwin => "kwin",
+            Compositor::Wlroots => "wlroots",
+            Compositor::Mutter => "mutter",
+            Compositor::Gamescope => "gamescope",
+        }
+    }
+
+    /// Human label for UIs.
+    pub fn label(self) -> &'static str {
+        match self {
+            Compositor::Kwin => "KWin / KDE Plasma",
+            Compositor::Wlroots => "wlroots (Sway / Hyprland)",
+            Compositor::Mutter => "Mutter / GNOME",
+            Compositor::Gamescope => "gamescope",
+        }
+    }
+
+    /// The protocol [`punktfunk_core::CompositorPref`] naming this backend.
+    pub fn as_pref(self) -> punktfunk_core::CompositorPref {
+        use punktfunk_core::CompositorPref as P;
+        match self {
+            Compositor::Kwin => P::Kwin,
+            Compositor::Wlroots => P::Wlroots,
+            Compositor::Mutter => P::Mutter,
+            Compositor::Gamescope => P::Gamescope,
+        }
+    }
+
+    /// The concrete backend a [`punktfunk_core::CompositorPref`] names, or `None` for `Auto`.
+    pub fn from_pref(p: punktfunk_core::CompositorPref) -> Option<Compositor> {
+        use punktfunk_core::CompositorPref as P;
+        Some(match p {
+            P::Auto => return None,
+            P::Kwin => Compositor::Kwin,
+            P::Wlroots => Compositor::Wlroots,
+            P::Mutter => Compositor::Mutter,
+            P::Gamescope => Compositor::Gamescope,
+        })
+    }
+
+    /// Every backend, in a stable display order (for enumeration / UIs).
+    pub fn all() -> [Compositor; 4] {
+        [
+            Compositor::Kwin,
+            Compositor::Gamescope,
+            Compositor::Mutter,
+            Compositor::Wlroots,
+        ]
+    }
+}
+
+/// The compositor backends usable on this host *right now*: gamescope wherever its binary is
+/// installed (it spawns a nested session — independent of the running desktop), plus the live
+/// session's own compositor (KWin / Mutter / wlroots) when the host runs inside it. Cheap,
+/// side-effect-free probes — safe to call per management request. A concrete client preference
+/// is validated against this set before it's honored (see the m3 handshake's resolution).
+pub fn available() -> Vec<Compositor> {
+    #[cfg(target_os = "linux")]
+    {
+        let mut v = Vec::new();
+        if kwin::is_available() {
+            v.push(Compositor::Kwin);
+        }
+        if gamescope::is_available() {
+            v.push(Compositor::Gamescope);
+        }
+        if mutter::is_available() {
+            v.push(Compositor::Mutter);
+        }
+        if wlroots::is_available() {
+            v.push(Compositor::Wlroots);
+        }
+        v
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        Vec::new()
+    }
+}
+
 /// Detect the compositor to drive: `PUNKTFUNK_COMPOSITOR` override, else `XDG_CURRENT_DESKTOP`.
 pub fn detect() -> Result<Compositor> {
     if let Ok(v) = std::env::var("PUNKTFUNK_COMPOSITOR") {

crates/punktfunk-host/src/vdisplay/gamescope.rs

@@ -198,6 +198,17 @@ fn find_gamescope_node() -> Option<u32> {
     None
 }
 
+/// gamescope is usable wherever its binary runs — it spawns its own nested session, so it does
+/// not require any particular desktop to be running. Quiet (no version warning — that's for the
+/// create path); just checks the binary executes.
+pub fn is_available() -> bool {
+    std::process::Command::new("gamescope")
+        .arg("--version")
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
 /// Minimum gamescope that captures reliably: below 3.16.22, headless PipeWire capture deadlocks
 /// against PipeWire ≥ 1.6 (a loop-lock bug) and a stuck link head-blocks the whole daemon.
 const MIN_GAMESCOPE: (u32, u32, u32) = (3, 16, 22);
@@ -255,7 +266,10 @@ mod tests {
 
     #[test]
     fn parses_version_banner() {
-        assert_eq!(parse_version("gamescope version 3.16.22"), Some((3, 16, 22)));
+        assert_eq!(
+            parse_version("gamescope version 3.16.22"),
+            Some((3, 16, 22))
+        );
         assert_eq!(
             parse_version("gamescope: version v3.15.9 (no PipeWire)"),
             Some((3, 15, 9))

crates/punktfunk-host/src/vdisplay/kwin.rs

@@ -307,6 +307,12 @@ pub fn probe() -> Result<()> {
     Ok(())
 }
 
+/// KWin is usable iff we're inside a KWin session exposing `zkde_screencast` — exactly what
+/// [`probe`] checks, surfaced as a bool for compositor enumeration.
+pub fn is_available() -> bool {
+    probe().is_ok()
+}
+
 fn run(
     width: u32,
     height: u32,

crates/punktfunk-host/src/vdisplay/mutter.rs

@@ -46,6 +46,15 @@ impl MutterDisplay {
     }
 }
 
+/// Mutter is usable when the host runs inside a GNOME session (its `RecordVirtual` D-Bus API
+/// drives the *live* compositor). Cheap signal: `XDG_CURRENT_DESKTOP` names GNOME — same basis
+/// as [`super::detect`], avoiding a blocking D-Bus round-trip on the enumeration path.
+pub fn is_available() -> bool {
+    std::env::var("XDG_CURRENT_DESKTOP")
+        .map(|d| d.to_ascii_uppercase().contains("GNOME"))
+        .unwrap_or(false)
+}
+
 impl VirtualDisplay for MutterDisplay {
     fn name(&self) -> &'static str {
         "mutter"

crates/punktfunk-host/src/vdisplay/wlroots.rs

@@ -62,6 +62,12 @@ impl WlrootsDisplay {
     }
 }
 
+/// wlroots/Sway is usable when the host runs inside a Sway session — signalled by `SWAYSOCK`
+/// (the IPC socket `swaymsg create_output` needs). Cheap env check for the enumeration path.
+pub fn is_available() -> bool {
+    std::env::var_os("SWAYSOCK").is_some()
+}
+
 impl VirtualDisplay for WlrootsDisplay {
     fn name(&self) -> &'static str {
         "wlroots"

docs/api/openapi.json

@@ -103,6 +103,41 @@
         }
       }
     },
+    "/api/v1/compositors": {
+      "get": {
+        "tags": [
+          "host"
+        ],
+        "summary": "Available compositor backends",
+        "description": "Lists every backend the host knows how to drive, flags which are usable right now, and marks\nthe one an unspecified (`Auto`) client request resolves to. Clients pass an `id` to their\n`--compositor` flag (or `PUNKTFUNK_COMPOSITOR_*` over the C ABI) to request it.",
+        "operationId": "listCompositors",
+        "responses": {
+          "200": {
+            "description": "Compositor backends with availability + the auto-detected default",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/AvailableCompositor"
+                  }
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Missing or invalid bearer token",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ApiError"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/health": {
       "get": {
         "tags": [
@@ -381,6 +416,34 @@
           }
         }
       },
+      "AvailableCompositor": {
+        "type": "object",
+        "description": "A compositor backend the host can drive a virtual output on, and whether it's usable now.",
+        "required": [
+          "id",
+          "label",
+          "available",
+          "default"
+        ],
+        "properties": {
+          "available": {
+            "type": "boolean",
+            "description": "Usable on this host right now: the live session's own compositor, or gamescope wherever\nits binary is installed."
+          },
+          "default": {
+            "type": "boolean",
+            "description": "True for the backend an `Auto` (unspecified) request resolves to right now."
+          },
+          "id": {
+            "type": "string",
+            "description": "Stable identifier (`\"kwin\"` | `\"wlroots\"` | `\"mutter\"` | `\"gamescope\"`) — pass this to a\nclient's `--compositor` flag."
+          },
+          "label": {
+            "type": "string",
+            "description": "Human-readable label for UIs."
+          }
+        }
+      },
       "Health": {
         "type": "object",
         "description": "Liveness + version probe.",

include/punktfunk_core.h

@@ -19,6 +19,24 @@
 // added `punktfunk_pair` / `punktfunk_generate_identity` / `punktfunk_connection_request_mode`.
 #define ABI_VERSION 2
 
+// Compositor preference for [`punktfunk_connect_ex`] (`compositor` arg). `AUTO` lets the host
+// pick (auto-detect from its running desktop); a concrete value is honored only if that backend
+// is available on the host right now, else the host falls back to auto-detect. The resolved
+// choice is reported back over the protocol (see `punktfunk/1` `Welcome`).
+#define PUNKTFUNK_COMPOSITOR_AUTO 0
+
+// KWin / KDE Plasma.
+#define PUNKTFUNK_COMPOSITOR_KWIN 1
+
+// wlroots (Sway / Hyprland).
+#define PUNKTFUNK_COMPOSITOR_WLROOTS 2
+
+// Mutter / GNOME.
+#define PUNKTFUNK_COMPOSITOR_MUTTER 3
+
+// gamescope (spawned nested).
+#define PUNKTFUNK_COMPOSITOR_GAMESCOPE 4
+
 // 16-byte AEAD authentication tag appended by GCM.
 #define TAG_LEN 16
 
@@ -350,7 +368,8 @@ PunktfunkStatus punktfunk_get_stats(PunktfunkSession *s, PunktfunkStats *out);
 
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Connect to a `punktfunk/1` host and start a session at `width`x`height`@`refresh_hz`.
-// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure.
+// Blocks up to `timeout_ms` for the handshake. Returns NULL on failure. Equivalent to
+// [`punktfunk_connect_ex`] with `compositor = PUNKTFUNK_COMPOSITOR_AUTO`.
 //
 // Trust: `pin_sha256` (NULL or 32 bytes) is the expected SHA-256 fingerprint of the host's
 // certificate — a mismatching host is rejected. NULL = trust on first use; persist the
@@ -378,6 +397,27 @@ PunktfunkConnection *punktfunk_connect(const char *host,
                                        uint32_t timeout_ms);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Like [`punktfunk_connect`], but requests a specific `compositor` backend on the host (one of
+// the `PUNKTFUNK_COMPOSITOR_*` values). `PUNKTFUNK_COMPOSITOR_AUTO` (or any unrecognized value)
+// lets the host decide; a concrete value is honored only if available, else the host falls back
+// to auto-detect. The resolved choice is logged host-side and returned over the protocol.
+//
+// # Safety
+// Same as [`punktfunk_connect`].
+PunktfunkConnection *punktfunk_connect_ex(const char *host,
+                                          uint16_t port,
+                                          uint32_t width,
+                                          uint32_t height,
+                                          uint32_t refresh_hz,
+                                          uint32_t compositor,
+                                          const uint8_t *pin_sha256,
+                                          uint8_t *observed_sha256_out,
+                                          const char *client_cert_pem,
+                                          const char *client_key_pem,
+                                          uint32_t timeout_ms);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Generate a persistent client identity: a self-signed certificate + private key, both
 // PEM, NUL-terminated, written into the caller's buffers. Generate ONCE, store both

web/messages/de.json

@@ -31,6 +31,11 @@
   "host_codecs": "Codecs",
   "host_ports": "Ports",
   "host_uniqueid": "Eindeutige ID",
+  "host_compositors": "Compositoren",
+  "host_compositors_help": "Backends, auf denen der Host eine virtuelle Ausgabe erzeugen kann. Übergib eine ID an das --compositor-Flag eines Clients; der Host nutzt sie, falls verfügbar, sonst per Auto-Erkennung.",
+  "compositor_available": "Verfügbar",
+  "compositor_unavailable": "Nicht verfügbar",
+  "compositor_default": "Standard",
   "clients_title": "Gekoppelte Geräte",
   "clients_empty": "Noch keine gekoppelten Geräte.",
   "clients_name": "Name",

web/messages/en.json

@@ -31,6 +31,11 @@
   "host_codecs": "Codecs",
   "host_ports": "Ports",
   "host_uniqueid": "Unique ID",
+  "host_compositors": "Compositors",
+  "host_compositors_help": "Backends the host can drive a virtual output on. Pass an id to a client's --compositor flag; the host honors it if available, else auto-detects.",
+  "compositor_available": "Available",
+  "compositor_unavailable": "Unavailable",
+  "compositor_default": "Default",
   "clients_title": "Paired clients",
   "clients_empty": "No paired clients yet.",
   "clients_name": "Name",

web/src/routes/host.tsx

@@ -1,5 +1,5 @@
 import { createFileRoute } from '@tanstack/react-router'
-import { useGetHostInfo } from '@/api/gen/host/host'
+import { useGetHostInfo, useListCompositors } from '@/api/gen/host/host'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
 import { Badge } from '@/components/ui/badge'
 import { QueryState } from '@/components/query-state'
@@ -11,6 +11,7 @@ export const Route = createFileRoute('/host')({ component: HostPage })
 function HostPage() {
   useLocale()
   const host = useGetHostInfo()
+  const compositors = useListCompositors()
   const h = host.data
 
   return (
@@ -65,6 +66,37 @@ function HostPage() {
           </div>
         )}
       </QueryState>
+
+      <Card>
+        <CardHeader>
+          <CardTitle>{m.host_compositors()}</CardTitle>
+        </CardHeader>
+        <CardContent className="space-y-4">
+          <p className="text-sm text-muted-foreground">{m.host_compositors_help()}</p>
+          <QueryState
+            isLoading={compositors.isLoading}
+            error={compositors.error}
+            refetch={compositors.refetch}
+          >
+            <ul className="divide-y rounded-md border">
+              {compositors.data?.map((c) => (
+                <li key={c.id} className="flex items-center justify-between gap-4 px-4 py-3">
+                  <div className="min-w-0">
+                    <div className="flex items-center gap-2">
+                      <span className="font-medium">{c.label}</span>
+                      {c.default && <Badge variant="secondary">{m.compositor_default()}</Badge>}
+                    </div>
+                    <code className="text-xs text-muted-foreground">{c.id}</code>
+                  </div>
+                  <Badge variant={c.available ? 'default' : 'outline'}>
+                    {c.available ? m.compositor_available() : m.compositor_unavailable()}
+                  </Badge>
+                </li>
+              ))}
+            </ul>
+          </QueryState>
+        </CardContent>
+      </Card>
     </div>
   )
 }