diff --git a/DeepDrftPublic.Client/Services/AudioInteropService.cs b/DeepDrftPublic.Client/Services/AudioInteropService.cs
index 966fd99..d463aa3 100644
--- a/DeepDrftPublic.Client/Services/AudioInteropService.cs
+++ b/DeepDrftPublic.Client/Services/AudioInteropService.cs
@@ -159,6 +159,25 @@ public class AudioInteropService : IAsyncDisposable
         }
     }
 
+    /// <summary>
+    /// Phase 21.2a back-pressure poll: ask whether the scheduler is still over its forward
+    /// high/low-water band. The read loop calls this only WHILE already throttled, to learn when it
+    /// may resume reading — the steady-state loop reads the piggybacked <c>ProductionPaused</c> flag
+    /// off each chunk result instead. Defaults to false on any interop failure so a torn-down player
+    /// never wedges a loop that is exiting anyway.
+    /// </summary>
+    public async Task<bool> IsProductionPaused(string playerId)
+    {
+        try
+        {
+            return await _jsRuntime.InvokeAsync<bool>("DeepDrftAudio.isProductionPaused", playerId);
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     public async Task<AudioOperationResult> ReinitializeFromOffset(string playerId, long totalStreamLength, double seekPosition)
     {
         return await InvokeJsAsync<AudioOperationResult>("DeepDrftAudio.reinitializeFromOffset", playerId, totalStreamLength, seekPosition);
@@ -419,6 +438,11 @@ public class StreamingResult : AudioOperationResult
     public bool HeaderParsed { get; set; }
     public int BufferCount { get; set; }
     public double? Duration { get; set; } // Duration in seconds calculated from WAV header
+
+    // Phase 21.2a back-pressure: true when the scheduler's forward decoded fill is over the
+    // high-water mark and the C# read loop should stop calling ReadAsync until it drains. Read off
+    // the chunk result the loop already awaits — no extra interop hop in the unthrottled steady state.
+    public bool ProductionPaused { get; set; }
 }
 
 public class AudioPlayerState
diff --git a/DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs b/DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs
index 9b6f856..71346b0 100644
--- a/DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs
+++ b/DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs
@@ -16,6 +16,14 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
     // Adaptive chunk sizing
     private const int MinBufferSize = 16 * 1024;  // 16KB minimum
     private const int MaxBufferSize = 64 * 1024;  // 64KB maximum
+
+    // Phase 21.2a back-pressure poll interval. While the scheduler is over its forward high-water
+    // mark, the read loop stops calling ReadAsync and polls IsProductionPaused at this cadence
+    // until the fill drains below low-water. 100 ms is well under the low-water lookahead (seconds),
+    // so resume is prompt relative to the playhead — no starvation (AC3) — while keeping the poll
+    // cheap. The poll honors the loop's cancellation token, so a track switch/seek during a pause
+    // exits through the same drain discipline as a pause during ReadAsync (C6).
+    private const int BackpressurePollMs = 100;
     private int _currentBufferSize = DefaultBufferSize;
     private int _consecutiveSlowReads = 0;
     
@@ -465,6 +473,26 @@ public class StreamingAudioPlayerService : AudioPlayerService, IStreamingPlayerS
                     }
                     
                     await ThrottledNotifyStateChanged();
+
+                    // Phase 21.2a back-pressure (serves BOTH paths). The chunk we just processed
+                    // reported the scheduler's forward fill is over the high-water mark — stop
+                    // reading the socket so the unplayed decoded region stays bounded. Pausing
+                    // ReadAsync lets the kernel TCP window close (we are working WITH transport flow
+                    // control, not against it). Poll until the fill drains below low-water, then
+                    // resume the loop. For WAV this is the whole story (StreamDecoder decodes
+                    // synchronously into the scheduler); the Opus feed additionally self-throttles
+                    // its demux/decode off the SAME signal (21.2b), so its upstream queues stay
+                    // near-empty too. The poll awaits on cancellationToken, so a track switch/seek
+                    // mid-pause throws OCE and unwinds through the existing drain discipline (C6) —
+                    // no separate cancellation path, no stale read racing a reinit.
+                    if (chunkResult.ProductionPaused)
+                    {
+                        while (await _audioInterop.IsProductionPaused(PlayerId))
+                        {
+                            cancellationToken.ThrowIfCancellationRequested();
+                            await Task.Delay(BackpressurePollMs, cancellationToken);
+                        }
+                    }
                 }
             } while (currentBytes > 0);
 
diff --git a/DeepDrftPublic/Interop/audio/AudioPlayer.ts b/DeepDrftPublic/Interop/audio/AudioPlayer.ts
index 06ce602..9e6f2e1 100644
--- a/DeepDrftPublic/Interop/audio/AudioPlayer.ts
+++ b/DeepDrftPublic/Interop/audio/AudioPlayer.ts
@@ -30,6 +30,10 @@ export interface StreamingResult extends AudioResult {
     headerParsed?: boolean;
     bufferCount?: number;
     duration?: number;
+    // Phase 21.2a back-pressure signal piggybacked on the chunk result the C# read loop already
+    // awaits — true means the scheduler's forward fill is over the high-water mark and the loop
+    // should stop calling ReadAsync until it drains (no extra interop hop in the common case).
+    productionPaused?: boolean;
 }
 
 export interface AudioState {
@@ -133,7 +137,14 @@ export class AudioPlayer {
             // selects Opus when the sidecar parsed, so the null branch is defensive.
             if (this.isOpusContentType(contentType) && this.pendingOpusSidecar) {
                 this.activeOpusSidecar = this.pendingOpusSidecar;
-                this.opusDecoder = new OpusStreamDecoder(this.contextManager, this.pendingOpusSidecar);
+                // Pass the shared back-pressure signal (21.2b): the Opus decoder stops demuxing/
+                // decoding new packets while the scheduler is full, so the WebCodecs decode queue
+                // and decodedQueue do not balloon behind a throttled socket (OQ7). Same signal the
+                // C# read loop honors — one policy, two thin hooks.
+                this.opusDecoder = new OpusStreamDecoder(
+                    this.contextManager,
+                    this.pendingOpusSidecar,
+                    () => this.scheduler.isProductionPaused());
                 return { success: true };
             }
 
@@ -263,7 +274,8 @@ export class AudioPlayer {
                 canStartStreaming: canStart,
                 headerParsed,
                 bufferCount: this.scheduler.getBufferCount(),
-                duration: this.duration
+                duration: this.duration,
+                productionPaused: this.scheduler.isProductionPaused()
             };
         } catch (error) {
             return { success: false, error: (error as Error).message };
@@ -307,7 +319,8 @@ export class AudioPlayer {
                 canStartStreaming: canStart,
                 headerParsed: this.streamDecoder.headerParsed,
                 bufferCount: this.scheduler.getBufferCount(),
-                duration: this.duration
+                duration: this.duration,
+                productionPaused: this.scheduler.isProductionPaused()
             };
         } catch (error) {
             return { success: false, error: (error as Error).message };
@@ -508,6 +521,17 @@ export class AudioPlayer {
         return this.scheduler.getTotalDuration() + this.scheduler.getPlaybackOffset();
     }
 
+    /**
+     * The shared back-pressure signal (Phase 21.2a), polled by the C# read loop WHILE it is
+     * already throttled to learn when the forward fill has drained below the low-water mark and it
+     * may resume reading. The steady-state (unthrottled) loop never calls this — it reads the
+     * piggybacked productionPaused flag off each chunk result instead, so there is no extra
+     * interop hop until back-pressure actually engages.
+     */
+    isProductionPaused(): boolean {
+        return this.scheduler.isProductionPaused();
+    }
+
     /**
      * Calculate byte offset for a time position (for C# layer)
      */
diff --git a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
index 44f716b..da9d72e 100644
--- a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
+++ b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.test.ts
@@ -448,6 +448,65 @@ test('OpusStreamDecoder.totalDuration is null when the sidecar carries no positi
     assertEqual(decoder.totalDuration, null, 'no positive duration -> null');
 });
 
+// --- Phase 21.2b: Opus decode-ahead back-pressure (the stash-while-full half) ------------------
+//
+// When the shared scheduler is full, push() must NOT demux/decode ahead — it stashes the raw bytes
+// and returns nothing, so the WebCodecs decode queue and decodedQueue stay near-empty (OQ7). The
+// stash-while-full branch returns BEFORE ensureConfigured(), so it is testable without WebCodecs
+// (no AudioDecoder is constructed). The drain-on-resume path needs the real WebCodecs decoder and
+// stays browser-verified; here we pin the bound itself and the lifecycle resets.
+
+// Access the private stash for white-box assertions (same idiom the scheduler tests use).
+function stashLength(decoder: OpusStreamDecoder): number {
+    return (decoder as unknown as { pendingBytes: Uint8Array[] }).pendingBytes.length;
+}
+
+// The stash-while-full branch returns synchronously at the top of push() (before any real await),
+// so the stash is observable immediately without awaiting the returned promise — keeping these
+// tests inside the synchronous inline harness (which does not await test bodies).
+test('push stashes bytes and decodes nothing while the scheduler is full (no decode-ahead)', () => {
+    const sidecar = sidecarFrom({
+        setupHeader: [0x4f, 0x70, 0x75, 0x73, 0x48, 0x65, 0x61, 0x64],
+        totalByteLength: 500_000, totalDuration: 100, preSkip: 312,
+        points: [{ granule: 312, byteOffset: 4096 }],
+    });
+    // Scheduler reports "full" → push must short-circuit before touching WebCodecs.
+    const decoder = new OpusStreamDecoder(stubContextManager, sidecar, () => true);
+
+    void decoder.push(new Uint8Array([1, 2, 3]));
+    void decoder.push(new Uint8Array([4, 5]));
+
+    assertEqual(stashLength(decoder), 2, 'both chunks stashed in arrival order');
+    assertEqual(decoder.ready, false, 'decoder not even configured while throttled');
+});
+
+test('reinitializeForRangeContinuation drops the pre-seek stash (C6 — no stale feed across reset)', () => {
+    const sidecar = sidecarFrom({
+        setupHeader: [0x4f, 0x70, 0x75, 0x73, 0x48, 0x65, 0x61, 0x64],
+        totalByteLength: 500_000, totalDuration: 100, preSkip: 312,
+        points: [{ granule: 312, byteOffset: 4096 }],
+    });
+    const decoder = new OpusStreamDecoder(stubContextManager, sidecar, () => true);
+    void decoder.push(new Uint8Array([1, 2, 3])); // stash one chunk while full
+    assertEqual(stashLength(decoder), 1, 'one chunk stashed pre-seek');
+
+    decoder.reinitializeForRangeContinuation(0, 5); // a seek
+    assertEqual(stashLength(decoder), 0, 'pre-seek stash dropped on range-continuation');
+});
+
+test('dispose clears the stash', () => {
+    const sidecar = sidecarFrom({
+        setupHeader: [0x4f, 0x70, 0x75, 0x73, 0x48, 0x65, 0x61, 0x64],
+        totalByteLength: 500_000, totalDuration: 100, preSkip: 312,
+        points: [{ granule: 312, byteOffset: 4096 }],
+    });
+    const decoder = new OpusStreamDecoder(stubContextManager, sidecar, () => true);
+    void decoder.push(new Uint8Array([9]));
+    assertEqual(stashLength(decoder), 1, 'stashed');
+    decoder.dispose();
+    assertEqual(stashLength(decoder), 0, 'stash cleared on dispose');
+});
+
 function concat(arrs: Uint8Array[]): Uint8Array {
     let len = 0;
     for (const a of arrs) len += a.length;
diff --git a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.ts b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.ts
index 01c61f2..a3db462 100644
--- a/DeepDrftPublic/Interop/audio/OpusStreamDecoder.ts
+++ b/DeepDrftPublic/Interop/audio/OpusStreamDecoder.ts
@@ -39,6 +39,17 @@ const MAX_PACKET_FRAMES = 5760;
 export class OpusStreamDecoder implements IStreamingDecoder {
     private readonly contextManager: AudioContextManager;
     private readonly sidecar: OpusSeekData;
+    // Phase 21.2b back-pressure hook: returns true when the shared scheduler is full (forward fill
+    // over high-water). While full, push() stashes raw bytes WITHOUT demuxing/decoding so the
+    // WebCodecs decode queue and decodedQueue stay near-empty behind a throttled socket (OQ7).
+    // Null = no back-pressure (e.g. unit tests), in which case the decoder feeds eagerly as before.
+    private readonly isSchedulerFull: (() => boolean) | null;
+
+    // Raw bytes received while the scheduler was full, held undemuxed until it drains. The C# read
+    // loop also pauses above high-water, so this stash is bounded to at most the in-flight chunks
+    // between the loop reading the productionPaused flag and actually stopping — a handful of KB,
+    // not a decode-ahead. Drained (demuxed + decoded) on the next push once below high-water.
+    private pendingBytes: Uint8Array[] = [];
 
     private demuxer = new OggDemuxer();
     private decoder: AudioDecoder | null = null;
@@ -66,9 +77,13 @@ export class OpusStreamDecoder implements IStreamingDecoder {
     private emittedFrames = 0;
     private readonly totalFrames: number;
 
-    constructor(contextManager: AudioContextManager, sidecar: OpusSeekData) {
+    constructor(
+        contextManager: AudioContextManager,
+        sidecar: OpusSeekData,
+        isSchedulerFull: (() => boolean) | null = null) {
         this.contextManager = contextManager;
         this.sidecar = sidecar;
+        this.isSchedulerFull = isSchedulerFull;
         this.totalFrames = sidecar.totalDurationSeconds > 0
             ? Math.round(sidecar.totalDurationSeconds * OPUS_SAMPLE_RATE)
             : Number.POSITIVE_INFINITY;
@@ -136,17 +151,59 @@ export class OpusStreamDecoder implements IStreamingDecoder {
 
     async push(chunk: Uint8Array): Promise<AudioBuffer[]> {
         if (this.fatalError) return [];
+
+        // 21.2b back-pressure: while the scheduler is full, do NOT demux/decode ahead. Stash the
+        // raw bytes in arrival order and return nothing — the WebCodecs decode queue and
+        // decodedQueue stay near-empty (OQ7). The bytes are demuxed/decoded on a later push once
+        // the scheduler has drained below low-water, in exactly the order received (Ogg demux is
+        // order-sensitive). configure() is deferred too — no need to spin up the decoder while
+        // throttled. The C# loop also stops reading above high-water, so the stash stays small.
+        if (this.isSchedulerFull?.()) {
+            this.pendingBytes.push(chunk);
+            return [];
+        }
+
         if (!(await this.ensureConfigured())) return [];
 
-        const packets = this.demuxer.push(chunk);
-        this.decodePackets(packets);
+        // Drained below high-water: replay any stashed bytes first (preserving stream order), then
+        // the new chunk, through the demuxer as one contiguous feed.
+        const out: AudioBuffer[] = [];
+        if (this.pendingBytes.length > 0) {
+            const stashed = this.pendingBytes;
+            this.pendingBytes = [];
+            for (const bytes of stashed) {
+                this.decodePackets(this.demuxer.push(bytes));
+            }
+        }
+
+        this.decodePackets(this.demuxer.push(chunk));
         // Wait until the WebCodecs decoder has processed the queued packets before draining.
         await this.yieldToDecoder();
-        return this.drainDecoded();
+        out.push(...this.drainDecoded());
+        return out;
     }
 
     async complete(): Promise<AudioBuffer[]> {
-        if (this.fatalError || !this.decoder || this.decoder.state !== 'configured') {
+        if (this.fatalError) {
+            return this.drainDecoded();
+        }
+
+        // End-of-stream may arrive while still throttled with bytes stashed (e.g. a short track
+        // that finished sending before the scheduler drained). Configure if needed and replay the
+        // stash so the tail is decoded before flush — otherwise the final seconds would be lost.
+        if (this.pendingBytes.length > 0) {
+            if (await this.ensureConfigured()) {
+                const stashed = this.pendingBytes;
+                this.pendingBytes = [];
+                for (const bytes of stashed) {
+                    this.decodePackets(this.demuxer.push(bytes));
+                }
+            } else {
+                this.pendingBytes = [];
+            }
+        }
+
+        if (!this.decoder || this.decoder.state !== 'configured') {
             return this.drainDecoded();
         }
         try {
@@ -184,6 +241,10 @@ export class OpusStreamDecoder implements IStreamingDecoder {
         // continuation mode (treat the first page's packets as audio — no setup pages in a 206 body).
         this.demuxer.reset(true);
         this.decodedQueue = [];
+        // Drop any bytes stashed by back-pressure: they belong to the PRE-seek stream position and
+        // must never be replayed against the post-seek (range-continuation) demux state (C6 — no
+        // stale feed racing the reset).
+        this.pendingBytes = [];
         this.emittedFrames = 0; // post-seek buffers are positioned by the scheduler's playbackOffset
         // Arm the lead trim: skip enough decoded frames to land at targetTimeSeconds, not at
         // landingTimeSeconds (the page start). Clamp to ≥0 to guard against floating-point rounding.
@@ -199,6 +260,7 @@ export class OpusStreamDecoder implements IStreamingDecoder {
             try { d.close(); } catch { /* already closed */ }
         }
         this.decodedQueue = [];
+        this.pendingBytes = [];
         if (this.decoder && this.decoder.state !== 'closed') {
             try { this.decoder.close(); } catch { /* already closed */ }
         }
diff --git a/DeepDrftPublic/Interop/audio/PlaybackScheduler.test.ts b/DeepDrftPublic/Interop/audio/PlaybackScheduler.test.ts
index ec75d0e..8395147 100644
--- a/DeepDrftPublic/Interop/audio/PlaybackScheduler.test.ts
+++ b/DeepDrftPublic/Interop/audio/PlaybackScheduler.test.ts
@@ -85,6 +85,14 @@ function buf(duration: number): AudioBuffer {
     return { duration } as AudioBuffer;
 }
 
+/**
+ * A decoded buffer carrying realistic byte-footprint fields (length + numberOfChannels) for the
+ * OQ3 byte-ceiling test. Models 48 kHz stereo float PCM: length = duration × 48000 frames, 2 ch.
+ */
+function bufBytes(duration: number): AudioBuffer {
+    return { duration, length: Math.round(duration * 48000), numberOfChannels: 2 } as AudioBuffer;
+}
+
 function makeScheduler(cm: FakeContextManager): PlaybackScheduler {
     // The scheduler only uses the subset FakeContextManager implements.
     return new PlaybackScheduler(cm as unknown as AudioContextManager);
@@ -325,6 +333,114 @@ test('eviction via handleSourceEnded: position exact, live bufferIndex decrement
     }
 });
 
+// === Phase 21.2 back-pressure: the forward water-mark signal =================================
+//
+// The signal is pure given the clock + buffer durations + the playhead position, so it is
+// testable in Node with the same fakes. We drive forward lookahead by adding buffers (fill) and
+// advancing the clock (drain), and assert the hysteresis latch and the OQ3 byte ceiling.
+
+/**
+ * Fill the scheduler with `count` 1 s buffers, start playback at t=0, and advance the schedule
+ * cursor to the end so nextBufferIndex does not pin anything. Leaves all `count` buffers decoded
+ * and the playhead at the clock position the caller sets afterwards.
+ */
+function fillAndStart(s: PlaybackScheduler, cm: FakeContextManager, count: number): void {
+    for (let i = 0; i < count; i++) s.addBuffer(buf(1));
+    cm.now = 0;
+    s.playFromPosition(0);
+    advanceCursorToEnd(s, cm);
+}
+
+// High-water reached → production pauses; the signal reflects the forward lookahead.
+test('isProductionPaused latches true when forward lookahead reaches high-water', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    s.setForwardWindow(10, 5, 0); // high 10s, low 5s, byte cap disabled
+    fillAndStart(s, cm, 40); // 40s decoded, track [0,40)
+
+    cm.now = 0; // playhead at 0 → forward lookahead = 40s ≥ 10s high-water
+    assertEqual(s.getForwardLookaheadSeconds(), 40, 'lookahead is full decoded tail at t=0');
+    assertEqual(s.isProductionPaused(), true, 'pauses at/above high-water');
+});
+
+// Below high-water but above low-water while NOT yet paused → stays unpaused (no premature pause).
+test('isProductionPaused stays false in the hysteresis band before the high-water crossing', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    s.setForwardWindow(10, 5, 0);
+    fillAndStart(s, cm, 8); // 8s decoded
+
+    cm.now = 0; // lookahead 8s: between low(5) and high(10), never latched → unpaused
+    assertEqual(s.isProductionPaused(), false, 'no pause until high-water is actually reached');
+});
+
+// Hysteresis: once paused at high-water, stays paused through the band until lookahead drains
+// below low-water, then resumes. Drain is modeled by advancing the clock (playhead moves forward,
+// shrinking forward lookahead).
+test('isProductionPaused holds through the band and resumes only below low-water', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    s.setForwardWindow(10, 5, 0);
+    fillAndStart(s, cm, 40); // track [0,40)
+
+    cm.now = 0;
+    assertEqual(s.isProductionPaused(), true, 'latched at high-water (40s ahead)');
+
+    // Playhead at 32 → lookahead 8s: in the band (5..10) → must STAY paused (hysteresis).
+    cm.now = 32;
+    assertEqual(s.getForwardLookaheadSeconds(), 8, 'lookahead drained to 8s');
+    assertEqual(s.isProductionPaused(), true, 'still paused inside the band');
+
+    // Playhead at 36 → lookahead 4s ≤ low-water 5 → resume.
+    cm.now = 36;
+    assertEqual(s.getForwardLookaheadSeconds(), 4, 'lookahead below low-water');
+    assertEqual(s.isProductionPaused(), false, 'resumes below low-water');
+
+    // Refill back over high-water re-latches (the next chunk would re-pause).
+    for (let i = 0; i < 20; i++) s.addBuffer(buf(1)); // +20s decoded ahead
+    assertEqual(s.isProductionPaused(), true, 're-latches when fill exceeds high-water again');
+});
+
+// OQ3 hard byte ceiling pauses production independent of the time window, and releases as soon as
+// the footprint is back under the cap (no separate low-water band on the byte guard).
+test('OQ3 byte ceiling pauses regardless of the time window', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    // Each 1s buffer here is 48000 frames × 2 ch × 4 bytes = 384000 bytes. Cap at ~1.5 MB ≈ 4 buffers.
+    const perBuffer = 48000 * 2 * 4;
+    s.setForwardWindow(1000, 500, perBuffer * 4); // time window huge so only the byte cap can fire
+    for (let i = 0; i < 6; i++) s.addBuffer(bufBytes(1)); // 6 buffers > 4-buffer cap
+    cm.now = 0;
+    s.playFromPosition(0);
+    advanceCursorToEnd(s, cm);
+
+    cm.now = 0;
+    if (s.getDecodedByteEstimate() <= perBuffer * 4) {
+        throw new Error('test setup: byte estimate should exceed the cap');
+    }
+    assertEqual(s.isProductionPaused(), true, 'byte ceiling pauses even with a huge time window');
+});
+
+// clear() / clearForSeek() release the latch so a fresh stream/seek starts unthrottled (C2).
+test('clear and clearForSeek release the back-pressure latch (C2 latency parity)', () => {
+    const cm = new FakeContextManager();
+    const s = makeScheduler(cm);
+    s.setForwardWindow(10, 5, 0);
+    fillAndStart(s, cm, 40);
+    cm.now = 0;
+    assertEqual(s.isProductionPaused(), true, 'latched');
+
+    s.clear();
+    // After clear there are no buffers, lookahead is 0, and the latch is reset → unpaused.
+    assertEqual(s.isProductionPaused(), false, 'clear resets the latch and empties fill');
+
+    fillAndStart(s, cm, 40);
+    cm.now = 0;
+    assertEqual(s.isProductionPaused(), true, 'latched again after refill');
+    s.clearForSeek();
+    assertEqual(s.isProductionPaused(), false, 'clearForSeek resets the latch');
+});
+
 // --- run -------------------------------------------------------------------------------------
 if (failures.length > 0) {
     console.error(failures.join('\n'));
diff --git a/DeepDrftPublic/Interop/audio/PlaybackScheduler.ts b/DeepDrftPublic/Interop/audio/PlaybackScheduler.ts
index 975dc45..f0043fb 100644
--- a/DeepDrftPublic/Interop/audio/PlaybackScheduler.ts
+++ b/DeepDrftPublic/Interop/audio/PlaybackScheduler.ts
@@ -36,6 +36,32 @@ import { AudioContextManager } from './AudioContextManager.js';
  */
 const DEFAULT_BACK_RETAIN_SECONDS = 10;
 
+/**
+ * Forward back-pressure water-marks (Phase 21.2 — the bound on the *unplayed* region).
+ *
+ * The single back-pressure signal is the scheduler's decoded forward lookahead: how many
+ * seconds of decoded audio sit AHEAD of the playhead (OQ7). Production (the C# read loop and,
+ * for Opus, the demux/decode feed) pauses above the high-water mark and resumes below the
+ * low-water mark — classic hysteresis so the two producers do not chatter on/off per chunk.
+ *
+ * Provisional time-based defaults (OQ1 — 21.4 tunes them):
+ *   - HIGH (30 s): the most decoded lookahead we hold ahead of the playhead before throttling.
+ *     Comfortably above the playback-start minimum (6 buffers ≈ a second or two), so C2 holds —
+ *     first audio never waits on a throttle (the high-water is reached only well after playback
+ *     is already running).
+ *   - LOW (15 s): resume producing here. Kept generous so the forward fill never drains to the
+ *     ~500 ms scheduler lookahead under normal network jitter (AC3 — no starvation).
+ *
+ * OQ3 hard memory ceiling: an absolute byte cap on total decoded float held, independent of the
+ * time window. This is the guard-rail that makes "1 GB never OOMs" a guarantee rather than a
+ * tuning hope — if a pathological stream packs an unusual amount of decoded audio into the time
+ * window, the byte cap still pauses production. Estimated as channels × frames × 4 bytes (f32).
+ */
+const DEFAULT_FORWARD_HIGH_WATER_SECONDS = 30;
+const DEFAULT_FORWARD_LOW_WATER_SECONDS = 15;
+const DEFAULT_MAX_DECODED_BYTES = 96 * 1024 * 1024; // ~96 MB of decoded float PCM
+const BYTES_PER_FLOAT_SAMPLE = 4;
+
 interface ScheduledSource {
     source: AudioBufferSourceNode;
     bufferIndex: number;
@@ -66,6 +92,19 @@ export class PlaybackScheduler {
     // 21.2 will drive this from the window policy. Not a hardcoded eviction decision.
     private backRetainSeconds: number = DEFAULT_BACK_RETAIN_SECONDS;
 
+    // Forward back-pressure water-marks + the OQ3 hard byte ceiling (Phase 21.2). This is the
+    // single shared window policy (OQ6): both producers read isProductionPaused() and honor it
+    // in their own way — the C# read loop stops ReadAsync, the Opus feed stops demux/decode.
+    private forwardHighWaterSeconds: number = DEFAULT_FORWARD_HIGH_WATER_SECONDS;
+    private forwardLowWaterSeconds: number = DEFAULT_FORWARD_LOW_WATER_SECONDS;
+    private maxDecodedBytes: number = DEFAULT_MAX_DECODED_BYTES;
+
+    // Hysteresis latch for the production pause. Once forward fill crosses the high-water mark we
+    // stay paused until it drains below the low-water mark, so the two producers do not flap
+    // on/off around a single threshold (and a paused producer does not resume for one chunk only
+    // to re-pause immediately). False until first crossing; flips on the band edges.
+    private productionPaused_: boolean = false;
+
     // Callbacks
     public onPlaybackEnded: (() => void) | null = null;
 
@@ -132,6 +171,68 @@ export class PlaybackScheduler {
         this.backRetainSeconds = Math.max(0, seconds);
     }
 
+    /**
+     * Configure the forward back-pressure water-marks (seconds of decoded lookahead) and the OQ3
+     * hard byte ceiling. Provisional config seam — 21.4 tunes the numbers. Low is clamped below
+     * high so the hysteresis band is always valid; non-positive byte cap disables the OQ3 guard.
+     */
+    setForwardWindow(highWaterSeconds: number, lowWaterSeconds: number, maxDecodedBytes: number): void {
+        this.forwardHighWaterSeconds = Math.max(0, highWaterSeconds);
+        this.forwardLowWaterSeconds = Math.max(0, Math.min(lowWaterSeconds, this.forwardHighWaterSeconds));
+        this.maxDecodedBytes = maxDecodedBytes;
+    }
+
+    /**
+     * Seconds of decoded audio sitting AHEAD of the current playhead — the forward fill. This is
+     * the single back-pressure signal (OQ7): the absolute end time of the last decoded buffer
+     * minus the current playback position. Never negative (clamped at 0 when the playhead has
+     * caught up to or passed the decoded tail).
+     */
+    getForwardLookaheadSeconds(): number {
+        const decodedEnd = this.getTotalDuration() + this.playbackOffset;
+        return Math.max(0, decodedEnd - this.getCurrentPosition());
+    }
+
+    /**
+     * Estimated bytes of decoded float PCM currently retained (OQ3 input). Web Audio AudioBuffers
+     * are 32-bit float per sample per channel; frames = duration × sampleRate. Summed across the
+     * retained buffers only — evicted buffers are already reclaimed, so this tracks the live
+     * footprint, not the whole track.
+     */
+    getDecodedByteEstimate(): number {
+        let bytes = 0;
+        for (const b of this.buffers) {
+            bytes += b.length * b.numberOfChannels * BYTES_PER_FLOAT_SAMPLE;
+        }
+        return bytes;
+    }
+
+    /**
+     * The single shared production-pause decision (Phase 21.2, OQ6/OQ7). Both producers — the C#
+     * read loop (21.2a) and the Opus demux/decode feed (21.2b) — call this and stop producing
+     * while it returns true. Hysteresis: pause when forward lookahead exceeds the high-water mark
+     * OR the decoded byte estimate exceeds the OQ3 ceiling; resume only once forward lookahead has
+     * drained below the low-water mark AND the byte estimate is back under the ceiling. The
+     * byte-ceiling test has no separate low-water band — it is the hard guard rail, so it releases
+     * as soon as eviction brings the footprint back under the cap.
+     */
+    isProductionPaused(): boolean {
+        const lookahead = this.getForwardLookaheadSeconds();
+        const overByteCeiling = this.maxDecodedBytes > 0 && this.getDecodedByteEstimate() > this.maxDecodedBytes;
+
+        if (this.productionPaused_) {
+            // Stay paused until BOTH the time window has drained below low-water AND the byte
+            // footprint is back under the ceiling.
+            if (lookahead <= this.forwardLowWaterSeconds && !overByteCeiling) {
+                this.productionPaused_ = false;
+            }
+        } else if (lookahead >= this.forwardHighWaterSeconds || overByteCeiling) {
+            this.productionPaused_ = true;
+        }
+
+        return this.productionPaused_;
+    }
+
     /**
      * Drop already-played buffers from the front of the array, reclaiming their decoded float
      * memory, and advance the time anchor so all position/index bookkeeping stays exact.
@@ -418,6 +519,9 @@ export class PlaybackScheduler {
         this.nextBufferIndex = 0;
         this.nextScheduleTime = 0;
         this.playbackOffset = 0;
+        // Release the back-pressure latch — a fresh stream must start unthrottled so its first
+        // chunks decode immediately (C2: no throttle-induced first-audio stall).
+        this.productionPaused_ = false;
     }
 
     /**
@@ -432,6 +536,9 @@ export class PlaybackScheduler {
         this.nextBufferIndex = 0;
         this.nextScheduleTime = 0;
         // Note: playbackOffset is NOT reset - it will be set by the caller
+        // Release the back-pressure latch — the post-seek continuation must refill from the new
+        // offset without inheriting the pre-seek paused state.
+        this.productionPaused_ = false;
     }
 
     /**
diff --git a/DeepDrftPublic/Interop/audio/index.ts b/DeepDrftPublic/Interop/audio/index.ts
index dd863b9..136b329 100644
--- a/DeepDrftPublic/Interop/audio/index.ts
+++ b/DeepDrftPublic/Interop/audio/index.ts
@@ -117,6 +117,14 @@ const DeepDrftAudio = {
         return player?.calculateByteOffset(positionSeconds) ?? 0;
     },
 
+    // Phase 21.2a back-pressure poll: the C# read loop calls this WHILE throttled to learn when
+    // the scheduler has drained below low-water and reading may resume. A missing player reads as
+    // "not paused" so a torn-down player never wedges a loop that is already exiting.
+    isProductionPaused: (playerId: string): boolean => {
+        const player = audioPlayers.get(playerId);
+        return player?.isProductionPaused() ?? false;
+    },
+
     reinitializeFromOffset: (playerId: string, totalStreamLength: number, seekPosition: number): AudioResult => {
         const player = audioPlayers.get(playerId);
         if (!player) return { success: false, error: 'Player not found' };