diff --git a/src-tauri/src/player/conformance.rs b/src-tauri/src/player/conformance.rs
new file mode 100644
index 00000000..9ae1c8a3
--- /dev/null
+++ b/src-tauri/src/player/conformance.rs
@@ -0,0 +1,277 @@
+//! The conformance suite every [`MediaPlayer`] must pass.
+//!
+//! One set of behaviours, run against every engine: `FakePlayer` and `MpvPlayer`
+//! in `cargo test`, `ExoPlayerPlayer` instrumented on a device, `WebviewPlayer`
+//! in vitest. A new engine is finished when it passes this.
+//!
+//! Written *before* the second engine on purpose. A suite written afterwards
+//! encodes whatever the first engine happened to do, which is how three separate
+//! playback implementations drifted apart in the first place.
+//!
+//! Each case names the defect it exists to prevent. Two of them —
+//! [`opens_at_a_start_position`] and [`seek_while_opening_is_honoured`] — fail
+//! against the pre-migration mpv path, which is what makes them a reproduction
+//! of DR-241 rather than a restatement of it.
+//!
+//! Engines differ in *when* an open completes, so the suite drives that through
+//! a [`Harness`] rather than sleeping: the fake completes on demand, mpv waits
+//! for its `FileLoaded` event, ExoPlayer for `STATE_READY`.
+//!
+//! Available to `cargo test` and, behind the `conformance` feature, to the
+//! `player-conformance` binary — so an engine that cannot run in-process
+//! (ExoPlayer on a device) is driven by exactly the same cases rather than by a
+//! second, drifting checklist.
+//!
+//! TRACES: UR-081 | DR-243 | UT-220
+
+use std::time::Duration;
+
+use super::media_player::{MediaPlayer, OpenRequest, Phase};
+
+/// How the suite drives one engine.
+pub trait Harness {
+    type Player: MediaPlayer;
+
+    fn player(&mut self) -> &mut Self::Player;
+
+    /// A request this engine can actually open, at `start`.
+    fn request(&self, start: Duration) -> OpenRequest;
+
+    /// Block until an in-flight `open` has finished (or failed).
+    ///
+    /// The fake completes on demand; a real engine waits for its own readiness
+    /// event. Never a sleep — a timing-dependent suite is worse than none.
+    fn settle(&mut self);
+
+    /// Whether the engine is producing audio. Engines that cannot answer may
+    /// return `None`, which skips the silence assertions rather than passing
+    /// them vacuously.
+    fn audible(&mut self) -> Option<bool>;
+
+    /// How far a landed position may differ from the one asked for. Keyframe
+    /// granularity makes exactness the wrong bar for a real decoder.
+    fn seek_tolerance(&self) -> Duration {
+        Duration::from_secs(5)
+    }
+}
+
+fn assert_near(actual: Duration, expected: Duration, tolerance: Duration, what: &str) {
+    let delta = actual.abs_diff(expected);
+    assert!(
+        delta <= tolerance,
+        "{what}: expected ~{expected:?}, got {actual:?} (tolerance {tolerance:?})"
+    );
+}
+
+/// Opening at zero reaches a usable state and starts near the beginning.
+pub fn opens_from_the_beginning<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    let s = h.player().snapshot();
+    assert!(
+        matches!(s.phase, Phase::Playing | Phase::Ready),
+        "after open the engine should hold media, phase was {:?}",
+        s.phase
+    );
+    assert_near(
+        s.position,
+        Duration::ZERO,
+        h.seek_tolerance(),
+        "start of item",
+    );
+}
+
+/// **DR-241.** Opening at a position starts *there*, not at zero.
+///
+/// The whole reason `OpenRequest` carries `start`. Under the previous contract a
+/// caller had to `load()` then `seek()`, and because `loadfile` is asynchronous
+/// the seek was issued against a player with nothing loaded, failed, and was
+/// discarded — so resume and transcoded skip both played from the beginning.
+pub fn opens_at_a_start_position<H: Harness>(h: &mut H) {
+    let start = Duration::from_secs(600);
+    let req = h.request(start);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    let s = h.player().snapshot();
+    assert_ne!(
+        s.position,
+        Duration::ZERO,
+        "opened at {start:?} but playback began at zero - the start position was dropped"
+    );
+    assert_near(s.position, start, h.seek_tolerance(), "start position");
+}
+
+/// **DR-241.** A seek issued while opening is honoured, not lost.
+///
+/// The engine owns this window; no caller can avoid it, because a caller cannot
+/// see when the pipeline becomes ready.
+pub fn seek_while_opening_is_honoured<H: Harness>(h: &mut H) {
+    let target = Duration::from_secs(300);
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+
+    // Deliberately before settle(): this is the race, expressed on purpose.
+    h.player().seek(target).expect("seek during open failed");
+    h.settle();
+
+    let s = h.player().snapshot();
+    assert_near(
+        s.position,
+        target,
+        h.seek_tolerance(),
+        "seek issued while opening",
+    );
+}
+
+/// A later intent wins: the seek replaces the start position it overtook.
+pub fn seek_while_opening_overrides_start<H: Harness>(h: &mut H) {
+    let start = Duration::from_secs(600);
+    let target = Duration::from_secs(120);
+    let req = h.request(start);
+    h.player().open(req).expect("open failed");
+    h.player().seek(target).expect("seek during open failed");
+    h.settle();
+
+    assert_near(
+        h.player().snapshot().position,
+        target,
+        h.seek_tolerance(),
+        "seek should override the start position it overtook",
+    );
+}
+
+/// Seeking a settled item lands where asked.
+pub fn seeks_after_open<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    let target = Duration::from_secs(420);
+    h.player().seek(target).expect("seek failed");
+
+    assert_near(
+        h.player().snapshot().position,
+        target,
+        h.seek_tolerance(),
+        "seek after open",
+    );
+}
+
+/// **DR-239.** Pause and play are reflected in the engine's own state.
+///
+/// An engine that changes nothing observable is indistinguishable from one that
+/// ignored the call — which is exactly how a handler for mpv's `pause` property
+/// sat unreachable while the UI waited for an event that never came.
+pub fn pause_and_play_are_observable<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    h.player().pause().expect("pause failed");
+    assert_eq!(
+        h.player().snapshot().phase,
+        Phase::Paused,
+        "pause must be visible in the snapshot"
+    );
+    if let Some(audible) = h.audible() {
+        assert!(!audible, "a paused engine must be silent");
+    }
+
+    h.player().play().expect("play failed");
+    assert_eq!(
+        h.player().snapshot().phase,
+        Phase::Playing,
+        "play must be visible in the snapshot"
+    );
+}
+
+/// `close()` reaches Idle, is silent, and can be called twice.
+pub fn close_is_silent_and_idempotent<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    h.player().close().expect("close failed");
+    assert_eq!(h.player().snapshot().phase, Phase::Idle);
+    if let Some(audible) = h.audible() {
+        assert!(!audible, "a closed engine must be silent");
+    }
+
+    h.player().close().expect("close must be idempotent");
+    assert_eq!(h.player().snapshot().phase, Phase::Idle);
+}
+
+/// Closing during an open must not let playback start afterwards.
+///
+/// The shape of the "audio keeps playing after leaving the player" report: an
+/// open still in flight completed after the stop, and nothing was left to tell
+/// it not to.
+pub fn close_during_open_never_plays<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.player().close().expect("close during open failed");
+    h.settle();
+
+    let s = h.player().snapshot();
+    assert!(
+        !s.phase.is_active(),
+        "an open cancelled by close must not start playing, phase was {:?}",
+        s.phase
+    );
+    if let Some(audible) = h.audible() {
+        assert!(!audible, "an engine closed during open must be silent");
+    }
+}
+
+/// Volume, mute and rate round-trip through the snapshot.
+pub fn transport_settings_round_trip<H: Harness>(h: &mut H) {
+    let req = h.request(Duration::ZERO);
+    h.player().open(req).expect("open failed");
+    h.settle();
+
+    h.player().set_volume(0.25).expect("set_volume failed");
+    h.player().set_muted(true).expect("set_muted failed");
+    h.player().set_rate(1.5).expect("set_rate failed");
+
+    let s = h.player().snapshot();
+    assert!((s.volume - 0.25).abs() < 0.01, "volume did not round-trip");
+    assert!(s.muted, "mute did not round-trip");
+    assert!((s.rate - 1.5).abs() < 0.01, "rate did not round-trip");
+}
+
+/// Run every case against one engine.
+///
+/// Each case gets a fresh harness, because a suite whose cases depend on each
+/// other's leftovers is one that hides state bugs instead of finding them.
+#[macro_export]
+macro_rules! media_player_conformance {
+    ($name:ident, $make:expr) => {
+        mod $name {
+            use super::*;
+            use $crate::player::conformance as c;
+
+            macro_rules! case {
+                ($case:ident) => {
+                    #[test]
+                    fn $case() {
+                        let mut h = $make;
+                        c::$case(&mut h);
+                    }
+                };
+            }
+
+            case!(opens_from_the_beginning);
+            case!(opens_at_a_start_position);
+            case!(seek_while_opening_is_honoured);
+            case!(seek_while_opening_overrides_start);
+            case!(seeks_after_open);
+            case!(pause_and_play_are_observable);
+            case!(close_is_silent_and_idempotent);
+            case!(close_during_open_never_plays);
+            case!(transport_settings_round_trip);
+        }
+    };
+}
diff --git a/src-tauri/src/player/fake_player.rs b/src-tauri/src/player/fake_player.rs
new file mode 100644
index 00000000..27aafa35
--- /dev/null
+++ b/src-tauri/src/player/fake_player.rs
@@ -0,0 +1,230 @@
+//! A deterministic in-memory [`MediaPlayer`], for tests.
+//!
+//! Two jobs:
+//!
+//! 1. Give the conformance suite something that is correct by construction, so a
+//!    failure there means the *suite* is wrong rather than an engine.
+//! 2. Let everything above the engine — controller, queue, autoplay, sleep
+//!    timer, session — be tested with no mpv, no device and no network. Most of
+//!    that logic is currently only reachable through a real engine, which is why
+//!    so little of it is covered.
+//!
+//! It models the one behaviour that matters most: **opening is not
+//! instantaneous**. `open()` lands in [`Phase::Opening`] and stays there until
+//! [`FakePlayer::complete_open`] is called, so a test can put a `seek` into that
+//! window on purpose. That is the window DR-241 lived in.
+//!
+//! TRACES: UR-081 | DR-243
+
+// `tick` and `fail_open` are for tests not yet written — the controller-level
+// ones DR-245 unlocks. Remove this allow once those exist.
+#![allow(dead_code)]
+
+use std::time::Duration;
+
+use super::backend::PlayerError;
+use super::media_player::{Capabilities, MediaPlayer, OpenRequest, Phase, PlaybackSnapshot};
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum FakeEvent {
+    Opened { url: String, start: Duration },
+    Played,
+    Paused,
+    Closed,
+    Sought(Duration),
+}
+
+pub struct FakePlayer {
+    snapshot: PlaybackSnapshot,
+    /// Set while `Opening`; applied when the open completes.
+    pending_start: Duration,
+    /// A seek that arrived while opening. Honoured on completion, never dropped.
+    deferred_seek: Option<Duration>,
+    autoplay: bool,
+    duration: Duration,
+    /// Every call, in order — so tests can assert what an engine was *asked* to
+    /// do, not only where it ended up.
+    pub log: Vec<FakeEvent>,
+    /// Whether audio is being produced. `close()` must clear it; the bug that
+    /// motivated all this had a "stopped" player that was still audible.
+    pub audible: bool,
+    pub capabilities: Capabilities,
+}
+
+impl Default for FakePlayer {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl FakePlayer {
+    pub fn new() -> Self {
+        Self {
+            snapshot: PlaybackSnapshot::default(),
+            pending_start: Duration::ZERO,
+            deferred_seek: None,
+            autoplay: true,
+            duration: Duration::from_secs(3600),
+            log: Vec::new(),
+            audible: false,
+            capabilities: Capabilities {
+                video: true,
+                audio_settings: true,
+                subtitle_switching: true,
+                audio_track_switching: true,
+            },
+        }
+    }
+
+    /// The item this fake will report once opened.
+    pub fn with_duration(mut self, duration: Duration) -> Self {
+        self.duration = duration;
+        self
+    }
+
+    /// Finish an in-flight `open`, as a real engine's "file loaded" would.
+    ///
+    /// Applies the requested start position, then any seek that arrived while
+    /// opening — the later intent wins.
+    pub fn complete_open(&mut self) {
+        if self.snapshot.phase != Phase::Opening {
+            return;
+        }
+        self.snapshot.duration = Some(self.duration);
+        self.snapshot.seekable = true;
+        self.snapshot.position = self.deferred_seek.take().unwrap_or(self.pending_start);
+        if self.autoplay {
+            self.snapshot.phase = Phase::Playing;
+            self.audible = true;
+        } else {
+            self.snapshot.phase = Phase::Ready;
+        }
+    }
+
+    /// Advance playback, for tests that care about time passing.
+    pub fn tick(&mut self, by: Duration) {
+        if self.snapshot.phase.is_active() {
+            self.snapshot.position = (self.snapshot.position + by).min(self.duration);
+            if self.snapshot.position >= self.duration {
+                self.snapshot.phase = Phase::Ended;
+                self.audible = false;
+            }
+        }
+    }
+
+    pub fn fail_open(&mut self, why: &str) {
+        self.snapshot.phase = Phase::Failed(why.to_string());
+        self.audible = false;
+    }
+}
+
+impl MediaPlayer for FakePlayer {
+    fn open(&mut self, req: OpenRequest) -> Result<(), PlayerError> {
+        self.log.push(FakeEvent::Opened {
+            url: req.selection.url.clone(),
+            start: req.start,
+        });
+        self.snapshot = PlaybackSnapshot {
+            phase: Phase::Opening,
+            volume: self.snapshot.volume,
+            muted: self.snapshot.muted,
+            rate: self.snapshot.rate,
+            audio_track: req.audio_track,
+            subtitle_track: req.subtitle_track,
+            ..PlaybackSnapshot::default()
+        };
+        self.pending_start = req.start;
+        self.deferred_seek = None;
+        self.autoplay = req.autoplay;
+        self.audible = false;
+        Ok(())
+    }
+
+    fn play(&mut self) -> Result<(), PlayerError> {
+        self.log.push(FakeEvent::Played);
+        if self.snapshot.phase.has_media() {
+            if self.snapshot.phase == Phase::Opening {
+                self.autoplay = true;
+            } else {
+                self.snapshot.phase = Phase::Playing;
+                self.audible = true;
+            }
+        }
+        Ok(())
+    }
+
+    fn pause(&mut self) -> Result<(), PlayerError> {
+        self.log.push(FakeEvent::Paused);
+        if self.snapshot.phase == Phase::Opening {
+            self.autoplay = false;
+        } else if self.snapshot.phase.has_media() {
+            self.snapshot.phase = Phase::Paused;
+            self.audible = false;
+        }
+        Ok(())
+    }
+
+    fn close(&mut self) -> Result<(), PlayerError> {
+        self.log.push(FakeEvent::Closed);
+        self.snapshot = PlaybackSnapshot {
+            volume: self.snapshot.volume,
+            muted: self.snapshot.muted,
+            rate: self.snapshot.rate,
+            ..PlaybackSnapshot::default()
+        };
+        self.pending_start = Duration::ZERO;
+        self.deferred_seek = None;
+        // An open that was still in flight must not come back to life.
+        self.autoplay = false;
+        self.audible = false;
+        Ok(())
+    }
+
+    fn seek(&mut self, to: Duration) -> Result<(), PlayerError> {
+        self.log.push(FakeEvent::Sought(to));
+        match self.snapshot.phase {
+            // The window DR-241 lived in: hold it, do not discard it.
+            Phase::Opening => self.deferred_seek = Some(to),
+            Phase::Idle | Phase::Failed(_) => {
+                return Err(PlayerError {
+                    message: "seek with nothing open".to_string(),
+                })
+            }
+            _ => self.snapshot.position = to.min(self.duration),
+        }
+        Ok(())
+    }
+
+    fn set_volume(&mut self, volume: f32) -> Result<(), PlayerError> {
+        self.snapshot.volume = volume.clamp(0.0, 1.0);
+        Ok(())
+    }
+
+    fn set_muted(&mut self, muted: bool) -> Result<(), PlayerError> {
+        self.snapshot.muted = muted;
+        Ok(())
+    }
+
+    fn set_rate(&mut self, rate: f64) -> Result<(), PlayerError> {
+        self.snapshot.rate = rate;
+        Ok(())
+    }
+
+    fn select_audio_track(&mut self, index: Option<i32>) -> Result<(), PlayerError> {
+        self.snapshot.audio_track = index;
+        Ok(())
+    }
+
+    fn select_subtitle_track(&mut self, index: Option<i32>) -> Result<(), PlayerError> {
+        self.snapshot.subtitle_track = index;
+        Ok(())
+    }
+
+    fn snapshot(&self) -> PlaybackSnapshot {
+        self.snapshot.clone()
+    }
+
+    fn capabilities(&self) -> Capabilities {
+        self.capabilities
+    }
+}
diff --git a/src-tauri/src/player/fake_player_conformance.rs b/src-tauri/src/player/fake_player_conformance.rs
new file mode 100644
index 00000000..64c2f10c
--- /dev/null
+++ b/src-tauri/src/player/fake_player_conformance.rs
@@ -0,0 +1,56 @@
+//! `FakePlayer` runs the conformance suite.
+//!
+//! It is correct by construction, so a failure here means the *suite* is wrong,
+//! not an engine. That is what makes it safe to trust the same cases when they
+//! fail against a real one.
+//!
+//! TRACES: UR-081 | DR-243 | UT-220
+
+use std::time::Duration;
+
+use super::conformance::Harness;
+use super::fake_player::FakePlayer;
+use super::media::MediaItem;
+use super::media_player::OpenRequest;
+use crate::repository::stream_selection::StreamSelection;
+
+struct FakeHarness {
+    player: FakePlayer,
+}
+
+impl FakeHarness {
+    fn new() -> Self {
+        Self {
+            player: FakePlayer::new().with_duration(Duration::from_secs(7200)),
+        }
+    }
+}
+
+impl Harness for FakeHarness {
+    type Player = FakePlayer;
+
+    fn player(&mut self) -> &mut FakePlayer {
+        &mut self.player
+    }
+
+    fn request(&self, start: Duration) -> OpenRequest {
+        let selection = StreamSelection::local_file("http://example.invalid/stream.mp4");
+        let media = MediaItem::sample("fake-item", &selection.url);
+        OpenRequest::new(media, selection).starting_at(start)
+    }
+
+    fn settle(&mut self) {
+        self.player.complete_open();
+    }
+
+    fn audible(&mut self) -> Option<bool> {
+        Some(self.player.audible)
+    }
+
+    /// Exact: the fake has no keyframes to round to, so any drift is a bug.
+    fn seek_tolerance(&self) -> Duration {
+        Duration::ZERO
+    }
+}
+
+crate::media_player_conformance!(fake, FakeHarness::new());
diff --git a/src-tauri/src/player/media.rs b/src-tauri/src/player/media.rs
index 2cba4c47..781cb073 100644
--- a/src-tauri/src/player/media.rs
+++ b/src-tauri/src/player/media.rs
@@ -198,6 +198,48 @@ impl MediaItem {
     }
 }
 
+impl MediaItem {
+    /// A minimal item for tests.
+    ///
+    /// The struct has twenty-odd fields, almost none of which any given test
+    /// cares about, and repeating the literal per test is how a new field ends
+    /// up added in thirty places. Set what matters on the result.
+    ///
+    /// TRACES: UR-081 | DR-243
+    #[cfg(any(test, feature = "conformance"))]
+    pub fn sample(id: &str, url: &str) -> Self {
+        Self {
+            transport: None,
+            id: id.to_string(),
+            title: id.to_string(),
+            name: None,
+            artist: None,
+            album: None,
+            album_name: None,
+            album_id: None,
+            artist_items: None,
+            artists: None,
+            primary_image_tag: None,
+            image_id: None,
+            item_type: None,
+            playlist_id: None,
+            duration: None,
+            artwork_url: None,
+            media_type: MediaType::Video,
+            source: MediaSource::DirectUrl {
+                url: url.to_string(),
+            },
+            video_codec: None,
+            needs_transcoding: false,
+            video_width: None,
+            video_height: None,
+            subtitles: vec![],
+            series_id: None,
+            server_id: None,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src-tauri/src/player/mod.rs b/src-tauri/src/player/mod.rs
index a81e30fb..12fb99a2 100644
--- a/src-tauri/src/player/mod.rs
+++ b/src-tauri/src/player/mod.rs
@@ -5,8 +5,17 @@
 pub mod autoplay;
 pub mod backend;
 pub mod background_policy;
+#[cfg(any(test, feature = "conformance"))]
+pub mod conformance;
 pub mod events;
+#[cfg(any(test, feature = "conformance"))]
+pub mod fake_player;
+#[cfg(test)]
+mod fake_player_conformance;
 pub mod media;
+pub mod media_player;
+#[cfg(target_os = "linux")]
+pub mod mpv_player;
 pub mod queue;
 pub mod seek;
 pub mod session;