feat(codec): make client codec selection real — GPU-aware native advertisement

The clients' codec pickers sent a preference the host threw away:
host_wire_caps() hardcoded HEVC for every GPU backend, so the native
path never emitted H.264/AV1 regardless of what the user chose.

Host: advertise what the backend actually encodes, mirroring the
GameStream serverinfo logic — software openh264 emits H.264; probed
backends (Linux VAAPI, Windows AMF/QSV) advertise their per-GPU
CodecSupport via the new wire_mask(); NVENC keeps the
Moonlight-validated H.264|HEVC|AV1 static superset; an empty probe
falls back to the superset so auto clients still resolve HEVC. Gate
10-bit to HEVC (like the 4:4:4 gate) now that a client can steer the
codec — Main10 is the only 10-bit encode path. Fix the web-console
stats label hardcoded to "hevc" (new Codec::label(), shared with the
GameStream register_session mapping).

Android: replace the hardcoded H264|HEVC Hello advertisement with a
videoCodecs param fed by VideoDecoders.decodableCodecBits() (AV1 bit
only when a real hardware video/av01 decoder exists — the decode loop
was already mime-driven); offer AV1 in the codec picker on capable
devices.

Decky: add the missing "Video codec" dropdown (auto/hevc/h264/av1) to
the plugin settings, round-tripping the same codec key the flatpak
client reads.

Apple: unchanged by design (AnnexB.swift is NAL-only, AV1 is never
advertised); refresh the stale "hosts don't emit AV1 on the native
path yet" comments here and host-side.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/android/app/src/main/kotlin/io/unom/punktfunk/HostConnect.kt

@@ -3,6 +3,7 @@ package io.unom.punktfunk
 import android.content.Context
 import io.unom.punktfunk.kit.Gamepad
 import io.unom.punktfunk.kit.NativeBridge
+import io.unom.punktfunk.kit.VideoDecoders
 import io.unom.punktfunk.kit.security.ClientIdentity
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.withContext
@@ -41,7 +42,10 @@ suspend fun connectToHost(
             host, port, w, h, hz,
             identity.certPem, identity.privateKeyPem, pinHex,
             settings.bitrateKbps, settings.compositor, gamepadPref,
-            hdrEnabled, settings.audioChannels, settings.preferredCodec(), timeoutMs,
+            hdrEnabled, settings.audioChannels,
+            // What this device can decode (H.264|HEVC always, AV1 when a real decoder exists) +
+            // the user's soft codec preference — the host resolves the emitted codec from both.
+            VideoDecoders.decodableCodecBits(), settings.preferredCodec(), timeoutMs,
             launch,
         )
     }

clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt

@@ -26,8 +26,9 @@ data class Settings(
     /** Requested audio channel count: 2 (stereo), 6 (5.1) or 8 (7.1). The host clamps to what it
      * can capture; the resolved count drives the decoder + AAudio layout. */
     val audioChannels: Int = 2,
-    /** Preferred video codec: `"auto"` (host decides), `"hevc"`, or `"h264"`. A soft preference — the
+    /** Preferred video codec: `"auto"` (host decides), `"hevc"`, `"h264"`, or `"av1"`. A soft
-     * host emits it when it can, else falls back. AMediaCodec decodes whichever the host resolves. */
+     * preference — the host emits it when it can, else falls back. AMediaCodec decodes whichever
+     * the host resolves (AV1 is only advertised/offered when the device has a real AV1 decoder). */
     val codec: String = "auto",
     val micEnabled: Boolean = false,
     /**
@@ -271,14 +272,17 @@ val AUDIO_CHANNEL_OPTIONS = listOf(
     8 to "7.1 Surround",
 )
 
-/** (stored value, label) for the preferred video codec. `"auto"` = host decides. */
+/** (stored value, label) for the preferred video codec. `"auto"` = host decides. The `"av1"` row
+ * only makes sense on a device with a real AV1 decoder — SettingsScreen filters it out otherwise. */
 val CODEC_OPTIONS = listOf(
     "auto" to "Automatic",
     "hevc" to "HEVC (H.265)",
     "h264" to "H.264 (AVC)",
+    "av1" to "AV1",
 )
 
-/** The [Settings.codec] string as a `quic::CODEC_*` preference byte (`0` = auto). H264=1, HEVC=2. */
+/** The [Settings.codec] string as a `quic::CODEC_*` preference byte (`0` = auto). H264=1, HEVC=2,
+ * AV1=4. */
 fun Settings.preferredCodec(): Int = when (codec) {
     "h264" -> 1
     "hevc" -> 2

clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt

@@ -65,6 +65,7 @@ import androidx.compose.ui.graphics.vector.ImageVector
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.dp
 import androidx.core.content.ContextCompat
+import io.unom.punktfunk.kit.VideoDecoders
 
 /**
  * Stream settings, organised as an iOS-Settings / Android-system-settings style list of category
@@ -300,7 +301,12 @@ private fun DisplaySettings(s: Settings, update: (Settings) -> Unit, context: an
             update(s.copy(bitrateKbps = kbps))
         }
 
-        SettingDropdown(label = "Video codec", options = CODEC_OPTIONS, selected = s.codec) { c ->
+        // AV1 is only offered when the device has a real AV1 decoder (it's never advertised to the
+        // host otherwise, so preferring it would be a dead setting). A stored "av1" from a capable
+        // device stays visible so the selection is always representable.
+        val av1Capable = remember { VideoDecoders.pickDecoder("video/av01") != null }
+        val codecOptions = CODEC_OPTIONS.filter { (v, _) -> v != "av1" || av1Capable || s.codec == "av1" }
+        SettingDropdown(label = "Video codec", options = codecOptions, selected = s.codec) { c ->
             update(s.copy(codec = c))
         }
 

clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt

@@ -48,6 +48,9 @@ object NativeBridge {
         gamepadPref: Int,
         hdrEnabled: Boolean,
         audioChannels: Int,
+        /** `quic::CODEC_*` bitfield of codecs this device decodes ([VideoDecoders.decodableCodecBits]);
+         *  `0` falls back to H.264|HEVC. The host resolves the emitted codec from this ∩ its GPU. */
+        videoCodecs: Int,
         /** Preferred video codec as a `quic::CODEC_*` bit (`0` = auto). Soft — the host falls back. */
         preferredCodec: Int,
         timeoutMs: Int,

clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/VideoDecoders.kt

@@ -44,6 +44,15 @@ object VideoDecoders {
      * Pick the best decoder for [mime] (`"video/hevc"` / `"video/avc"` / `"video/av01"`), or `null`
      * to let the platform resolve its default. Enumerates once — call at stream start.
      */
+    /**
+     * The `quic::CODEC_*` bitfield of codecs this device can decode, advertised in the Hello so the
+     * host never emits a codec the decode loop can't open: H.264 (1) and HEVC (2) always (universal
+     * on Android hardware), plus AV1 (4) only when [pickDecoder] finds a real (hardware, non-blocked)
+     * `video/av01` decoder. Enumerates `MediaCodecList` — call at connect time, not per frame.
+     */
+    fun decodableCodecBits(): Int =
+        1 or 2 or (if (pickDecoder("video/av01") != null) 4 else 0)
+
     fun pickDecoder(mime: String): DecoderChoice? {
         if (mime.isEmpty()) return null
         val infos = runCatching { MediaCodecList(MediaCodecList.REGULAR_CODECS).codecInfos }

clients/android/native/src/session/connect.rs

@@ -76,6 +76,7 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeConnect<'lo
     gamepad_pref: jint,
     hdr_enabled: jboolean,
     audio_channels: jint,
+    video_codecs: jint,
     preferred_codec: jint,
     timeout_ms: jint,
     launch: JString<'local>,
@@ -142,10 +143,23 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeConnect<'lo
         // decoder + AAudio layout (read in `crate::audio::AudioPlayback::start`). Anything else
         // normalizes to stereo here.
         punktfunk_core::audio::normalize_channels(audio_channels.clamp(0, u8::MAX as jint) as u8),
-        // Codecs this device can decode — AMediaCodec decodes both HEVC and H.264 (AV1 isn't wired;
+        // Codecs this device can decode, ranked on the Kotlin side (`VideoDecoders.decodableCodecBits`:
-        // hosts don't emit it on the native path yet). The host resolves the emitted codec from these
+        // H.264 + HEVC always, AV1 when a real `video/av01` decoder exists — AMediaCodec is
-        // + the soft `preferred_codec` and echoes it in `connector.codec`, which drives the mime below.
+        // mime-driven, see `codec_mime`). Mask to the known bits and fall back to the pre-AV1
-        punktfunk_core::quic::CODEC_H264 | punktfunk_core::quic::CODEC_HEVC,
+        // H.264|HEVC pair on 0 so a bogus value can't advertise nothing and kill the handshake.
+        // The host resolves the emitted codec from these + the soft `preferred_codec` and echoes it
+        // in `connector.codec`, which drives the mime below.
+        {
+            let bits = (video_codecs.clamp(0, u8::MAX as jint) as u8)
+                & (punktfunk_core::quic::CODEC_H264
+                    | punktfunk_core::quic::CODEC_HEVC
+                    | punktfunk_core::quic::CODEC_AV1);
+            if bits == 0 {
+                punktfunk_core::quic::CODEC_H264 | punktfunk_core::quic::CODEC_HEVC
+            } else {
+                bits
+            }
+        },
         preferred_codec.clamp(0, u8::MAX as jint) as u8,
         launch,   // a store-qualified library id to boot into a game, or None for the desktop
         pin,      // Some → Crypto on host-fp mismatch

clients/apple/Sources/PunktfunkClient/Session/SessionModel.swift

@@ -194,9 +194,10 @@ final class SessionModel: ObservableObject {
             if want444, canDecode444 {
                 videoCaps |= PunktfunkConnection.videoCap444
             }
-            // This client's VideoToolbox path decodes H.264 and HEVC (AV1 isn't wired — hosts don't
+            // This client's VideoToolbox path decodes H.264 and HEVC (AV1 depacketization isn't
-            // emit it on the native path yet). The host resolves the emitted codec from these + the
+            // wired — AnnexB.swift is NAL-only — so it must never be advertised here). The host
-            // soft `preferredCodec`; `resolvedCodec` reflects what it chose.
+            // resolves the emitted codec from these + the soft `preferredCodec`; `resolvedCodec`
+            // reflects what it chose.
             let videoCodecs = PunktfunkConnection.codecH264 | PunktfunkConnection.codecHEVC
             let result = Result { try PunktfunkConnection(
                 host: host.address, port: host.port,

clients/apple/Sources/PunktfunkClient/Settings/SettingsOptions.swift

@@ -38,8 +38,8 @@ enum SettingsOptions {
         HUDPlacement.allCases.map { ($0.label, $0.rawValue) }
 
     /// Video-codec preference (`DefaultsKey.codec`) — a soft preference the host falls back from.
-    /// No AV1: this client's VideoToolbox path decodes H.264/HEVC only (hosts don't emit AV1 on
+    /// No AV1: this client's VideoToolbox path decodes H.264/HEVC only (AnnexB.swift is NAL-only),
-    /// the native path yet).
+    /// so it never advertises AV1 — offering it here would be a dead setting.
     static let codecs: [(label: String, tag: String)] = [
         ("Automatic", "auto"),
         ("HEVC (H.265)", "hevc"),

clients/apple/Sources/PunktfunkKit/Video/AnnexB.swift

@@ -6,8 +6,8 @@
 // buffers whose NALs are 4-byte-length-prefixed. This file converts between the two, for
 // the codec the host resolved in the Welcome (`connection.videoCodec`) — HEVC and H.264
 // differ only in NAL-header layout and which parameter sets exist (HEVC adds a VPS). AV1
-// is not an Annex-B/NAL codec and isn't handled here (hosts don't emit it on the native
+// is not an Annex-B/NAL codec and isn't handled here — this client never advertises it in
-// path yet).
+// the Hello, so a host never emits it at us.
 //
 // HOT PATH: both pumps run `formatDescription(fromIDR:codec:)` + `sampleBuffer(au:format:codec:)`
 // once per AU, so the conversion is built on `forEachNAL` — a zero-copy scan over the AU's bytes

clients/decky/main.py

@@ -669,7 +669,7 @@ class Plugin:
             # The client's own defaults (native display, host-default bitrate, auto pad).
             return {
                 "width": 0, "height": 0, "refresh_hz": 0, "bitrate_kbps": 0,
-                "gamepad": "auto", "compositor": "auto",
+                "codec": "auto", "gamepad": "auto", "compositor": "auto",
                 "inhibit_shortcuts": True, "mic_enabled": False,
             }
 

clients/decky/src/backend.ts

@@ -61,13 +61,14 @@ export interface RunnerInfo {
 }
 
 // The slice of the flatpak client's settings JSON this UI surfaces. The file can hold more
-// keys (codec, decoder, … set from the desktop client's own UI) — they round-trip untouched
+// keys (decoder, … set from the desktop client's own UI) — they round-trip untouched
 // because get_settings returns the whole parsed file and patches are object spreads.
 export interface StreamSettings {
   width: number; // 0 = native
   height: number; // 0 = native
   refresh_hz: number; // 0 = native
   bitrate_kbps: number; // 0 = host default
+  codec?: string; // "auto" | "hevc" | "h264" | "av1" — soft preference (absent in pre-codec files)
   gamepad: string; // "auto" | "xbox360" | "xboxone" | "dualsense" | "dualshock4" | "steamdeck"
   compositor: string; // "auto" | "kwin" | "wlroots" | "mutter" | "gamescope"
   inhibit_shortcuts: boolean;

clients/decky/src/settings.tsx

@@ -34,6 +34,15 @@ const GAMEPAD_LABELS: Record<string, string> = {
   dualshock4: "DualShock 4",
   steamdeck: "Steam Deck",
 };
+// Mirrors the desktop client's picker (ui_settings.rs CODECS) — a soft preference the host
+// falls back from when its GPU can't encode it.
+const CODECS = ["auto", "hevc", "h264", "av1"];
+const CODEC_LABELS: Record<string, string> = {
+  auto: "Automatic",
+  hevc: "HEVC (H.265)",
+  h264: "H.264 (AVC)",
+  av1: "AV1",
+};
 const COMPOSITORS = ["auto", "kwin", "wlroots", "mutter", "gamescope"];
 const COMPOSITOR_LABELS: Record<string, string> = {
   auto: "Automatic",
@@ -108,6 +117,21 @@ export const SettingsSection: FC = () => {
         valueSuffix=" Mbit/s"
         onChange={(v) => patch({ bitrate_kbps: v * 1000 })}
       />
+      <Field
+        label="Video codec"
+        description="Preferred stream codec — the host falls back when its GPU can't encode it"
+        childrenContainerWidth="max"
+      >
+        <RowActions>
+          <div style={selectShell}>
+            <Dropdown
+              rgOptions={CODECS.map((c) => ({ data: c, label: CODEC_LABELS[c] ?? c }))}
+              selectedOption={s.codec ?? "auto"}
+              onChange={(o) => patch({ codec: o.data as string })}
+            />
+          </div>
+        </RowActions>
+      </Field>
       <Field
         label="Gamepad type"
         description="Which virtual controller the host creates for your inputs"

crates/punktfunk-host/src/encode.rs

@@ -77,14 +77,68 @@ impl Codec {
     }
 
     /// The `quic` codec bitfield the host can currently **emit** on the punktfunk/1 native path,
-    /// given the resolved encode backend. The GPU-less software encoder (openh264) produces H.264
+    /// given the resolved encode backend — the same GPU-aware advertisement GameStream builds for
-    /// only; every GPU backend emits HEVC today (per-GPU H.264/AV1 negotiation on the native path is
+    /// Moonlight ([`crate::gamestream::serverinfo`]), in `quic::CODEC_*` bits. The GPU-less software
-    /// future work — GameStream already negotiates codecs with Moonlight separately). Fed to
+    /// encoder (openh264) produces H.264 only; the probed backends (Linux VAAPI, Windows AMF/QSV)
+    /// advertise exactly what the GPU encodes ([`vaapi_codec_support`] / [`windows_codec_support`] —
+    /// AV1 encode is narrow, an old iGPU might lack HEVC); NVENC keeps the Moonlight-validated
+    /// static superset. An empty probe means the GPU wasn't usable at probe time (GPU-less CI,
+    /// wrong-vendor pref), not that it encodes nothing — fall back to the superset so `resolve_codec`
+    /// still lands on HEVC for an auto client, exactly the pre-probe behaviour. Fed to
     /// [`punktfunk_core::quic::resolve_codec`] against the client's advertised codecs.
     pub fn host_wire_caps() -> u8 {
-        match crate::config::config().encoder_pref.as_str() {
+        /// The static GPU superset (H.264 | HEVC | AV1) — mirrors the GameStream
-            "software" | "sw" | "openh264" => punktfunk_core::quic::CODEC_H264,
+        /// `SERVER_CODEC_MODE_SUPPORT` advertisement for the unprobed backends.
-            _ => punktfunk_core::quic::CODEC_HEVC,
+        const GPU_SUPERSET: u8 = punktfunk_core::quic::CODEC_H264
+            | punktfunk_core::quic::CODEC_HEVC
+            | punktfunk_core::quic::CODEC_AV1;
+        #[cfg(target_os = "linux")]
+        {
+            if matches!(
+                crate::config::config().encoder_pref.as_str(),
+                "software" | "sw" | "openh264"
+            ) {
+                return punktfunk_core::quic::CODEC_H264;
+            }
+            if linux_zero_copy_is_vaapi() {
+                if let Some(m) = vaapi_codec_support().wire_mask() {
+                    return m;
+                }
+            }
+            // NVENC (static superset, like GameStream) — or an empty VAAPI probe (see above).
+            GPU_SUPERSET
+        }
+        #[cfg(target_os = "windows")]
+        {
+            if windows_resolved_backend() == WindowsBackend::Software {
+                return punktfunk_core::quic::CODEC_H264;
+            }
+            if windows_backend_is_probed() {
+                if let Some(m) = windows_codec_support().wire_mask() {
+                    return m;
+                }
+            }
+            // NVENC (static superset, like GameStream) — or an empty AMF/QSV probe (see above).
+            GPU_SUPERSET
+        }
+        // The macOS dev/test host has no GPU encode backend — keep the pre-probe advertisement.
+        #[cfg(not(any(target_os = "linux", target_os = "windows")))]
+        {
+            let _ = GPU_SUPERSET;
+            match crate::config::config().encoder_pref.as_str() {
+                "software" | "sw" | "openh264" => punktfunk_core::quic::CODEC_H264,
+                _ => punktfunk_core::quic::CODEC_HEVC,
+            }
+        }
+    }
+
+    /// Lowercase stats/console label (`"h264"` / `"hevc"` / `"av1"`) — the codec string seeded into
+    /// the web console's session meta ([`crate::stats_recorder::StatsRecorder::register_session`]).
+    pub fn label(self) -> &'static str {
+        match self {
+            Codec::H264 => "h264",
+            Codec::H265 => "hevc",
+            Codec::Av1 => "av1",
         }
     }
 
@@ -741,6 +795,27 @@ pub struct CodecSupport {
     pub av1: bool,
 }
 
+#[cfg(any(target_os = "linux", target_os = "windows"))]
+impl CodecSupport {
+    /// The probed codecs as a `quic::CODEC_*` bitfield, or `None` when the probe found nothing —
+    /// meaning the GPU wasn't usable at probe time (GPU-less CI, a wrong-vendor pref), NOT that it
+    /// encodes zero codecs; the caller then falls back to the static superset (the native-path
+    /// analogue of `gamestream::serverinfo::probed_mask`).
+    pub fn wire_mask(self) -> Option<u8> {
+        let mut m = 0u8;
+        if self.h264 {
+            m |= punktfunk_core::quic::CODEC_H264;
+        }
+        if self.h265 {
+            m |= punktfunk_core::quic::CODEC_HEVC;
+        }
+        if self.av1 {
+            m |= punktfunk_core::quic::CODEC_AV1;
+        }
+        (m != 0).then_some(m)
+    }
+}
+
 /// Probe the active Linux GPU backend for its encodable codecs (cached; opens a tiny encoder per
 /// codec, once). Only the VAAPI (AMD/Intel) backend is probed — NVENC keeps its Moonlight-validated
 /// static advertisement (callers gate on [`linux_zero_copy_is_vaapi`]).
@@ -1042,4 +1117,42 @@ mod tests {
             }
         }
     }
+
+    /// The probed-capability → wire-bitfield mapping the native codec advertisement is built from.
+    #[cfg(any(target_os = "linux", target_os = "windows"))]
+    #[test]
+    fn codec_support_wire_mask() {
+        use punktfunk_core::quic::{CODEC_AV1, CODEC_H264, CODEC_HEVC};
+        let all = CodecSupport {
+            h264: true,
+            h265: true,
+            av1: true,
+        };
+        assert_eq!(all.wire_mask(), Some(CODEC_H264 | CODEC_HEVC | CODEC_AV1));
+        let hevc_only = CodecSupport {
+            h264: false,
+            h265: true,
+            av1: false,
+        };
+        assert_eq!(hevc_only.wire_mask(), Some(CODEC_HEVC));
+        // An all-false probe means "GPU unusable at probe time", not "zero codecs" — `None` tells
+        // the caller to advertise the static superset instead of refusing every handshake.
+        let none = CodecSupport {
+            h264: false,
+            h265: false,
+            av1: false,
+        };
+        assert_eq!(none.wire_mask(), None);
+    }
+
+    /// Wire round-trip and the stats label stay in lockstep with the `quic::CODEC_*` bits.
+    #[test]
+    fn codec_wire_roundtrip_and_label() {
+        for c in [Codec::H264, Codec::H265, Codec::Av1] {
+            assert_eq!(Codec::from_wire(c.to_wire()), c);
+        }
+        assert_eq!(Codec::H264.label(), "h264");
+        assert_eq!(Codec::H265.label(), "hevc");
+        assert_eq!(Codec::Av1.label(), "av1");
+    }
 }

crates/punktfunk-host/src/gamestream/stream.rs

@@ -663,11 +663,7 @@ fn stream_body(
     // Web-console stats accumulation (active when `perf` OR a capture is armed): per-stage vectors
     // for p50/p99, the goodput bytes queued to the sender this window, the previous window's
     // dropped-frame count for delta computation, and the registration id cached on the first sample.
-    let codec_name = match cfg.codec {
+    let codec_name = cfg.codec.label();
-        Codec::H264 => "h264",
-        Codec::H265 => "hevc",
-        Codec::Av1 => "av1",
-    };
     let mut sid: Option<u32> = None;
     let (mut v_cap, mut v_enc, mut v_pkt, mut v_send): (Vec<u32>, Vec<u32>, Vec<u32>, Vec<u32>) =
         (Vec::new(), Vec::new(), Vec::new(), Vec::new());

crates/punktfunk-host/src/punktfunk1.rs

@@ -732,9 +732,10 @@ async fn serve_session(
         // The pairing gate (require_pairing → paired? else park for delegated approval) ran above,
         // before this future, so a client reaching here is paired (or the host is `--open`).
 
-        // Codec negotiation: pick the one codec this host will emit (its backend capability ∩ the
+        // Codec negotiation: pick the one codec this host will emit (its GPU-probed backend
-        // client's advertised codecs). A GPU-less software host emits H.264, so an HEVC-only client
+        // capability ∩ the client's advertised codecs, honoring the client's soft preference).
-        // shares nothing with it → refuse honestly rather than send a stream it can't decode.
+        // A GPU-less software host emits H.264 only, so an HEVC-only client shares nothing with
+        // it → refuse honestly rather than send a stream it can't decode.
         let host_codecs = crate::encode::Codec::host_wire_caps();
         let codec_bit =
             punktfunk_core::quic::resolve_codec(hello.video_codecs, host_codecs, hello.preferred_codec)
@@ -888,10 +889,16 @@ async fn serve_session(
         // Resolve the encode bit depth: HEVC Main10 only when the client advertised it AND the host
         // opted in (PUNKTFUNK_10BIT). A client that can't decode 10-bit (caps bit clear, or an older
         // client) always gets the 8-bit stream. PUNKTFUNK_10BIT is the host policy gate until a
-        // mgmt/console toggle replaces it.
+        // mgmt/console toggle replaces it. 10-bit is HEVC-only (like the 4:4:4 gate below): now that
+        // the client can steer the codec to H.264/AV1, a non-HEVC session must stay 8-bit — the
+        // encoders' 10-bit path is Main10, and AV1 10-bit stays off until live-confirmed (the same
+        // stance as the GameStream Main10 advertisement).
         let host_wants_10bit = crate::config::config().ten_bit;
         let client_supports_10bit = hello.video_caps & punktfunk_core::quic::VIDEO_CAP_10BIT != 0;
-        let bit_depth: u8 = if host_wants_10bit && client_supports_10bit {
+        let bit_depth: u8 = if host_wants_10bit
+            && client_supports_10bit
+            && codec == crate::encode::Codec::H265
+        {
             10
         } else {
             8
@@ -1010,8 +1017,9 @@ async fn serve_session(
             // The resolved audio channel count the audio thread will capture + Opus-(multi)stream
             // encode (2/6/8). The client builds its decoder from this echoed value.
             audio_channels,
-            // The negotiated codec the encoder will emit (H.264 for a software host, else HEVC). The
+            // The negotiated codec the encoder will emit (client preference ∩ GPU capability;
-            // client builds its decoder from this instead of assuming HEVC.
+            // HEVC-precedence tie-break). The client builds its decoder from this instead of
+            // assuming HEVC.
             codec: codec_bit,
         };
         io::write_msg(&mut send, &welcome.encode()).await?;
@@ -1027,7 +1035,7 @@ async fn serve_session(
             .await
             .map_err(|_| anyhow!("handshake timed out after {HANDSHAKE_TIMEOUT:?}"))??;
     let (mut ctrl_send, mut ctrl_recv) = (send, recv);
-    // Negotiated codec (HEVC / H.264), derived from the Welcome. `Copy`, so the control task's
+    // Negotiated codec (HEVC / H.264 / AV1), derived from the Welcome. `Copy`, so the control task's
     // `async move` captures a copy and it stays usable for the data-plane SessionContext below.
     let codec = crate::encode::Codec::from_wire(welcome.codec);
     let client_udp = std::net::SocketAddr::new(peer.ip(), start.client_udp_port);
@@ -3080,8 +3088,9 @@ struct SessionContext {
     bit_depth: u8,
     /// Negotiated chroma subsampling (4:2:0, or 4:4:4 when the client + host + GPU all support it).
     chroma: crate::encode::ChromaFormat,
-    /// Negotiated video codec the encoder emits (HEVC by default; H.264 for a software host). Also
+    /// Negotiated video codec the encoder emits (HEVC by default; H.264 / AV1 when the client
-    /// used to rebuild the encoder at the same codec across a mid-stream mode reconfigure.
+    /// prefers one the GPU encodes; H.264 for a software host). Also used to rebuild the encoder
+    /// at the same codec across a mid-stream mode reconfigure.
     codec: crate::encode::Codec,
     /// Speed-test burst requests (see [`service_probes`]).
     probe_rx: std::sync::mpsc::Receiver<ProbeRequest>,
@@ -3236,7 +3245,7 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
         width: mode.width,
         height: mode.height,
         fps: mode.refresh_hz,
-        codec: "hevc",
+        codec: plan.codec.label(),
         client: client_label,
         bitrate_kbps,
     };