From 3678c182d5002c5f5d7eb4f156ec111b2c4fa613 Mon Sep 17 00:00:00 2001
From: enricobuehler <buehler@unom.io>
Date: Thu, 2 Jul 2026 00:29:38 +0000
Subject: [PATCH] feat(clients): codec preference on Windows/Apple/Android
 clients (Phase 2b)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Rounds out codec negotiation across the last three clients — each advertises what it can decode,
builds its decoder from the resolved Welcome.codec, and exposes a "Video codec" preference picker.

**Windows** (Rust, mirrors Linux): `decodable_codecs()` + `ffmpeg_codec_id()`; the D3D11VA and
software FFmpeg decoders (and the mid-session D3D11VA→software demotion) open the negotiated codec
instead of hardcoding HEVC; settings gain a `codec` field + reactor ComboBox; `--codec` CLI flag.

**Apple** (Swift/C-ABI): AnnexB is now codec-aware — a `VideoCodec` enum drives H.264 vs HEVC NAL
parsing / parameter-set extraction (`CMVideoFormatDescriptionCreateFromH264ParameterSets` for H.264,
no VPS) and AVCC repacking; `PunktfunkConnection` advertises H264|HEVC via `punktfunk_connect_ex7`,
reads `resolvedCodec` (`punktfunk_connection_codec`), and threads `videoCodec` into the stage-1/2
pipelines + `VideoDecoder`; SettingsView "Video codec" Picker (auto/HEVC/H.264). AV1 is left out
(hosts don't emit it on the native path, and it's not an AnnexB codec). Test call sites updated.

**Android** (Kotlin + Rust JNI): the JNI `nativeConnect` gains `preferredCodec`; the native decode
loop picks the AMediaCodec MIME (`video/hevc`|`video/avc`) from `connector.codec` and advertises
H264|HEVC; Settings `codec` field + Compose dropdown.

Core/host/probe/Linux clippy + tests green (unchanged from 2a). Windows/Apple/Android compile on
their platform CI (this Linux box can't build them — Windows toolchain / Xcode / the Android NDK's
opus-cmake toolchain). All follow the Linux client's validated pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 .../kotlin/io/unom/punktfunk/ConnectScreen.kt |   3 +-
 .../main/kotlin/io/unom/punktfunk/Settings.kt |  21 +++
 .../io/unom/punktfunk/SettingsScreen.kt       |   6 +
 .../io/unom/punktfunk/kit/NativeBridge.kt     |   2 +
 clients/android/native/src/decode.rs          |  13 +-
 clients/android/native/src/session.rs         |   6 +
 .../Sources/PunktfunkClient/ContentView.swift |  12 ++
 .../PunktfunkClient/SessionModel.swift        |   8 +-
 .../PunktfunkClient/SettingsView.swift        |  17 ++-
 .../apple/Sources/PunktfunkKit/AnnexB.swift   | 122 +++++++++++++-----
 .../Sources/PunktfunkKit/DefaultsKeys.swift   |   3 +
 .../PunktfunkKit/PunktfunkConnection.swift    |  27 +++-
 .../Sources/PunktfunkKit/Stage2Pipeline.swift |   3 +-
 .../Sources/PunktfunkKit/StreamPump.swift     |   4 +-
 .../Sources/PunktfunkKit/VideoDecoder.swift   |  14 +-
 .../Tests/PunktfunkKitTests/AnnexBTests.swift |   4 +-
 .../RemoteFirstLightTests.swift               |   4 +-
 .../PunktfunkKitTests/Stage444Tests.swift     |   2 +-
 .../VideoToolboxRoundTripTests.swift          |   8 +-
 clients/windows/src/app.rs                    |  25 ++++
 clients/windows/src/main.rs                   |   7 +
 clients/windows/src/session.rs                |  16 ++-
 clients/windows/src/trust.rs                  |  21 +++
 clients/windows/src/video.rs                  |  54 ++++++--
 24 files changed, 328 insertions(+), 74 deletions(-)

diff --git a/clients/android/app/src/main/kotlin/io/unom/punktfunk/ConnectScreen.kt b/clients/android/app/src/main/kotlin/io/unom/punktfunk/ConnectScreen.kt
index a380c10..ad110dd 100644
--- a/clients/android/app/src/main/kotlin/io/unom/punktfunk/ConnectScreen.kt
+++ b/clients/android/app/src/main/kotlin/io/unom/punktfunk/ConnectScreen.kt
@@ -187,7 +187,8 @@ fun ConnectScreen(settings: Settings, onConnected: (Long) -> Unit) {
                     targetHost, targetPort, w, h, hz,
                     id.certPem, id.privateKeyPem, pinHex ?: "",
                     settings.bitrateKbps, settings.compositor, gamepadPref,
-                    hdrEnabled, settings.audioChannels, CONNECT_TIMEOUT_MS,
+                    hdrEnabled, settings.audioChannels, settings.preferredCodec(),
+                    CONNECT_TIMEOUT_MS,
                 )
             }
             connecting = false
diff --git a/clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt b/clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt
index 01479be..1499d16 100644
--- a/clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt
+++ b/clients/android/app/src/main/kotlin/io/unom/punktfunk/Settings.kt
@@ -26,6 +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
+     * host emits it when it can, else falls back. AMediaCodec decodes whichever the host resolves. */
+    val codec: String = "auto",
     val micEnabled: Boolean = false,
     /** Show the live stats overlay (FPS / throughput / latency) during a stream. */
     val statsHudEnabled: Boolean = true,
@@ -51,6 +54,7 @@ class SettingsStore(context: Context) {
         compositor = prefs.getInt(K_COMPOSITOR, 0),
         gamepad = prefs.getInt(K_GAMEPAD, 0),
         audioChannels = prefs.getInt(K_AUDIO_CH, 2),
+        codec = prefs.getString(K_CODEC, "auto") ?: "auto",
         micEnabled = prefs.getBoolean(K_MIC, false),
         statsHudEnabled = prefs.getBoolean(K_HUD, true),
         trackpadMode = prefs.getBoolean(K_TRACKPAD, true),
@@ -66,6 +70,7 @@ class SettingsStore(context: Context) {
             .putInt(K_COMPOSITOR, s.compositor)
             .putInt(K_GAMEPAD, s.gamepad)
             .putInt(K_AUDIO_CH, s.audioChannels)
+            .putString(K_CODEC, s.codec)
             .putBoolean(K_MIC, s.micEnabled)
             .putBoolean(K_HUD, s.statsHudEnabled)
             .putBoolean(K_TRACKPAD, s.trackpadMode)
@@ -81,6 +86,7 @@ class SettingsStore(context: Context) {
         const val K_COMPOSITOR = "compositor"
         const val K_GAMEPAD = "gamepad"
         const val K_AUDIO_CH = "audio_channels"
+        const val K_CODEC = "codec"
         const val K_MIC = "mic_enabled"
         const val K_HUD = "stats_hud_enabled"
         const val K_TRACKPAD = "trackpad_mode"
@@ -156,6 +162,21 @@ val AUDIO_CHANNEL_OPTIONS = listOf(
     8 to "7.1 Surround",
 )
 
+/** (stored value, label) for the preferred video codec. `"auto"` = host decides. */
+val CODEC_OPTIONS = listOf(
+    "auto" to "Automatic",
+    "hevc" to "HEVC (H.265)",
+    "h264" to "H.264 (AVC)",
+)
+
+/** The [Settings.codec] string as a `quic::CODEC_*` preference byte (`0` = auto). H264=1, HEVC=2. */
+fun Settings.preferredCodec(): Int = when (codec) {
+    "h264" -> 1
+    "hevc" -> 2
+    "av1" -> 4
+    else -> 0
+}
+
 /** (kbps, label). `0` = host default. */
 val BITRATE_OPTIONS = listOf(
     0 to "Automatic",
diff --git a/clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt b/clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt
index 5447fd5..f2bc874 100644
--- a/clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt
+++ b/clients/android/app/src/main/kotlin/io/unom/punktfunk/SettingsScreen.kt
@@ -95,6 +95,12 @@ fun SettingsScreen(initial: Settings, onChange: (Settings) -> Unit, onBack: () -
                 selected = s.bitrateKbps,
             ) { kbps -> update(s.copy(bitrateKbps = kbps)) }
 
+            SettingDropdown(
+                label = "Video codec",
+                options = CODEC_OPTIONS,
+                selected = s.codec,
+            ) { c -> update(s.copy(codec = c)) }
+
             // HDR is only meaningful on a panel that can present HDR10; on an SDR display the toggle
             // is disabled (and HDR is never advertised regardless) so the host doesn't send PQ the
             // panel would mis-tone-map. The capability is fixed for the device, so read it once.
diff --git a/clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt b/clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt
index 2cf3625..e4bb368 100644
--- a/clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt
+++ b/clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt
@@ -48,6 +48,8 @@ object NativeBridge {
         gamepadPref: Int,
         hdrEnabled: Boolean,
         audioChannels: Int,
+        /** Preferred video codec as a `quic::CODEC_*` bit (`0` = auto). Soft — the host falls back. */
+        preferredCodec: Int,
         timeoutMs: Int,
     ): Long
 
diff --git a/clients/android/native/src/decode.rs b/clients/android/native/src/decode.rs
index f483956..7968ef0 100644
--- a/clients/android/native/src/decode.rs
+++ b/clients/android/native/src/decode.rs
@@ -27,16 +27,23 @@ pub fn run(
 ) {
     boost_thread_priority();
     let mode = client.mode();
-    let codec = match MediaCodec::from_decoder_type("video/hevc") {
+    // The MediaCodec MIME for the codec the host resolved (`Welcome.codec`): HEVC or H.264. AMediaCodec
+    // needs no out-of-band extradata — the in-band VPS/SPS/PPS on every IDR configure it either way.
+    let mime = match client.codec {
+        punktfunk_core::quic::CODEC_H264 => "video/avc",
+        _ => "video/hevc",
+    };
+    let codec = match MediaCodec::from_decoder_type(mime) {
         Some(c) => c,
         None => {
-            log::error!("decode: no HEVC decoder on this device");
+            log::error!("decode: no {mime} decoder on this device");
             return;
         }
     };
+    log::info!("decode: codec mime = {mime}");
 
     let mut format = MediaFormat::new();
-    format.set_str("mime", "video/hevc");
+    format.set_str("mime", mime);
     format.set_i32("width", mode.width as i32);
     format.set_i32("height", mode.height as i32);
     // Generous input buffer so a large keyframe AU is never truncated.
diff --git a/clients/android/native/src/session.rs b/clients/android/native/src/session.rs
index c12f72e..7de2abf 100644
--- a/clients/android/native/src/session.rs
+++ b/clients/android/native/src/session.rs
@@ -167,6 +167,7 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeConnect<'lo
     gamepad_pref: jint,
     hdr_enabled: jboolean,
     audio_channels: jint,
+    preferred_codec: jint,
     timeout_ms: jint,
 ) -> jlong {
     let host: String = match env.get_string(&host) {
@@ -224,6 +225,11 @@ 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;
+        // hosts don't emit it on the native path yet). The host resolves the emitted codec from these
+        // + the soft `preferred_codec` and echoes it in `connector.codec`, which drives the mime below.
+        punktfunk_core::quic::CODEC_H264 | punktfunk_core::quic::CODEC_HEVC,
+        preferred_codec.clamp(0, u8::MAX as jint) as u8,
         None,     // launch: default app
         pin,      // Some → Crypto on host-fp mismatch
         identity, // owned (cert, key) PEM, or None (anonymous)
diff --git a/clients/apple/Sources/PunktfunkClient/ContentView.swift b/clients/apple/Sources/PunktfunkClient/ContentView.swift
index 52d42e4..e238d85 100644
--- a/clients/apple/Sources/PunktfunkClient/ContentView.swift
+++ b/clients/apple/Sources/PunktfunkClient/ContentView.swift
@@ -28,10 +28,20 @@ struct ContentView: View {
     @AppStorage(DefaultsKey.gamepadType) private var gamepadType = 0
     @AppStorage(DefaultsKey.bitrateKbps) private var bitrateKbps = 0
     @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
+    @AppStorage(DefaultsKey.codec) private var codec = "auto"
     @AppStorage(DefaultsKey.hdrEnabled) private var hdrEnabled = true
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
+    /// The `codec` setting as a `PUNKTFUNK_CODEC_*` soft-preference byte (`0` = auto).
+    private var preferredCodecByte: UInt8 {
+        switch codec {
+        case "h264": return PunktfunkConnection.codecH264
+        case "hevc": return PunktfunkConnection.codecHEVC
+        case "av1": return PunktfunkConnection.codecAV1
+        default: return 0
+        }
+    }
     @State private var showAddHost = false
     @State private var pairingTarget: StoredHost?
     /// A fresh `pair=required`/unknown host the user tapped: drives the choice between no-PIN
@@ -378,6 +388,7 @@ struct ContentView: View {
             bitrateKbps: UInt32(clamping: bitrateKbps),
             audioChannels: UInt8(clamping: audioChannels),
             hdrEnabled: hdrEnabled,
+            preferredCodec: preferredCodecByte,
             launchID: launchID,
             allowTofu: allowTofu,
             requestAccess: requestAccess)
@@ -521,6 +532,7 @@ struct ContentView: View {
             bitrateKbps: bitrate,
             audioChannels: UInt8(clamping: audioChannels),
             hdrEnabled: hdrEnabled,
+            preferredCodec: preferredCodecByte,
             autoTrust: true)
     }
 }
diff --git a/clients/apple/Sources/PunktfunkClient/SessionModel.swift b/clients/apple/Sources/PunktfunkClient/SessionModel.swift
index ea96df0..87a0725 100644
--- a/clients/apple/Sources/PunktfunkClient/SessionModel.swift
+++ b/clients/apple/Sources/PunktfunkClient/SessionModel.swift
@@ -108,6 +108,7 @@ final class SessionModel: ObservableObject {
                  bitrateKbps: UInt32 = 0,
                  audioChannels: UInt8 = 2,
                  hdrEnabled: Bool = true,
+                 preferredCodec: UInt8 = 0,
                  launchID: String? = nil,
                  allowTofu: Bool = false,
                  autoTrust: Bool = false,
@@ -155,12 +156,17 @@ 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
+            // emit it on the native path yet). The host 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,
                 width: width, height: height, refreshHz: hz,
                 pinSHA256: pin, identity: identity, compositor: compositor,
                 gamepad: gamepad, bitrateKbps: bitrateKbps, videoCaps: videoCaps,
-                audioChannels: audioChannels, launchID: launchID,
+                audioChannels: audioChannels,
+                videoCodecs: videoCodecs, preferredCodec: preferredCodec, launchID: launchID,
                 // Delegated approval: the host holds this connect open until the operator approves
                 // it (~180 s) — outwait that window so a slow approval still lands here. Normal
                 // connects keep the snappy default.
diff --git a/clients/apple/Sources/PunktfunkClient/SettingsView.swift b/clients/apple/Sources/PunktfunkClient/SettingsView.swift
index e6aef36..a57fb21 100644
--- a/clients/apple/Sources/PunktfunkClient/SettingsView.swift
+++ b/clients/apple/Sources/PunktfunkClient/SettingsView.swift
@@ -30,6 +30,7 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.fullscreenWhileStreaming) private var fullscreenWhileStreaming = true
     @AppStorage(DefaultsKey.micEnabled) private var micEnabled = true
     @AppStorage(DefaultsKey.audioChannels) private var audioChannels = 2
+    @AppStorage(DefaultsKey.codec) private var codec = "auto"
     @AppStorage(DefaultsKey.hudEnabled) private var hudEnabled = true
     @AppStorage(DefaultsKey.hudPlacement) private var hudPlacement = HUDPlacement.topTrailing.rawValue
     @ObservedObject private var gamepads = GamepadManager.shared
@@ -671,16 +672,22 @@ struct SettingsView: View {
 
     @ViewBuilder private var hdrSection: some View {
         Section {
+            Picker("Video codec", selection: $codec) {
+                Text("Automatic").tag("auto")
+                Text("HEVC (H.265)").tag("hevc")
+                Text("H.264 (AVC)").tag("h264")
+            }
             Toggle("10-bit HDR", isOn: $hdrEnabled)
             Toggle("Full chroma (4:4:4)", isOn: $enable444)
         } header: {
             Text("Video quality")
         } footer: {
-            Text("HDR requests a 10-bit BT.2020 PQ (HDR10) stream — it only engages when the host is "
-                + "sending HDR content AND this display supports HDR. 4:4:4 requests full chroma "
-                + "(sharper text/UI, more bandwidth) — it only engages when this device can "
-                + "hardware-decode it AND the host opted in. Otherwise the stream stays 8-bit "
-                + "4:2:0 SDR. Applies from the next session.")
+            Text("Codec is a preference — the host falls back if it can't encode the one you pick "
+                + "(and 10-bit/4:4:4 are HEVC-only). HDR requests a 10-bit BT.2020 PQ (HDR10) stream — "
+                + "it only engages when the host is sending HDR content AND this display supports HDR. "
+                + "4:4:4 requests full chroma (sharper text/UI, more bandwidth) — it only engages when "
+                + "this device can hardware-decode it AND the host opted in. Otherwise the stream stays "
+                + "8-bit 4:2:0 SDR. Applies from the next session.")
                 .font(.geist(12, relativeTo: .caption))
                 .foregroundStyle(.secondary)
         }
diff --git a/clients/apple/Sources/PunktfunkKit/AnnexB.swift b/clients/apple/Sources/PunktfunkKit/AnnexB.swift
index ed9436d..9dfdd09 100644
--- a/clients/apple/Sources/PunktfunkKit/AnnexB.swift
+++ b/clients/apple/Sources/PunktfunkKit/AnnexB.swift
@@ -10,6 +10,20 @@
 import CoreMedia
 import Foundation
 
+/// The video codec of the host's elementary stream — negotiated in the Welcome and read via
+/// `punktfunk_connection_codec`. Both are Annex-B with in-band parameter sets on every IDR; they
+/// 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 path yet).
+public enum VideoCodec: Equatable {
+    case h264
+    case hevc
+
+    /// Resolve from the wire `Welcome.codec` byte (`PUNKTFUNK_CODEC_*`; unknown → HEVC).
+    public init(wire: UInt8) {
+        self = wire == 0x01 ? .h264 : .hevc // 0x01 = PUNKTFUNK_CODEC_H264
+    }
+}
+
 public enum AnnexB {
     /// Split an Annex-B stream into NAL units (start codes 00 00 01 / 00 00 00 01 stripped).
     /// All zeros immediately preceding a start code are dropped: they're either the
@@ -47,40 +61,83 @@ public enum AnnexB {
         return (first >> 1) & 0x3F
     }
 
-    /// Build a format description from an IDR AU's in-band VPS(32)/SPS(33)/PPS(34).
-    /// Returns nil when the AU carries no parameter sets (non-IDR).
-    public static func formatDescription(fromIDR au: Data) -> CMVideoFormatDescription? {
+    /// H.264 NAL unit type (bits 0..4 of the first byte).
+    public static func h264NalType(_ nal: Data) -> UInt8 {
+        guard let first = nal.first else { return 0xFF }
+        return first & 0x1F
+    }
+
+    /// True if this NAL is a parameter set for `codec` (dropped from AVCC; kept for the format desc).
+    /// HEVC: VPS 32 / SPS 33 / PPS 34. H.264: SPS 7 / PPS 8 (no VPS).
+    private static func isParameterSet(_ nal: Data, _ codec: VideoCodec) -> Bool {
+        switch codec {
+        case .hevc: let t = hevcNalType(nal); return t == 32 || t == 33 || t == 34
+        case .h264: let t = h264NalType(nal); return t == 7 || t == 8
+        }
+    }
+
+    /// Build a format description from an IDR AU's in-band parameter sets (HEVC: VPS/SPS/PPS;
+    /// H.264: SPS/PPS). Returns nil when the AU carries no parameter sets (non-IDR).
+    public static func formatDescription(fromIDR au: Data, codec: VideoCodec)
+        -> CMVideoFormatDescription?
+    {
+        // Collect the parameter-set NALs in the order VideoToolbox wants them (HEVC: VPS,SPS,PPS;
+        // H.264: SPS,PPS).
         var vps: Data?, sps: Data?, pps: Data?
         for nal in nalUnits(in: au) {
-            switch hevcNalType(nal) {
-            case 32: vps = nal
-            case 33: sps = nal
-            case 34: pps = nal
-            default: break
+            switch codec {
+            case .hevc:
+                switch hevcNalType(nal) {
+                case 32: vps = nal
+                case 33: sps = nal
+                case 34: pps = nal
+                default: break
+                }
+            case .h264:
+                switch h264NalType(nal) {
+                case 7: sps = nal
+                case 8: pps = nal
+                default: break
+                }
             }
         }
-        guard let vps, let sps, let pps else { return nil }
+        guard let sps, let pps else { return nil }
+        let sets: [Data] = codec == .hevc ? [vps, sps, pps].compactMap { $0 } : [sps, pps]
+        guard codec == .h264 || sets.count == 3 else { return nil } // HEVC needs the VPS too
 
         var format: CMVideoFormatDescription?
-        let sets = [vps, sps, pps]
-        let status: OSStatus = sets[0].withUnsafeBytes { v in
-            sets[1].withUnsafeBytes { s in
-                sets[2].withUnsafeBytes { p in
-                    let pointers: [UnsafePointer<UInt8>] = [
-                        v.bindMemory(to: UInt8.self).baseAddress!,
-                        s.bindMemory(to: UInt8.self).baseAddress!,
-                        p.bindMemory(to: UInt8.self).baseAddress!,
-                    ]
-                    let sizes = [vps.count, sps.count, pps.count]
-                    return CMVideoFormatDescriptionCreateFromHEVCParameterSets(
-                        allocator: kCFAllocatorDefault,
-                        parameterSetCount: 3,
-                        parameterSetPointers: pointers,
-                        parameterSetSizes: sizes,
-                        nalUnitHeaderLength: 4,
-                        extensions: nil,
-                        formatDescriptionOut: &format)
-                }
+        // Pin every parameter set's bytes for the duration of the create call, then hand
+        // VideoToolbox parallel pointer/size arrays.
+        var pointers: [UnsafePointer<UInt8>] = []
+        var sizes: [Int] = []
+        func withAll(_ i: Int, _ body: () -> Void) {
+            if i == sets.count { body(); return }
+            sets[i].withUnsafeBytes { raw in
+                pointers.append(raw.bindMemory(to: UInt8.self).baseAddress!)
+                sizes.append(sets[i].count)
+                withAll(i + 1, body)
+            }
+        }
+        var status: OSStatus = -1
+        withAll(0) {
+            switch codec {
+            case .hevc:
+                status = CMVideoFormatDescriptionCreateFromHEVCParameterSets(
+                    allocator: kCFAllocatorDefault,
+                    parameterSetCount: pointers.count,
+                    parameterSetPointers: pointers,
+                    parameterSetSizes: sizes,
+                    nalUnitHeaderLength: 4,
+                    extensions: nil,
+                    formatDescriptionOut: &format)
+            case .h264:
+                status = CMVideoFormatDescriptionCreateFromH264ParameterSets(
+                    allocator: kCFAllocatorDefault,
+                    parameterSetCount: pointers.count,
+                    parameterSetPointers: pointers,
+                    parameterSetSizes: sizes,
+                    nalUnitHeaderLength: 4,
+                    formatDescriptionOut: &format)
             }
         }
         return status == noErr ? format : nil
@@ -88,11 +145,10 @@ public enum AnnexB {
 
     /// Re-pack an Annex-B AU as AVCC (4-byte big-endian length before each NAL), dropping
     /// the parameter-set NALs (they live in the format description).
-    public static func avcc(from au: Data) -> Data {
+    public static func avcc(from au: Data, codec: VideoCodec) -> Data {
         var out = Data(capacity: au.count + 16)
         for nal in nalUnits(in: au) {
-            let t = hevcNalType(nal)
-            if t == 32 || t == 33 || t == 34 { continue } // VPS/SPS/PPS
+            if isParameterSet(nal, codec) { continue }
             var len = UInt32(nal.count).bigEndian
             withUnsafeBytes(of: &len) { out.append(contentsOf: $0) }
             out.append(nal)
@@ -102,9 +158,9 @@ public enum AnnexB {
 
     /// Wrap one AU as a decode-ready CMSampleBuffer.
     public static func sampleBuffer(
-        au: AccessUnit, format: CMVideoFormatDescription
+        au: AccessUnit, format: CMVideoFormatDescription, codec: VideoCodec
     ) -> CMSampleBuffer? {
-        let avccData = avcc(from: au.data)
+        let avccData = avcc(from: au.data, codec: codec)
         var blockBuffer: CMBlockBuffer?
         guard CMBlockBufferCreateWithMemoryBlock(
             allocator: kCFAllocatorDefault, memoryBlock: nil,
diff --git a/clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift b/clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift
index 9b85529..370ac57 100644
--- a/clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift
+++ b/clients/apple/Sources/PunktfunkKit/DefaultsKeys.swift
@@ -18,6 +18,9 @@ public enum DefaultsKey {
     /// 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 in-core decode + AVAudioEngine layout.
     public static let audioChannels = "punktfunk.audioChannels"
+    /// Preferred video codec: `"auto"` (host decides), `"hevc"`, or `"h264"`. A soft preference —
+    /// the host emits it when it can, else falls back. Drives the decoder via `Welcome.codec`.
+    public static let codec = "punktfunk.codec"
     public static let micEnabled = "punktfunk.micEnabled"
     public static let speakerUID = "punktfunk.speakerUID"
     public static let micUID = "punktfunk.micUID"
diff --git a/clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift b/clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift
index 564954d..172a5ed 100644
--- a/clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift
+++ b/clients/apple/Sources/PunktfunkKit/PunktfunkConnection.swift
@@ -255,6 +255,13 @@ public final class PunktfunkConnection {
     /// PCM from `nextAudioPcm` is interleaved in the canonical wire order FL FR FC LFE RL RR SL SR.
     public private(set) var resolvedAudioChannels: UInt8 = 2
 
+    /// The video codec the host resolved for this session (`Welcome.codec`, `PUNKTFUNK_CODEC_*`):
+    /// `2` = HEVC (default / older host), `1` = H.264, `4` = AV1. Build the decoder from THIS. The
+    /// resolved value honors the client's `preferredCodec` when the host could emit it.
+    public private(set) var resolvedCodec: UInt8 = 2 // PUNKTFUNK_CODEC_HEVC
+    /// The resolved codec as an `AnnexB.VideoCodec` (H.264 vs HEVC) — drives the NAL parsing.
+    public var videoCodec: VideoCodec { VideoCodec(wire: resolvedCodec) }
+
     /// Connect and start a session at the requested mode (the host creates a native virtual
     /// output at exactly this size/refresh). Blocks up to `timeoutMs`.
     ///
@@ -285,6 +292,8 @@ public final class PunktfunkConnection {
         bitrateKbps: UInt32 = 0,
         videoCaps: UInt8 = 0,
         audioChannels: UInt8 = 2,
+        videoCodecs: UInt8 = 0x02, // PUNKTFUNK_CODEC_HEVC — the codecs this client can decode
+        preferredCodec: UInt8 = 0, // 0 = auto; else PUNKTFUNK_CODEC_* soft preference
         launchID: String? = nil,
         timeoutMs: UInt32 = 10_000
     ) throws {
@@ -300,16 +309,18 @@ public final class PunktfunkConnection {
                     withOptionalCString(launchID) { launch in
                         if let pin = pinSHA256 {
                             return pin.withUnsafeBytes { p in
-                                punktfunk_connect_ex6(
+                                punktfunk_connect_ex7(
                                     cs, port, width, height, refreshHz, compositor.rawValue,
-                                    gamepad.rawValue, bitrateKbps, videoCaps, audioChannels, launch,
+                                    gamepad.rawValue, bitrateKbps, videoCaps, audioChannels,
+                                    videoCodecs, preferredCodec, launch,
                                     p.bindMemory(to: UInt8.self).baseAddress, &observed,
                                     cert, key, timeoutMs)
                             }
                         }
-                        return punktfunk_connect_ex6(
+                        return punktfunk_connect_ex7(
                             cs, port, width, height, refreshHz, compositor.rawValue,
-                            gamepad.rawValue, bitrateKbps, videoCaps, audioChannels, launch,
+                            gamepad.rawValue, bitrateKbps, videoCaps, audioChannels,
+                            videoCodecs, preferredCodec, launch,
                             nil, &observed, cert, key, timeoutMs)
                     }
                 }
@@ -347,6 +358,9 @@ public final class PunktfunkConnection {
         var ac: UInt8 = 2
         _ = punktfunk_connection_audio_channels(handle, &ac)
         resolvedAudioChannels = ac
+        var codec: UInt8 = 2 // PUNKTFUNK_CODEC_HEVC
+        _ = punktfunk_connection_codec(handle, &codec)
+        resolvedCodec = codec
     }
 
     /// A bandwidth speed-test measurement (see `startSpeedTest`). Partial until `done`.
@@ -620,6 +634,11 @@ public final class PunktfunkConnection {
     /// the host then emits 4:4:4 only if it too opted in. `chromaFormat` reflects the real value.
     public static let videoCap444: UInt8 = UInt8(PUNKTFUNK_VIDEO_CAP_444)
 
+    /// Codec bits for `videoCodecs` / `preferredCodec` and the value `resolvedCodec` returns.
+    public static let codecH264: UInt8 = UInt8(PUNKTFUNK_CODEC_H264)
+    public static let codecHEVC: UInt8 = UInt8(PUNKTFUNK_CODEC_HEVC)
+    public static let codecAV1: UInt8 = UInt8(PUNKTFUNK_CODEC_AV1)
+
     /// Static HDR mastering metadata (SMPTE ST.2086 + content light level) the host sent for an HDR
     /// session. Mirrors the wire/ABI `PunktfunkHdrMeta`; primaries are in ST.2086 **G, B, R** order,
     /// 1/50000 units; mastering luminance in 0.0001 cd/m²; MaxCLL/MaxFALL in nits.
diff --git a/clients/apple/Sources/PunktfunkKit/Stage2Pipeline.swift b/clients/apple/Sources/PunktfunkKit/Stage2Pipeline.swift
index 5c8741d..436d4ee 100644
--- a/clients/apple/Sources/PunktfunkKit/Stage2Pipeline.swift
+++ b/clients/apple/Sources/PunktfunkKit/Stage2Pipeline.swift
@@ -119,6 +119,7 @@ public final class Stage2Pipeline {
         // chroma subsampling drives only the decode pixel format (orthogonal to HDR/depth); the HDR
         // config is the Welcome's latched value, which a mid-session flip then overrides per-frame.
         decoder.setChroma444(connection.isChroma444)
+        decoder.setCodec(connection.videoCodec)
         presenter.configure(hdr: connection.isHDR)
 
         let token = token
@@ -159,7 +160,7 @@ public final class Stage2Pipeline {
                     }
                     guard let au = try connection.nextAU(timeoutMs: 100) else { continue }
                     onFrame?(au)
-                    if let f = AnnexB.formatDescription(fromIDR: au.data) {
+                    if let f = AnnexB.formatDescription(fromIDR: au.data, codec: connection.videoCodec) {
                         format = f          // refreshed on every IDR (mode changes included)
                         awaitingIDR = false // a fresh IDR re-anchored decode — recovery complete
                     }
diff --git a/clients/apple/Sources/PunktfunkKit/StreamPump.swift b/clients/apple/Sources/PunktfunkKit/StreamPump.swift
index 4b41c30..cecae03 100644
--- a/clients/apple/Sources/PunktfunkKit/StreamPump.swift
+++ b/clients/apple/Sources/PunktfunkKit/StreamPump.swift
@@ -95,7 +95,7 @@ final class StreamPump {
 
                     guard let au = try connection.nextAU(timeoutMs: 100) else { continue }
                     onFrame?(au)
-                    let idrFormat = AnnexB.formatDescription(fromIDR: au.data)
+                    let idrFormat = AnnexB.formatDescription(fromIDR: au.data, codec: connection.videoCodec)
                     if let f = idrFormat {
                         format = f          // refreshed on every IDR (mode changes included)
                         if awaitingIDR {
@@ -119,7 +119,7 @@ final class StreamPump {
                     }
                     wasFailed = failed
                     guard let f = format,
-                          let sample = AnnexB.sampleBuffer(au: au, format: f),
+                          let sample = AnnexB.sampleBuffer(au: au, format: f, codec: connection.videoCodec),
                           token.isLive // don't enqueue a stale frame after a restart
                     else { continue }
                     layer.enqueue(sample)
diff --git a/clients/apple/Sources/PunktfunkKit/VideoDecoder.swift b/clients/apple/Sources/PunktfunkKit/VideoDecoder.swift
index 127b3f9..e8e6761 100644
--- a/clients/apple/Sources/PunktfunkKit/VideoDecoder.swift
+++ b/clients/apple/Sources/PunktfunkKit/VideoDecoder.swift
@@ -54,6 +54,10 @@ public final class VideoDecoder: @unchecked Sendable {
     /// depth / HDR). Read inside `createSessionLocked` under `lock`.
     private var chroma444 = false
 
+    /// The negotiated codec (`connection.videoCodec`), set once at session start. Drives the AnnexB
+    /// NAL parsing (H.264 vs HEVC parameter sets). Read under `lock`.
+    private var codec: VideoCodec = .hevc
+
     public init(
         onDecoded: @escaping @Sendable (ReadyFrame) -> Void,
         onDecodeError: @escaping @Sendable (OSStatus) -> Void = { _ in }
@@ -73,6 +77,14 @@ public final class VideoDecoder: @unchecked Sendable {
         lock.unlock()
     }
 
+    /// Select the negotiated codec (H.264 vs HEVC). Call once at session start, before decoding,
+    /// from `connection.videoCodec`. Thread-safe.
+    public func setCodec(_ c: VideoCodec) {
+        lock.lock()
+        codec = c
+        lock.unlock()
+    }
+
     /// Submit one AU for asynchronous decode, (re)creating the session if `format` changed. The
     /// caller resolves `format` from the IDR exactly as stage-1 does (`AnnexB.formatDescription`).
     /// Returns false if the session couldn't be created or the frame couldn't be submitted.
@@ -93,7 +105,7 @@ public final class VideoDecoder: @unchecked Sendable {
         // invalidate the session between here and DecodeFrame. The VT output callback takes the
         // ring lock, not this one, so there's no re-entrancy. DecodeFrame is async — non-blocking.
         guard let session,
-              let sample = AnnexB.sampleBuffer(au: au, format: newFormat)
+              let sample = AnnexB.sampleBuffer(au: au, format: newFormat, codec: codec)
         else { lock.unlock(); return false }
         var infoOut = VTDecodeInfoFlags()
         let status = VTDecompressionSessionDecodeFrame(
diff --git a/clients/apple/Tests/PunktfunkKitTests/AnnexBTests.swift b/clients/apple/Tests/PunktfunkKitTests/AnnexBTests.swift
index 8bde705..e443b00 100644
--- a/clients/apple/Tests/PunktfunkKitTests/AnnexBTests.swift
+++ b/clients/apple/Tests/PunktfunkKitTests/AnnexBTests.swift
@@ -64,7 +64,7 @@ final class AnnexBTests: XCTestCase {
             au.append(n)
         }
 
-        let avcc = AnnexB.avcc(from: au)
+        let avcc = AnnexB.avcc(from: au, codec: .hevc)
         // Only the IDR survives: 4-byte BE length, then the NAL bytes.
         var expected = Data([0, 0, 0, UInt8(idr.count)])
         expected.append(idr)
@@ -74,6 +74,6 @@ final class AnnexBTests: XCTestCase {
     func testFormatDescriptionNilWithoutParameterSets() {
         let idr = nal(type: 19, payload: [0xDD])
         let au = Data(start4) + idr
-        XCTAssertNil(AnnexB.formatDescription(fromIDR: au))
+        XCTAssertNil(AnnexB.formatDescription(fromIDR: au, codec: .hevc))
     }
 }
diff --git a/clients/apple/Tests/PunktfunkKitTests/RemoteFirstLightTests.swift b/clients/apple/Tests/PunktfunkKitTests/RemoteFirstLightTests.swift
index d7a354d..cb81960 100644
--- a/clients/apple/Tests/PunktfunkKitTests/RemoteFirstLightTests.swift
+++ b/clients/apple/Tests/PunktfunkKitTests/RemoteFirstLightTests.swift
@@ -138,7 +138,7 @@ final class RemoteFirstLightTests: XCTestCase {
             if firstPtsNs == 0 { firstPtsNs = au.ptsNs }
             lastPtsNs = au.ptsNs
 
-            if let f = AnnexB.formatDescription(fromIDR: au.data) {
+            if let f = AnnexB.formatDescription(fromIDR: au.data, codec: .hevc) {
                 format = f
                 if decoder == nil {
                     let dims = CMVideoFormatDescriptionGetDimensions(f)
@@ -155,7 +155,7 @@ final class RemoteFirstLightTests: XCTestCase {
                 }
             }
             guard let f = format, let dec = decoder,
-                  let sample = AnnexB.sampleBuffer(au: au, format: f)
+                  let sample = AnnexB.sampleBuffer(au: au, format: f, codec: .hevc)
             else { continue }
 
             var gotPixels = false
diff --git a/clients/apple/Tests/PunktfunkKitTests/Stage444Tests.swift b/clients/apple/Tests/PunktfunkKitTests/Stage444Tests.swift
index e78d9f3..328152a 100644
--- a/clients/apple/Tests/PunktfunkKitTests/Stage444Tests.swift
+++ b/clients/apple/Tests/PunktfunkKitTests/Stage444Tests.swift
@@ -30,7 +30,7 @@ final class Stage444Tests: XCTestCase {
             Stage444Probe.hwDecode444_8bit, "no hardware 4:4:4 decode on this device")
         let data = Data(Probe444Blobs.au444_8bit)
         let format = try XCTUnwrap(
-            AnnexB.formatDescription(fromIDR: data), "the 4:4:4 blob must yield a format description")
+            AnnexB.formatDescription(fromIDR: data, codec: .hevc), "the 4:4:4 blob must yield a format description")
         let au = AccessUnit(data: data, ptsNs: 7_000_000, frameIndex: 0, flags: 0)
 
         let box = FrameBox()
diff --git a/clients/apple/Tests/PunktfunkKitTests/VideoToolboxRoundTripTests.swift b/clients/apple/Tests/PunktfunkKitTests/VideoToolboxRoundTripTests.swift
index d501617..c989943 100644
--- a/clients/apple/Tests/PunktfunkKitTests/VideoToolboxRoundTripTests.swift
+++ b/clients/apple/Tests/PunktfunkKitTests/VideoToolboxRoundTripTests.swift
@@ -28,18 +28,18 @@ final class VideoToolboxRoundTripTests: XCTestCase {
 
         // 1) Parameter-set extraction → format description.
         let rebuilt = try XCTUnwrap(
-            AnnexB.formatDescription(fromIDR: annexB),
+            AnnexB.formatDescription(fromIDR: annexB, codec: .hevc),
             "in-band VPS/SPS/PPS should yield a format description")
         let dims = CMVideoFormatDescriptionGetDimensions(rebuilt)
         XCTAssertEqual(Int(dims.width), width)
         XCTAssertEqual(Int(dims.height), height)
 
         // 2) Annex-B → AVCC re-pack must reproduce the encoder's own sample bytes.
-        XCTAssertEqual(AnnexB.avcc(from: annexB), avccSample)
+        XCTAssertEqual(AnnexB.avcc(from: annexB, codec: .hevc), avccSample)
 
         // 3) Sample buffer → real decoder → pixels.
         let au = AccessUnit(data: annexB, ptsNs: 1_000_000, frameIndex: 0, flags: 0)
-        let sample = try XCTUnwrap(AnnexB.sampleBuffer(au: au, format: rebuilt))
+        let sample = try XCTUnwrap(AnnexB.sampleBuffer(au: au, format: rebuilt, codec: .hevc))
 
         var session: VTDecompressionSession?
         XCTAssertEqual(
@@ -72,7 +72,7 @@ final class VideoToolboxRoundTripTests: XCTestCase {
     func testVideoDecoderAsyncCallbackDeliversPixels() throws {
         let (formatDesc, avccSample) = try encodeOneHEVCKeyframe()
         let annexB = try annexBAU(formatDesc: formatDesc, avccSample: avccSample)
-        let format = try XCTUnwrap(AnnexB.formatDescription(fromIDR: annexB))
+        let format = try XCTUnwrap(AnnexB.formatDescription(fromIDR: annexB, codec: .hevc))
         let au = AccessUnit(data: annexB, ptsNs: 42_000_000, frameIndex: 0, flags: 0)
 
         let box = FrameBox()
diff --git a/clients/windows/src/app.rs b/clients/windows/src/app.rs
index b95629b..71713d8 100644
--- a/clients/windows/src/app.rs
+++ b/clients/windows/src/app.rs
@@ -44,6 +44,14 @@ const BITRATES_MBPS: &[u32] = &[0, 10, 20, 30, 50, 80, 150];
 /// Audio channel presets: `(channel count, display label)`. The host clamps to what it can
 /// capture; the resolved count drives the decoder + WASAPI render layout.
 const AUDIO_CHANNELS: &[(u8, &str)] = &[(2, "Stereo"), (6, "5.1 Surround"), (8, "7.1 Surround")];
+/// Preferred-codec presets: `(stored value, display label)`. Soft — the host falls back if it can't
+/// encode the chosen codec.
+const CODECS: &[(&str, &str)] = &[
+    ("auto", "Automatic"),
+    ("hevc", "HEVC (H.265)"),
+    ("h264", "H.264 (AVC)"),
+    ("av1", "AV1"),
+];
 
 /// punktfunk's own license (MIT OR Apache-2.0), shown on the Licenses screen.
 const APP_LICENSE: &str = concat!(
@@ -681,6 +689,7 @@ fn connect_with(
         mic_enabled: s.mic_enabled,
         hdr_enabled: s.hdr_enabled,
         decoder: DecoderPref::from_name(&s.decoder),
+        preferred_codec: s.preferred_codec(),
         pin,
         identity: ctx.identity.clone(),
         connect_timeout: opts.connect_timeout,
@@ -1039,6 +1048,21 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
             })
     };
 
+    let codec_i = CODECS.iter().position(|&(v, _)| v == s.codec).unwrap_or(0) as i32;
+    let codec_names: Vec<String> = CODECS.iter().map(|&(_, l)| l.to_string()).collect();
+    let codec_combo = {
+        let ctx = ctx.clone();
+        ComboBox::new(codec_names)
+            .header("Video codec")
+            .selected_index(codec_i)
+            .on_selection_changed(move |i: i32| {
+                let (v, _) = CODECS[(i.max(0) as usize).min(CODECS.len() - 1)];
+                let mut s = ctx.settings.lock().unwrap();
+                s.codec = v.to_string();
+                s.save();
+            })
+    };
+
     let br_i = BITRATES_MBPS
         .iter()
         .position(|&m| m * 1000 == s.bitrate_kbps)
@@ -1149,6 +1173,7 @@ fn settings_page(ctx: &Arc<AppCtx>, set_screen: &AsyncSetState<Screen>) -> Eleme
             .font_size(12.0)
             .foreground(ThemeRef::SecondaryText),
             decoder_combo,
+            codec_combo,
             bitrate_combo,
             hdr_toggle,
         ))
diff --git a/clients/windows/src/main.rs b/clients/windows/src/main.rs
index da2f239..bb83d79 100644
--- a/clients/windows/src/main.rs
+++ b/clients/windows/src/main.rs
@@ -182,6 +182,13 @@ fn run_headless_cli(args: &[String], identity: (String, String)) {
         mic_enabled: flag("--mic"),
         hdr_enabled: !flag("--no-hdr"),
         decoder,
+        // `--codec h264|hevc|av1` sets the soft preference; default auto (host decides).
+        preferred_codec: match arg("--codec").as_deref() {
+            Some("h264") | Some("avc") => punktfunk_core::quic::CODEC_H264,
+            Some("hevc") | Some("h265") => punktfunk_core::quic::CODEC_HEVC,
+            Some("av1") => punktfunk_core::quic::CODEC_AV1,
+            _ => 0,
+        },
         pin,
         identity,
         // Headless CLI uses the normal (short) handshake budget; the long request-access wait is a
diff --git a/clients/windows/src/session.rs b/clients/windows/src/session.rs
index e9e2052..a40cbca 100644
--- a/clients/windows/src/session.rs
+++ b/clients/windows/src/session.rs
@@ -31,6 +31,9 @@ pub struct SessionParams {
     pub hdr_enabled: bool,
     /// Which video decode backend to use (auto/hardware/software).
     pub decoder: DecoderPref,
+    /// The user's preferred video codec (a `quic::CODEC_*` bit, `0` = auto). Soft — the host honors
+    /// it when it can emit it, else falls back; the resolved codec drives the decoder.
+    pub preferred_codec: u8,
     /// Pinned host fingerprint; `None` = trust on first use (caller persists the observed one).
     pub pin: Option<[u8; 32]>,
     pub identity: (String, String),
@@ -166,7 +169,9 @@ fn pump(
             0
         },
         params.audio_channels,
-        None, // launch: the Windows client has no library picker yet
+        crate::video::decodable_codecs(), // codecs FFmpeg can decode (HEVC/H.264/AV1)
+        params.preferred_codec,           // the user's soft codec preference (0 = auto)
+        None,                             // launch: the Windows client has no library picker yet
         params.pin,
         Some(params.identity),
         params.connect_timeout,
@@ -195,7 +200,14 @@ fn pump(
         fingerprint: connector.host_fingerprint,
     });
 
-    let mut decoder = match Decoder::new(params.decoder) {
+    // Build the decoder for the codec the host resolved (never assume HEVC).
+    let codec_id = crate::video::ffmpeg_codec_id(connector.codec);
+    tracing::info!(
+        ?codec_id,
+        welcome_codec = connector.codec,
+        "negotiated video codec"
+    );
+    let mut decoder = match Decoder::new(params.decoder, codec_id) {
         Ok(d) => d,
         Err(e) => {
             let _ = ev_tx.send_blocking(SessionEvent::Ended(Some(format!("video decoder: {e}"))));
diff --git a/clients/windows/src/trust.rs b/clients/windows/src/trust.rs
index 5d578ae..a7c21db 100644
--- a/clients/windows/src/trust.rs
+++ b/clients/windows/src/trust.rs
@@ -138,6 +138,26 @@ pub struct Settings {
     pub hdr_enabled: bool,
     /// Video decode backend: `auto` (D3D11VA, fall back to software), `hardware`, or `software`.
     pub decoder: String,
+    /// Preferred video codec: `"auto"` (host decides), `"hevc"`, `"h264"`, or `"av1"`. A soft
+    /// preference — the host honors it when it can emit it, else falls back to the best shared codec.
+    #[serde(default = "default_codec")]
+    pub codec: String,
+}
+
+fn default_codec() -> String {
+    "auto".into()
+}
+
+impl Settings {
+    /// The `codec` setting as a `quic::CODEC_*` preference bit (`0` = auto).
+    pub fn preferred_codec(&self) -> u8 {
+        match self.codec.as_str() {
+            "h264" | "avc" => punktfunk_core::quic::CODEC_H264,
+            "hevc" | "h265" => punktfunk_core::quic::CODEC_HEVC,
+            "av1" => punktfunk_core::quic::CODEC_AV1,
+            _ => 0,
+        }
+    }
 }
 
 impl Default for Settings {
@@ -154,6 +174,7 @@ impl Default for Settings {
             audio_channels: 2,
             hdr_enabled: true,
             decoder: "auto".into(),
+            codec: "auto".into(),
         }
     }
 }
diff --git a/clients/windows/src/video.rs b/clients/windows/src/video.rs
index 18a5b67..05657a1 100644
--- a/clients/windows/src/video.rs
+++ b/clients/windows/src/video.rs
@@ -124,17 +124,46 @@ enum Backend {
 
 pub struct Decoder {
     backend: Backend,
+    /// The negotiated codec, so a mid-session D3D11VA→software demotion rebuilds for the same codec.
+    codec_id: ffmpeg::codec::Id,
+}
+
+/// Map a negotiated `quic` codec bit to the FFmpeg decoder id the client opens.
+pub fn ffmpeg_codec_id(wire: u8) -> ffmpeg::codec::Id {
+    match wire {
+        punktfunk_core::quic::CODEC_H264 => ffmpeg::codec::Id::H264,
+        punktfunk_core::quic::CODEC_AV1 => ffmpeg::codec::Id::AV1,
+        _ => ffmpeg::codec::Id::HEVC,
+    }
+}
+
+/// The `quic` codec bitfield this client can decode — whatever FFmpeg has a decoder for (HEVC/H.264
+/// always; AV1 when built in). Advertised to the host so it never emits a codec we can't decode.
+pub fn decodable_codecs() -> u8 {
+    let _ = ffmpeg::init();
+    let mut bits = 0u8;
+    for (id, bit) in [
+        (ffmpeg::codec::Id::HEVC, punktfunk_core::quic::CODEC_HEVC),
+        (ffmpeg::codec::Id::H264, punktfunk_core::quic::CODEC_H264),
+        (ffmpeg::codec::Id::AV1, punktfunk_core::quic::CODEC_AV1),
+    ] {
+        if ffmpeg::decoder::find(id).is_some() {
+            bits |= bit;
+        }
+    }
+    bits
 }
 
 impl Decoder {
-    pub fn new(pref: DecoderPref) -> Result<Decoder> {
+    pub fn new(pref: DecoderPref, codec_id: ffmpeg::codec::Id) -> Result<Decoder> {
         ffmpeg::init().context("ffmpeg init")?;
         if pref != DecoderPref::Software {
-            match D3d11vaDecoder::new() {
+            match D3d11vaDecoder::new(codec_id) {
                 Ok(d) => {
-                    tracing::info!("D3D11VA hardware decode active (zero-copy)");
+                    tracing::info!(?codec_id, "D3D11VA hardware decode active (zero-copy)");
                     return Ok(Decoder {
                         backend: Backend::D3d11va(d),
+                        codec_id,
                     });
                 }
                 Err(e) => {
@@ -146,7 +175,8 @@ impl Decoder {
             }
         }
         Ok(Decoder {
-            backend: Backend::Software(SoftwareDecoder::new()?),
+            backend: Backend::Software(SoftwareDecoder::new(codec_id)?),
+            codec_id,
         })
     }
 
@@ -164,7 +194,7 @@ impl Decoder {
                 Ok(f) => Ok(f.map(DecodedFrame::Gpu)),
                 Err(e) => {
                     tracing::warn!(error = %e, "D3D11VA decode failed — falling back to software");
-                    self.backend = Backend::Software(SoftwareDecoder::new()?);
+                    self.backend = Backend::Software(SoftwareDecoder::new(self.codec_id)?);
                     Ok(None)
                 }
             },
@@ -183,9 +213,9 @@ struct SoftwareDecoder {
 }
 
 impl SoftwareDecoder {
-    fn new() -> Result<SoftwareDecoder> {
-        let codec =
-            ffmpeg::decoder::find(ffmpeg::codec::Id::HEVC).ok_or(anyhow!("no HEVC decoder"))?;
+    fn new(codec_id: ffmpeg::codec::Id) -> Result<SoftwareDecoder> {
+        let codec = ffmpeg::decoder::find(codec_id)
+            .ok_or_else(|| anyhow!("no {codec_id:?} decoder in libavcodec"))?;
         let mut ctx = ffmpeg::codec::Context::new_with_codec(codec);
         unsafe {
             let raw = ctx.as_mut_ptr();
@@ -194,7 +224,7 @@ impl SoftwareDecoder {
             (*raw).thread_type = ffmpeg::ffi::FF_THREAD_SLICE;
             (*raw).thread_count = 0; // auto
         }
-        let decoder = ctx.decoder().video().context("open HEVC decoder")?;
+        let decoder = ctx.decoder().video().context("open video decoder")?;
         Ok(SoftwareDecoder { decoder, sws: None })
     }
 
@@ -359,7 +389,7 @@ struct D3d11vaDecoder {
 unsafe impl Send for D3d11vaDecoder {}
 
 impl D3d11vaDecoder {
-    fn new() -> Result<D3d11vaDecoder> {
+    fn new(codec_id: ffmpeg::codec::Id) -> Result<D3d11vaDecoder> {
         use ffmpeg::ffi;
         let shared = crate::gpu::shared().ok_or_else(|| anyhow!("no shared D3D11 device"))?;
         if !shared.hardware {
@@ -387,11 +417,11 @@ impl D3d11vaDecoder {
                 bail!("av_hwdevice_ctx_init: {}", ffmpeg::Error::from(r));
             }
 
-            let codec = ffi::avcodec_find_decoder(ffi::AVCodecID::AV_CODEC_ID_HEVC);
+            let codec = ffi::avcodec_find_decoder(codec_id.into());
             if codec.is_null() {
                 let mut hw = hw_device;
                 ffi::av_buffer_unref(&mut hw);
-                bail!("no HEVC decoder");
+                bail!("no {codec_id:?} decoder");
             }
             let ctx = ffi::avcodec_alloc_context3(codec);
             (*ctx).hw_device_ctx = ffi::av_buffer_ref(hw_device);