diff --git a/android-shared/src/androidMain/kotlin/org/siloserver/silo/common/player/PlaybackV3Session.kt b/android-shared/src/androidMain/kotlin/org/siloserver/silo/common/player/PlaybackV3Session.kt
index daa76ece6..d045d087d 100644
--- a/android-shared/src/androidMain/kotlin/org/siloserver/silo/common/player/PlaybackV3Session.kt
+++ b/android-shared/src/androidMain/kotlin/org/siloserver/silo/common/player/PlaybackV3Session.kt
@@ -134,6 +134,12 @@ internal fun PlaybackPlanV3.toSessionResponse(
         position = timeline.sourceStartSeconds,
         streamUrl = stream.url,
         audioTrackIndex = selectedTracks.audio?.index ?: 0,
+        // The server's runtime for the effective file, or null when it does
+        // not know. Callers must keep null as null rather than substituting
+        // the engine's duration: on an HLS copy remux the engine reports the
+        // window produced so far, so adopting it shows a feature film as a
+        // couple of minutes.
+        durationSeconds = source.durationSeconds,
         subtitleUrls = subtitles,
         playbackPlan = legacyPlan,
     )
diff --git a/android-shared/src/androidUnitTest/kotlin/org/siloserver/silo/common/player/PlaybackV3SessionTest.kt b/android-shared/src/androidUnitTest/kotlin/org/siloserver/silo/common/player/PlaybackV3SessionTest.kt
index 731b6d08f..4d563a6ae 100644
--- a/android-shared/src/androidUnitTest/kotlin/org/siloserver/silo/common/player/PlaybackV3SessionTest.kt
+++ b/android-shared/src/androidUnitTest/kotlin/org/siloserver/silo/common/player/PlaybackV3SessionTest.kt
@@ -3,6 +3,7 @@ package org.siloserver.silo.common.player
 import org.siloserver.silo.model.playback.PlaybackDelivery
 import org.siloserver.silo.model.playback.PlaybackEngineKind
 import org.siloserver.silo.model.playback.PlaybackPlanV3
+import org.siloserver.silo.model.playback.PlaybackSourceDescriptorV3
 import org.siloserver.silo.model.playback.PlaybackStreamProtocol
 import org.siloserver.silo.model.playback.PlaybackStreamV3
 import org.siloserver.silo.model.playback.PlaybackSubtitleArtifactV3
@@ -11,8 +12,10 @@ import org.siloserver.silo.model.playback.PlaybackSubtitleModeV3
 import org.siloserver.silo.model.playback.PlaybackTimelineV3
 import org.siloserver.silo.model.playback.PlaybackTrackIdentityV3
 import org.siloserver.silo.model.playback.SelectedPlaybackTracksV3
+import org.siloserver.silo.network.SiloJson
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNull
 
 class PlaybackV3SessionTest {
     @Test
@@ -75,12 +78,61 @@ class PlaybackV3SessionTest {
         assertEquals(timeline.seekRestoration, converted.seekRestoration)
     }
 
+    @Test
+    fun sourceRuntimeReachesTheSessionResponse() {
+        val response = plan(
+            mode = PlaybackSubtitleModeV3.OFF,
+            format = "",
+            url = "",
+            source = PlaybackSourceDescriptorV3(mediaFileId = 482, durationSeconds = 5400.0),
+        ).toSessionResponse("session", "profile", 482)
+
+        assertEquals(5400.0, response.durationSeconds)
+    }
+
+    // A server that does not know the runtime must leave the client knowing it
+    // does not know. Substituting 0.0 here is what let the playback engine's
+    // growing-HLS-window duration win and show a feature film as a minute.
+    @Test
+    fun unknownSourceRuntimeStaysUnknown() {
+        val response = plan(
+            mode = PlaybackSubtitleModeV3.OFF,
+            format = "",
+            url = "",
+            source = PlaybackSourceDescriptorV3(mediaFileId = 482),
+        ).toSessionResponse("session", "profile", 482)
+
+        assertNull(response.durationSeconds)
+    }
+
+    // Servers predating the descriptor omit it entirely; decoding must not fail
+    // and the runtime must read as unknown rather than zero.
+    @Test
+    fun planWithoutASourceDescriptorDecodesWithAnUnknownRuntime() {
+        val decoded = SiloJson.decodeFromString<PlaybackPlanV3>(
+            """
+            {
+              "plan_id": "plan",
+              "delivery": "original_http",
+              "engine": "media3_direct",
+              "stream": {"url": "/stream/session", "protocol": "http_progressive"},
+              "decision_reason": "test"
+            }
+            """.trimIndent(),
+        )
+
+        assertNull(decoded.source.durationSeconds)
+        assertNull(decoded.toSessionResponse("session", "profile", 482).durationSeconds)
+    }
+
     private fun plan(
         mode: PlaybackSubtitleModeV3,
         format: String,
         url: String,
         timeline: PlaybackTimelineV3 = PlaybackTimelineV3(),
+        source: PlaybackSourceDescriptorV3 = PlaybackSourceDescriptorV3(),
     ) = PlaybackPlanV3(
+        source = source,
         planId = "plan",
         delivery = PlaybackDelivery.ORIGINAL_HTTP,
         engine = PlaybackEngineKind.MEDIA3_DIRECT,
diff --git a/shared/src/commonMain/kotlin/org/siloserver/silo/model/playback/PlaybackProtocolV3.kt b/shared/src/commonMain/kotlin/org/siloserver/silo/model/playback/PlaybackProtocolV3.kt
index 6de30d3c2..70d92606d 100644
--- a/shared/src/commonMain/kotlin/org/siloserver/silo/model/playback/PlaybackProtocolV3.kt
+++ b/shared/src/commonMain/kotlin/org/siloserver/silo/model/playback/PlaybackProtocolV3.kt
@@ -134,6 +134,30 @@ data class PlaybackPlanV3(
     @SerialName("decision_reason") val decisionReason: String,
     @SerialName("requested_media_file_id") val requestedMediaFileId: Int? = null,
     @SerialName("effective_media_file_id") val effectiveMediaFileId: Int? = null,
+    val source: PlaybackSourceDescriptorV3 = PlaybackSourceDescriptorV3(),
+)
+
+/**
+ * Facts about the media file the plan resolved to, as opposed to the transport
+ * carrying it. Defaulted throughout so a server that predates the descriptor
+ * still decodes.
+ */
+@Serializable
+data class PlaybackSourceDescriptorV3(
+    @SerialName("media_file_id") val mediaFileId: Int? = null,
+    /**
+     * Full runtime of the source, or null when the server does not know it.
+     *
+     * Null must survive as null: `SiloJson` sets `coerceInputValues`, so a
+     * non-nullable `Double` here would silently become 0.0 — the very value
+     * this field exists to stop the player inventing.
+     *
+     * This is the whole file, never `total - sourceStartSeconds`, and it is
+     * never adjusted by `timelineOffsetSeconds`. Do not substitute the
+     * engine's reported duration for it: on an HLS copy remux the engine
+     * reports the window produced so far, not the runtime.
+     */
+    @SerialName("duration_seconds") val durationSeconds: Double? = null,
 )
 
 @Serializable