# Rutster slice 4 — Barge-in: VAD-driven playout kill on a dedicated media thread

- **Status:** Draft (pending review)
- **Date:** 2026-07-01
- **Spearhead step:** 4 of 6 (vision-revision §10 / PORT_PLAN "Phasing")
- **Origin:** brainstorming session 2026-07-01
- **Depends on:** [slice 1 — WebRTC media loopback](2026-06-28-slice-1-webrtc-loopback-design.md),
  [slice 2 — The agent tap](2026-06-28-slice-2-agent-tap-design.md), and slice-3's
  OpenAI Realtime brain (merged as `c30a452` — `MockRealtimeBrain` + translator + the
  `speech_started` / `speech_stopped` advisory events). All three must be landed and green.
- **Related:** [ADR-0002](../../adr/0002-north-star-and-fused-core.md) (fused vertical — the
  hot-path hop invariant this slice re-affirms), [ADR-0008](../../adr/0008-fob-and-green-zone.md)
  (FOB/green-zone doctrine — the reflex is a FOB member: hot-path, differentiating),
  [ARCHITECTURE.md §"Biggest technical risk"](../../ARCHITECTURE.md) (the reflex loop *is*
  the remaining long pole), [ARCHITECTURE.md §"Media plane"](../../ARCHITECTURE.md)
  ("Dedicated timing threads for the 20ms loop, **never the shared tokio pool**" — this
  slice finally lands that mandate).

---

## TL;DR

Stand up spearhead step 4: the **FOB reflex loop**. Slice 3 pre-paved the advisory signals
(`speech_started` / `speech_stopped` from the brain) and locked the turn-ownership decision
(OpenAI Realtime server-side VAD disabled; the FOB owns turn-taking). Slice 4 **acts** on
the caller's speech with a **local in-core VAD** as the primary trigger — an RMS/energy
detector running in `on_pcm_frame` on the dedicated thread, in the 20 ms loop, with **zero
brain round-trip** between caller speech and playout kill. This is the property ARCHITECTURE.md:79-81
demands ("Local real-time reflexes... live in-core because the brain round-trip is too slow
to enforce them") and the proof wedge #1 rests on ("VAD killing TTS the instant the caller
speaks, without the brain" — README:98-100). Slice-3's `speech_started`/`speech_stopped`
advisory becomes the **secondary/confirmation** signal — the brain's ASR-quality VAD
confirms the local kill slightly later, but the kill itself fires from the FOB's own
inspection of caller audio.

Slice 4 also **graduates the media loop off the tokio pool**: a single dedicated `std::thread`
owns all `RtcSession`s exclusively and drives the 20 ms tick via `Instant::sleep_until`. This
honors ARCHITECTURE.md's "never the shared tokio pool" mandate, which slice-1 explicitly
deferred to "step 4 (barge-in)" (`loop_driver.rs:18-23`). The graduation is load-bearing:
the reflex is the differentiator and the long pole, and its timing discipline demands a
thread that doesn't compete with the axum runtime for scheduling.

The **seam slice 1→3 preserved** (`loop_driver.rs` + `rtc_session.rs` byte-identical) holds
for slice-4 as well: the reflex is a pair of composing `AudioPipe` decorators (`Reflex<P>` +
`LocalVadReflex<P>`) in `rutster-media`, invisibly to `loop_driver::drive`. Only the
binary-side wiring (`session_map.rs` → `media_thread.rs`) changes shape; the media crate's
hot path stays untouched.

---

## 1. Scope

### 1.1 In scope

- Implementation of spearhead step 4: **barge-in / VAD-driven playout kill**, driven by a
  **local in-core VAD** (RMS/energy detector in `on_pcm_frame` — the primary trigger, zero
  brain round-trip) with the brain's `speech_started`/`speech_stopped` advisory as the
  secondary/confirmation signal. This proves wedge #1 ("VAD killing TTS the instant the
  caller speaks, without the brain" — README:98-100, ARCHITECTURE.md:79-81).
- A **new `Reflex<P: AudioPipe>` wrapper** (`rutster-media/src/reflex.rs`) that decorates the
  pipe the `RtcSession` holds. The reflex owns the mute state machine, the advisory channel
  receiver, and the barge-in flush trigger. It is the concrete embodiment of ARCHITECTURE.md's
  "local real-time reflexes" row for the barge-in case. Fed by BOTH the local VAD (via the
  outer `LocalVadReflex` wrapper that injects `AdvisoryEvent`s into the same mpsc) AND the
  brain's advisories.
- A **new `LocalVadReflex<P: AudioPipe>` wrapper** (`rutster-media/src/reflex.rs`) — the
  PRIMARY trigger. Decorates `AudioPipe`'s `on_pcm_frame` path: inspects the caller's decoded
  PCM samples (computes RMS, compares to a `const` threshold, debounces N consecutive
  above-threshold frames), fires `AdvisoryEvent::SpeechStarted { at: Instant::now() }` into
  the `Reflex`'s advisory mpsc when local VAD trips. Wraps `Reflex<TapAudioPipe>` (composition:
  `LocalVadReflex<Reflex<TapAudioPipe>>` — the decorator pattern from §6.4 pays off here).
  Forward-compatible: the threshold is a `const` for the MVP (no tuning framework — that's the
  only piece deferred, per §1.2).
- A **new `barge_in_flush` method on the `AudioPipe` trait** (default impl delegates to
  `clear_playout_ring`) — the seam object's "kill now" path: clear the playout ring AND drain
  the brain-bound `rx_audio_out` channel of any frames queued before the barge so the first
  `audio_out` observed post-barge is provably post-barge. `TapAudioPipe` overrides;
  `EchoAudioPipe` uses the default.
- A **new `AdvisoryEvent` enum** (`SpeechStarted { at }`, `SpeechStopped { at }`) flowing
  over a tokio mpsc from the TapEngine (tokio) to the Reflex (media thread). The engine
  pushes the events it already decodes from the brain (slice-3 wired these as log+count;
  slice-4 *forwards* them into the reflex).
- A **new dedicated media thread** (`rutster/src/media_thread.rs`) replacing the tokio
  `spawn_poll_task`. One `std::thread::spawn` at binary startup owns
  `HashMap<ChannelId, RtcSession>` exclusively; all access from axum is via a command
  channel (`AcceptOffer`, `Delete`, `Shutdown`). The 20 ms tick is `std::thread::sleep`.
- **Rewired `session_map.rs`** (binary): `SessionEntry.rtc: Arc<Mutex<RtcSession>>` →
  `cmd_tx: mpsc::Sender<MediaCmd>`. `create_session`, `post_offer`, `close`,
  `spawn_poll_task` all route through the command channel. The async handlers are cold-path;
  no cross-thread coordination happens on the 20 ms tick.
- **`MockRealtimeBrain` extension** (`rutster-brain-realtime/src/mock.rs`): gains the ability
  to emit `speech_started` / `speech_stopped` on a programmable schedule (e.g. "after N
  audio_in frames received, send `speech_started`; after M more, send `speech_stopped`").
- **Barge-in e2e integration test** (extends slice-3's
  `crates/rutster/tests/realtime_integration.rs` harness): synthetic WebRTC peer →
  MediaThread → TapEngine → MockRealtimeBrain; mock emits `speech_started`; assert playout
  goes silent within ≤1 tick (20 ms); mock emits fresh `audio_out`; assert playout resumes.
- New `ReflexMetrics` (`barge_in_count`, `advisory_dropped`, `frames_suppressed`) mirroring
  `TapMetrics` shape (atomics, snapshot fn). Threaded through the same `TapConn.metrics`
  surface where reasonable, or a new side-car.
- Thorough learner-facing comments on the new std-thread / channel-bridge / wrapper-decorator
  patterns (slice-1 §7 standard carries over).

### 1.2 Out of scope (with scheduled return)

| Deferred item | Returns in | Why deferred |
|---|---|---|
| Local VAD **tuning framework** (configurable thresholds, per-environment calibration, adaptive noise floor) | post-spearhead refinement | The VAD itself (RMS/energy detector + debounce) IS in scope for slice-4 (the primary barge-in trigger, proving wedge #1). Only the *tuning* framework — configurable thresholds, calibration UI, adaptive noise floors — is deferred. The MVP ships with a single `const` threshold + N-frame debounce, exercised by the e2e test with a synthetic loud signal. Tuning to real-world noise conditions is post-spearhead. |
| Per-session media threads / threadpool shard | later rung | Single thread covers spearhead scale (loopback dev + low-concurrency PSTN via slice-5). The command-channel seam between axum and the thread makes the graduation to a threadpool shard localized. |
| Trickle ICE | later | Unchanged from slice-1 deferral. |
| Min-mute floor / inter-word-gap debouncing | post-spearhead | `SpeechStopped` is a no-op for mute; a floor timer on resume would protect against brain-yield races (brain emits fresh `audio_out` before the caller's inter-word gap ends). Defer until observed in practice. |
| Brain-side `input_audio_buffer.interrupt` / `clear` on barge | slice-5 or brain-side | Whether the brain should clear its own input buffer on `speech_started` is a brain-UX decision, not a FOB one; the FOB only kills *playout* (its half-duplex gate). The advisory already tells the brain what happened; the brain's response is its own concern. |
| Half-duplex gating beyond playout kill | later rung | Barge-in is the first half-duplex reflex; full HD gating (mixing, jitter buffer interaction, multi-party) arrives with conferencing. |
| TLS on HTTP / WSS | slice-5 | Unchanged. |
| Authn / authz / multi-tenancy | slice-6 | Unchanged. |
| Spend cap / abuse gate | slice-6 | Unchanged. |
| Browser-based automated e2e (Playwright/Selenium) | post-spearhead | Unchanged. The synthetic-peer harness from slice-2/3 is the test vehicle. |

---

## 2. Architecture delta

### 2.1 The reflex wrapper

`Reflex<P: AudioPipe>` is a zero-cost-style decorator around any `AudioPipe`. It sits
between `RtcSession.pipe` (which `loop_driver::drive` calls via `session.pipe.next_pcm_frame()`)
and the concrete pipe (`TapAudioPipe` in production, `EchoAudioPipe` in slice-1's unit tests).
`loop_driver` is oblivious to the wrapper: it still calls `session.pipe.next_pcm_frame()`,
the dynamic dispatch through `Box<dyn AudioPipe>` lands in `Reflex::next_pcm_frame`, which
applies the state machine and delegates to `inner.next_pcm_frame()` per the table in §3.2.

The reflex owns three pieces of state:
- `advisory_rx: mpsc::Receiver<AdvisoryEvent>` — drained sync-non-blocking via `try_recv`
  on the 20 ms tick before delegating to `inner`. Fed by TWO senders on the same channel:
  the outer `LocalVadReflex` (local VAD, the primary trigger) and the TapEngine task (the
  brain's advisory, secondary/confirmation) — both over tokio mpsc.
- `muted: bool` — the kill state. `next_pcm_frame` returns `None` while muted, *unless* the
  inner returns `Some` (the resume condition — the first fresh `audio_out` clears mute).
- `barge_epoch: u64` — incremented on every `SpeechStarted`. Load-bearing this slice: the
  local VAD (primary) and the brain's advisory (secondary) can both fire on the same barge,
  and the epoch distinguishes a genuine re-barge from the slower confirmation landing on the
  same event (§3.2). The flush + drain keeps resume race-free regardless.

### 2.2 The dedicated media thread

A single `std::thread::spawn` replaces the tokio `spawn_poll_task`. The thread owns
`HashMap<ChannelId, RtcSession>` **exclusively** — no `Arc<Mutex<RtcSession>>` shared with
axum. All access from the axum handlers is via a command channel:

```rust
enum MediaCmd {
    AcceptOffer { id: ChannelId, sdp: String, reply: oneshot::Sender<Result<String, String>> },
    Delete     { id: ChannelId, reply: oneshot::Sender<()> },
    Shutdown   { reply: oneshot::Sender<()> },
}
```

The thread loop per 10 ms meta-tick:
1. Drain `cmd_rx` via `try_recv` loop — handle all pending commands before ticking.
2. For each session in the map: drain the per-session `flush_rx` side-channel (slice-2's
   existing disconnect-flush signal) BEFORE `run_poll_once`, then call
   `RtcSession::run_poll_once(now)` (the unchanged `loop_driver::drive`).
3. After `run_poll_once`, observe `channel.state`:
   - `Connected && tap.is_none()` → spawn the TapEngine (tokio task via the
     `tokio::runtime::Handle` captured at thread-start) + wire
     `Reflex<TapAudioPipe>` as the session's pipe. Mirror of slice-2's spawn seam,
     relocated from `session_map.rs::drive_all_sessions` to here.
   - `Closed` → remove the entry + drop the session.
4. `std::thread::sleep(Duration::from_millis(10))` — 10 ms meta-tick. (Stable API:
   `std::thread::sleep_until` is nightly-only; `sleep(dur)` is the stable path. The 20 ms
   outbound encode tick is driven inside `loop_driver::drive` (unchanged); the 10 ms
   meta-tick gives finer resolution so str0m's `Timeout` outputs are honored promptly.)

**The tokio ↔ std-thread bridge:** all channels are tokio mpsc/oneshot (constructable on
tokio, drainable via `try_recv`/`blocking_recv` from any thread). The `tokio::runtime::Handle`
captured at `MediaThread::spawn` time is used on the std thread to `handle.spawn(...)` the
TapEngine when the `Connected` transition fires. No async code runs on the std thread itself
— only sync channel ops + `RtcSession::run_poll_once`.

**Why a single thread, not per-session:** spearhead scale. One loopback peer at a time in
dev; even at low PSTN concurrency (slice-5) one thread drives dozens of sessions in 10 ms.
Per-session threads arrive when the threadpool shard model lands (deferred). The
command-channel seam between axum and the thread makes that graduation localized.

### 2.3 The hot-path audit (ADR-0002 honored)

ADR-0002's load-bearing rule: *"the control↔media gRPC hop on the per-call hot path is
removed."* Slice 4 does not re-introduce a hop:

- The reflex's kill decision happens **inside** `Reflex::next_pcm_frame` on the dedicated
  thread — no channel send, no cross-thread coordination on the 20 ms tick. The advisory
  arrives via a `try_recv` drain (sync, non-blocking).
- axum → media-thread is **cold-path only** (SDP accept, DELETE). None of it runs on the
  20 ms tick.
- The brain WS ↔ TapEngine (tokio) path is unchanged from slice-3. The advisory channel
  is a *third* mpsc alongside the existing `tx_pcm_in`/`rx_audio_out`/`flush_tx` — same
  pattern, additive.

**The fused vertical stays fused.** ADR-0002 honored.

---

## 3. Component design

### 3.1 `AdvisoryEvent` enum

```rust
// crates/rutster-media/src/reflex.rs

/// A turn-event advisory from the brain. The brain decodes its own
/// speech-to-text / VAD results and forwards these; the FOB *owns*
/// turn-taking and acts on them (slice-3 §4.3 — OpenAI Realtime
/// server-side VAD is DISABLED; the FOB's reflex is authoritative).
///
/// Carried over a tokio mpsc from the TapEngine (tokio task) to the
/// `Reflex` wrapper (media thread). Drained sync via `try_recv` on the
/// 20 ms tick — the kill decision lives in the loop, not in a handler.
#[derive(Debug)]
pub enum AdvisoryEvent {
    /// The brain detected caller speech. Trigger barge-in: kill playout.
    SpeechStarted { at: Instant },
    /// The brain detected caller speech ended. Observed + counted; does
    /// NOT toggle mute (the resume condition is "first fresh audio_out
    /// after the barge", not "speech_stopped" — see §3.2 state table).
    SpeechStopped { at: Instant },
}
```

### 3.2 `Reflex<P>` state machine

| Current state | Event | Action | New state |
|---|---|---|---|
| Playing | `SpeechStarted` | `muted=true`; `epoch++`; `inner.barge_in_flush()` (clear ring + drain `rx_audio_out` so stale brain frames queued pre-barge are dropped); `metrics.barge_in_count++` | Muted |
| Muted | `SpeechStarted` (duplicate/re-barge) | `epoch++`; `barge_in_flush()` again (fresh barge resets the "fresh audio" clock); `barge_in_count++` | Muted |
| Muted | `SpeechStopped` | increment `advisory_observed_speech_stopped` counter; **no state change** | Muted |
| Playing | `SpeechStopped` | increment counter; **no state change** | Playing |
| Muted | inner `next_pcm_frame()` returns `Some(f)` (fresh brain audio arrived post-barge) | `muted=false`; return `Some(f)` | Playing |
| Muted | inner `next_pcm_frame()` returns `None` | return `None` (silence); `metrics.frames_suppressed++` | Muted |

**Why `SpeechStopped` is a no-op for mute:** per the resume-semantics decision (resume on
first fresh `audio_out`). The brain's `speech_stopped` is *observed* (counter) but doesn't
gate — this avoids the inter-word-gap problem (caller pauses, VAD fires stopped, brain
un-mutes too early, brain's audio overlaps caller's next word). The resume condition is
"the brain has yielded and started a new response," which is provably signaled by the first
`audio_out` frame after the barge — not by the caller's silence.

**Why `epoch`:** with two concurrent trigger sources this slice — the local VAD (primary)
and the brain's advisory (secondary/confirmation, §6.1) — the epoch disambiguates "is this
barge a re-barge of the same event, or a new one" when both race into the same advisory
channel. The local VAD trips first; the brain's slower ASR-grade advisory lands ~300 ms
later on the *same* event and must not be counted as a fresh barge. The epoch is that
disambiguator — load-bearing now, not a forward-compat seam.

### 3.3 `AudioPipe` trait extension

```rust
// crates/rutster-media/src/pcm.rs — additive method on `AudioPipe`

/// Barge-in flush: clear the playout ring AND drain the inbound brain
/// audio queue of any frames queued before the barge. Called by `Reflex`
/// on `SpeechStarted`. The drain of `rx_audio_out` is what makes the
/// resume condition race-free: the first `audio_out` observed post-barge
/// is provably post-barge (frames queued pre-barge are dropped here).
///
/// Default impl delegates to `clear_playout_ring` — sufficient for
/// pipes without an inbound queue to drain (like `EchoAudioPipe`).
fn barge_in_flush(&mut self) {
    self.clear_playout_ring();
}
```

`TapAudioPipe` overrides:
```rust
// crates/rutster-tap/src/tap_audio_pipe.rs

fn barge_in_flush(&mut self) {
    // Clear the playout ring (drops buffered brain-proposed frames).
    self.playout_ring.clear();
    // Drain rx_audio_out of any frames the engine task queued before
    // the barge. Without this, a stale frame in the mpsc would un-mute
    // immediately on the next tick — defeating the "first fresh audio_out"
    // resume condition. Hot-path: try_recv loop, bounded, no blocking.
    while self.rx_audio_out.try_recv().is_ok() {
        self.metrics.barge_drained_inflight.fetch_add(1, Ordering::Relaxed);
    }
}
```

### 3.4 `LocalVadReflex<P>` — the primary trigger (FOB-local, zero brain round-trip)

The decorator that proves wedge #1. Wraps any `AudioPipe`; inspects the caller's decoded PCM
in `on_pcm_frame`, computes RMS energy, fires `AdvisoryEvent::SpeechStarted` into the inner
`Reflex`'s advisory channel when local VAD trips. Composes as
`LocalVadReflex<Reflex<TapAudioPipe>>` — the outer wrapper does local-VAD; the inner wrapper
applies the mute state machine to the advisory stream (which now has TWO sources: the local
VAD + the brain's advisory, both feeding the same mpsc).

```rust
// crates/rutster-media/src/reflex.rs

/// RMS energy threshold for caller-speech detection. The MVP ships with
/// a single tuned-for-synthetic-loud-signal const; the tuning framework
/// (per-environment calibration, adaptive noise floor) is deferred per
/// slice-4 §1.2. The synthetic-peer e2e test sends a frame with samples
/// well above this threshold so the trip is deterministic.
///
/// i16 max is 32767; a |sample| average of ~500 (~1.5% of full scale) is
/// a quiet-but-unmistakable signal; room-tone background sits well below.
/// The debounce (N consecutive frames above threshold) filters transient
/// spikes (clicks, nose-breaths) without slowing the trip materially.
pub const VAD_RMS_THRESHOLD: f64 = 500.0;

/// Number of consecutive above-threshold frames required before the VAD
/// trips. At 20 ms/frame, N=3 = 60 ms of above-threshold audio — well
/// below the brain's ~300 ms ASR-VAD latency, comfortably instant for
/// the wedge-#1 demonstration. Tunable in a later slice; const for MVP.
pub const VAD_DEBOUNCE_FRAMES: u32 = 3;

pub struct LocalVadReflex<P: AudioPipe> {
    inner: P,
    /// The advisory_tx the LocalVadReflex pushes into when local VAD
    /// trips. This is the SAME channel the brain's advisories arrive
    /// on (the Reflex holds the paired rx) — so the Reflex's mute state
    /// machine sees both sources uniformly via `drain_advisories`.
    advisory_tx: mpsc::Sender<AdvisoryEvent>,
    /// Consecutive above-threshold frame count (the debounce counter).
    above_threshold_streak: u32,
    /// True once the VAD has tripped for the current caller-speech
    /// burst. Reset to false when the streak breaks (caller stopped)
    /// — so a new burst trips a fresh `SpeechStarted`.
    vad_armed: bool,
}

impl<P: AudioPipe> LocalVadReflex<P> {
    pub fn new(inner: P, advisory_tx: mpsc::Sender<AdvisoryEvent>) -> Self {
        Self {
            inner,
            advisory_tx,
            above_threshold_streak: 0,
            vad_armed: true, // armed on construction
        }
    }

    /// Compute RMS energy of a PCM frame. The frame is 480 i16 samples
    /// @ 24 kHz; RMS = sqrt(mean(sample²)). Cheap: ~480 multiplications,
    /// one division, one sqrt — well under the 20 ms tick budget.
    fn rms(frame: &PcmFrame) -> f64 {
        let sum_sq: u64 = frame.samples.iter()
            .map(|&s| (s as i64 * s as i64) as u64)
            .sum();
        (sum_sq as f64 / frame.samples.len() as f64).sqrt()
    }

    /// Inspect a caller PCM frame + apply the debounce state machine.
    /// Called from `on_pcm_frame` (the sink path). Returns true if the
    /// VAD tripped THIS call (so the caller can push the advisory).
    fn observe(&mut self, frame: &PcmFrame) -> bool {
        let energy = Self::rms(frame);
        if energy >= VAD_RMS_THRESHOLD {
            self.above_threshold_streak += 1;
            if self.above_threshold_streak >= VAD_DEBOUNCE_FRAMES && self.vad_armed {
                self.vad_armed = false; // disarm until caller stops
                return true; // trip!
            }
        } else {
            // Caller went quiet → re-arm for the next speech burst.
            self.above_threshold_streak = 0;
            self.vad_armed = true;
        }
        false
    }
}

impl<P: AudioPipe> AudioSource for LocalVadReflex<P> {
    fn next_pcm_frame(&mut self) -> Option<PcmFrame> {
        // Pure delegation — the VAD only observes the SINK path
        // (inbound caller audio); playout is the inner Reflex's concern.
        self.inner.next_pcm_frame()
    }
}

impl<P: AudioPipe> AudioSink for LocalVadReflex<P> {
    fn on_pcm_frame(&mut self, frame: PcmFrame) {
        // THE PRIMARY TRIGGER: inspect the caller's audio BEFORE delegating.
        // If the local VAD trips, push an advisory into the same channel the
        // brain's advisories arrive on — the inner Reflex drains it on the
        // next next_pcm_frame call + applies the kill. Zero brain round-trip.
        if self.observe(&frame) {
            let _ = self.advisory_tx.try_send(AdvisoryEvent::SpeechStarted {
                at: Instant::now(),
            });
            // try_send failure (channel full) → drop + observe (hot-path
            // policy). The brain's advisory path is the backstop; a missed
            // local-VAD trip here is not catastrophic — the brain will fire
            // its ASR-VAD ~300 ms later.
        }
        // Delegate to inner — the caller's audio still reaches the brain.
        self.inner.on_pcm_frame(frame)
    }
}

impl<P: AudioPipe> AudioPipe for LocalVadReflex<P> {
    fn clear_playout_ring(&mut self) { self.inner.clear_playout_ring() }
    fn barge_in_flush(&mut self) { self.inner.barge_in_flush() }
}
```

**Why the VAD trips `on_pcm_frame` (sink), not `next_pcm_frame` (source):** the caller's
audio is what the VAD inspects — and caller audio arrives via the sink path (decoded from
the peer's RTP by `loop_driver::drive` → `session.pipe.on_pcm_frame(pcm)`). The source path
is the brain's `audio_out` — the playout being killed, not the signal being detected.

**Why the SAME advisory channel as the brain:** the inner `Reflex` drains all advisories
uniformly — it doesn't care whether the source is local VAD or the brain's ASR. The brain's
advisory arriving ~300 ms later either finds the kill already applied (no-op, the `muted`
flag is already true) or confirms it. This is the composition pattern §6.4 anticipated; the
`Reflex<P>` wrapper shape was designed for exactly this layering.

### 3.5 `Reflex<P>` struct + impl

```rust
// crates/rutster-media/src/reflex.rs

pub struct Reflex<P: AudioPipe> {
    inner: P,
    advisory_rx: mpsc::Receiver<AdvisoryEvent>,
    muted: bool,
    barge_epoch: u64,
    metrics: Arc<ReflexMetrics>,
}

impl<P: AudioPipe> Reflex<P> {
    pub fn new(inner: P, advisory_rx: mpsc::Receiver<AdvisoryEvent>, metrics: Arc<ReflexMetrics>) -> Self {
        Self { inner, advisory_rx, muted: false, barge_epoch: 0, metrics }
    }

    /// Drain all pending advisories + apply the state table. Called at
    /// the top of `next_pcm_frame`. Hot-path: try_recv loop, bounded.
    fn drain_advisories(&mut self) {
        while let Ok(ev) = self.advisory_rx.try_recv() {
            match ev {
                AdvisoryEvent::SpeechStarted { at } => {
                    self.muted = true;
                    self.barge_epoch = self.barge_epoch.wrapping_add(1);
                    self.inner.barge_in_flush();
                    self.metrics.barge_in_count.fetch_add(1, Ordering::Relaxed);
                    tracing::info!(epoch = self.barge_epoch, ?at, "barge-in");
                }
                AdvisoryEvent::SpeechStopped { at: _ } => {
                    self.metrics.advisory_observed_speech_stopped.fetch_add(1, Ordering::Relaxed);
                    // No state change — see §3.2.
                }
            }
        }
    }
}

impl<P: AudioPipe> AudioPipe for Reflex<P> {
    fn next_pcm_frame(&mut self) -> Option<PcmFrame> {
        self.drain_advisories();
        if self.muted {
            // Muted: pull from inner. Some(f) = fresh brain audio arrived
            // post-barge → un-mute + return. None = silence, stay muted.
            match self.inner.next_pcm_frame() {
                Some(f) => {
                    self.muted = false;
                    Some(f)
                }
                None => {
                    self.metrics.frames_suppressed.fetch_add(1, Ordering::Relaxed);
                    None
                }
            }
        } else {
            self.inner.next_pcm_frame()
        }
    }

    fn on_pcm_frame(&mut self, frame: PcmFrame) {
        // Inbound caller audio is NEVER gated by the reflex. The brain
        // still hears the caller during barge — that's the point (the
        // brain needs to know the caller interrupted; the FOB only kills
        // its OWN playout, not the caller's path to the brain).
        self.inner.on_pcm_frame(frame)
    }

    fn clear_playout_ring(&mut self) {
        // The reconnect-flush path (slice-2 §5.3) still works through the
        // wrapper. If it fires during mute, the ring stays empty and mute
        // clears on the next post-reconnect audio_out.
        self.inner.clear_playout_ring()
    }

    fn barge_in_flush(&mut self) {
        // Allow the outer `LocalVadReflex` (primary trigger) to barge the inner.
        self.inner.barge_in_flush()
    }
}
```

### 3.6 `ReflexMetrics`

Mirror of `TapMetrics` shape (atomics + snapshot struct):

```rust
// crates/rutster-media/src/reflex.rs

#[derive(Default)]
pub struct ReflexMetrics {
    pub barge_in_count: AtomicU64,
    pub advisory_dropped: AtomicU64,       // advisory channel full (e.g. 16-cap)
    pub frames_suppressed: AtomicU64,      // None returns while muted
    pub advisory_observed_speech_stopped: AtomicU64,
}

pub struct ReflexMetricsSnapshot {
    pub barge_in_count: u64,
    pub advisory_dropped: u64,
    pub frames_suppressed: u64,
    pub advisory_observed_speech_stopped: u64,
}

// `barge_drained_inflight` lives on `TapMetrics` (in `rutster-tap`), not
// `ReflexMetrics`, because the drain happens inside `TapAudioPipe::barge_in_flush`,
// not inside `Reflex`. The path: `Reflex::drain_advisories` calls
// `inner.barge_in_flush()` which is `TapAudioPipe::barge_in_flush`, which is
// where the `rx_audio_out` drain + the counter increment happen.
```

---

## 4. The dedicated media thread

### 4.1 `MediaThread`

```rust
// crates/rutster/src/media_thread.rs

pub struct MediaThread {
    cmd_tx: mpsc::Sender<MediaCmd>,
    join: Option<std::thread::JoinHandle<()>>,
}

enum MediaCmd {
    AcceptOffer { id: ChannelId, sdp: String, reply: oneshot::Sender<Result<String, String>> },
    Delete     { id: ChannelId, reply: oneshot::Sender<()> },
    Shutdown   { reply: oneshot::Sender<()> },
}
```

Spawned at binary startup (`main.rs`), before `axum::serve`. The thread captures a
`tokio::runtime::Handle` (to spawn TapEngine tasks when `Connected` transitions fire) and
owns `HashMap<ChannelId, RtcSession>` + (per-session, lazily) the TapConn / advisory_rx /
Reflex wrapper.

### 4.2 Thread loop (per 10 ms meta-tick)

1. `cmd_rx.try_recv()` loop — handle ALL pending commands before ticking. `AcceptOffer`
   calls `RtcSession::accept_offer(sdp)` and replies via the oneshot. `Delete` fires
   `close_tx` + bounded-await the engine task (750 ms cap via
   `tokio::runtime::Handle::block_on(timeout(...))`) — the std thread briefly enters the
   tokio runtime to await; cold-path, not the 20 ms tick. `Shutdown` drains + replies.
2. For each `RtcSession` in the map:
   - Drain per-session `flush_rx` side-channel (slice-2's existing disconnect-flush) BEFORE
     `run_poll_once`.
   - Call `RtcSession::run_poll_once(now)` — the unchanged `loop_driver::drive`.
   - Observe `channel.state`:
     - `Connected && tap.is_none()` → `handle.spawn(spawn_tap_engine(...))` to bring up the
       tokio task; construct `Reflex::new(TapAudioPipe::new(...), advisory_rx, metrics)`;
       call `RtcSession::set_pipe(reflex)`. Mirror of slice-2's spawn seam.
     - `Closed` → remove the entry (drops the `RtcSession` + its pipe + advisory ends).
3. `std::thread::sleep(Duration::from_millis(10))` — 10 ms meta-tick.

### 4.3 `session_map.rs` rewire

`SessionEntry` loses `rtc: Arc<Mutex<RtcSession>>`, gains `cmd_tx: mpsc::Sender<MediaCmd>`
(cloned per-entry; cheap). `tap_url` stays (the thread reads it when spawning the engine).
`tap_conn: Option<TapConn>` moves onto the media thread (the thread owns it after spawn).

- `AppState::create_session` → sends a `Register { tap_url, reply }` command to the media
  thread; the **thread** constructs `RtcSession::new()` (saves a cross-thread move of the
  struct + keeps all `RtcSession` construction on the thread that owns it). The thread
  replies with `(id, cmd_tx_for_this_session)`; axum stores `SessionEntry { cmd_tx, tap_url,
  tap_conn: None }`.
- `AppState::get(id)` (SDP path) → `cmd_tx.send(AcceptOffer { ... }).await` + `reply.await`.
  Cold-path; the axum handler is async.
- `AppState::close(id)` → `cmd_tx.send(Delete { id, reply }).await` + `reply.await`. The
  reply returns after the TapEngine teardown completes on the thread.
- `spawn_poll_task` → `spawn_media_thread`: constructs the channels, spawns the std thread,
  stores `cmd_tx` + `join` in `AppState`. Same idempotent-guard pattern.

### 4.4 TapEngine extension

`spawn_tap_engine` returns a third channel end: `advisory_tx: mpsc::Sender<AdvisoryEvent>`.
The pump loop, on receiving `speech_started` / `speech_stopped` from the brain (slice-3
already decodes these in the tap protocol layer — `protocol_events.rs`), pushes the
corresponding `AdvisoryEvent` into `advisory_tx`. If the channel is full, drop + count
(hot-path "drop + observe" policy; an advisory is a hint, not a command). The `Reflex`
wrapper holds `advisory_rx`.

### 4.5 `MockRealtimeBrain` extension

`rutster-brain-realtime/src/mock.rs` gains a programmable advisory schedule: the test can
register "after N `audio_in` frames received, send `speech_started`" and "after M more,
send `speech_stopped`". The mock already asserts `turn_detection: null` on
`session.update` (slice-3's S4 lock); slice-4 keeps that assertion.

---

## 5. Data flow

### 5.1 Barge-in (the kill) — primary path: local VAD, zero brain round-trip

```
1. caller speaks into mic → peer RTP → str0m decode → on_pcm_frame
   → LocalVadReflex::on_pcm_frame INSPECTS the frame FIRST:
     • rms(frame) computed (~480 muls, < 1 µs)
     • if rms ≥ VAD_RMS_THRESHOLD: streak++; if streak ≥ VAD_DEBOUNCE_FRAMES (3 = 60 ms) → TRIP
     • on trip: try_send(AdvisoryEvent::SpeechStarted) into advisory_tx (same channel as
       brain's advisories) → disarm until caller goes quiet
   → THEN delegates to inner.on_pcm_frame(frame) → tx_pcm_in → TapClient → audio_in (WS) → brain
2. [THREE HOPS HAPPEN HERE, ON THE BRAIN SIDE — NOT on the kill path]:
   - brain's ASR-VAD fires (~300 ms later, slower but more accurate)
   - brain sends speech_started back over WS
   - TapEngine → advisory_tx (the SAME mpsc; both sources feed one channel)
3. media thread next 20 ms tick → Reflex::next_pcm_frame → drain_advisories:
   • The LOCAL VAD's SpeechStarted arrives FIRST (it was pushed in step 1, same thread,
     no WS hop) → muted=true; epoch++; inner.barge_in_flush() (ring cleared + rx_audio_out drained)
   • The brain's SpeechStarted arrives ~300 ms LATER → Reflex sees muted=true already
     → re-barge (epoch++, barge_in_flush again — harmless, mute stays)
   → returns None (silence) for this + subsequent ticks while muted
4. loop_driver::drive pulls None from pipe → encodes Opus silence → peer hears silence
   (the brain's in-flight audio_out frames are dropped; no overlap with caller's speech)
```

**Latency budget:** caller speaks → kill fires in ≤ 60 ms (3 debounce frames × 20 ms tick) +
one tick to drain the advisory + apply the kill = ≤ 80 ms wallclock. Zero brain round-trip
on the primary path. (The brain's ASR-VAD advisory arrives ~300 ms later — it confirms the
kill but doesn't gate it.) This is what ARCHITECTURE.md:80 demands ("the brain round-trip is
too slow to enforce them") and the proof wedge #1 rests on.

### 5.2 Resume (the un-mute)

```
1. brain decides to yield/respond → sends a fresh audio_out frame
   (provably post-barge: barge_in_flush drained rx_audio_out)
2. TapClient → audio_out (WS) → TapEngine → tx_audio_out → rx_audio_out → playout ring
3. media thread 20 ms tick → Reflex::next_pcm_frame → drain_advisories (empty)
   → muted=true → inner.next_pcm_frame() returns Some(f) (fresh brain audio)
   → muted=false; return Some(f)
4. loop_driver encodes + writes → peer hears the brain's new response
```

### 5.3 Cold-path (axum ↔ media thread)

```
- POST /v1/sessions → AppState::create_session → MediaCmd::Register → thread constructs RtcSession → reply(id)
- POST /v1/sessions/{id}/offer → AppState::get + cmd_tx.send(AcceptOffer) → thread.lock(session).accept_offer(sdp) → reply(answer)
- DELETE /v1/sessions/{id} → AppState::close → cmd_tx.send(Delete) → thread: fire close_tx, bounded-await engine task teardown → reply
- graceful shutdown → cmd_tx.send(Shutdown) → thread drains + drops → reply → join
```

---

## 6. Why these decisions

### 6.1 Why both, local VAD primary (revised after adversarial review)

The initial brainstorming landed on advisory-only for MVP (cheapest path to a working
barge-in; the brain's VAD already runs for STT). The 2026-07-01 adversarial review surfaced
the load-bearing problem with that choice: README:98-100 + ARCHITECTURE.md:79-81 rest the
wedge on "local reflexes that don't need the brain — VAD killing TTS the instant the caller
speaks." Advisory-only puts the brain round-trip in the trigger path (brain VAD → WS →
TapEngine → mpsc → Reflex); the kill DECISION is in-core but the TRIGGER SOURCE crossed
the brain. The spearhead's step 4 would prove a reflex that depends on the brain — the
opposite of the property steps 1-4 exist to prove.

The revision: **local VAD in `on_pcm_frame` is the primary trigger** (RMS/energy detector on
the dedicated thread, in the 20 ms loop, zero brain round-trip — the actual wedge-#1 proof).
Slice-3's advisory becomes the **secondary/confirmation** signal — the brain's ASR-quality
VAD confirms the local kill ~300 ms later (slower but more accurate — it knows words, not
just energy). The two sources feed the SAME advisory mpsc; the `Reflex` wrapper drains them
uniformly. The composition is `LocalVadReflex<Reflex<TapAudioPipe>>` — the decorator pattern
from §6.4, which was originally specced as deferred to "when local VAD arrives." It arrived.

- The VAD itself (~25 lines: RMS, threshold, debounce) is in scope. The **tuning framework**
  (configurable thresholds, per-environment calibration, adaptive noise floor) is deferred —
  the MVP ships a single `const` threshold + N-frame debounce, exercised by the e2e test
  with a synthetic loud signal.
- Slice-3's advisory plumbing + `MockRealtimeBrain` schedule aren't wasted — they become
  the secondary path, still exercised by the e2e test (the brain's advisory fires SLIGHTLY
  after the local VAD; both feeds land in the Reflex's drain).
- The `Reflex<P>` wrapper shape already supported composition — no structural rework, just
  a new `LocalVadReflex<P>` decorator landing in scope (Task 2b in the plan).

### 6.2 Why resume on first fresh `audio_out` (not `speech_stopped`)

- The "the brain has yielded and started a new response" condition is provably signaled by
  the first `audio_out` frame after the barge — not by the caller's silence. `speech_stopped`
  fires between words; resuming on it un-mutes too early (inter-word-gap overlap).
- The `barge_in_flush` drain of `rx_audio_out` makes the resume race-free: the first
  `audio_out` observed post-barge is provably post-barge (frames queued pre-barge are dropped
  in the flush).

### 6.3 Why a single dedicated thread (not per-session)

- Spearhead scale: one loopback peer in dev; even at low PSTN concurrency (slice-5), one
  thread drives dozens of sessions in 10 ms.
- The command-channel seam between axum and the thread makes the graduation to a threadpool
  shard localized — when per-CPU-shard threading arrives, it's a fan-out of the
  `cmd_rx`/`HashMap` shape, not a redesign.
- Per-session threads arrive when load demands; the spearhead's "shortest blocking path"
  rule dislikes spawning work per session that may not need it (pre-ICE-connected sessions
  would redundantly spin).

### 6.4 Why `Reflex<P>` as a wrapper (not inline in `TapAudioPipe`)

- Composition: `LocalVadReflex<P>` composes outside the advisory `Reflex<P>` (§6.1), the
  same way `Reflex<TapAudioPipe>` composes. The pattern (decorator over `AudioPipe`) stacks
  the two trigger sources without restructuring either.
- The seam: `loop_driver.rs` byte-identical (still calls `pipe.next_pcm_frame()`). If the
  reflex lived inline in `TapAudioPipe`, the binary-side wiring would still change but the
  `TapAudioPipe` module itself would grow the reflex state — less isolated.
- The payoff is realized this slice, not deferred: two stacked reflexes (local-VAD primary +
  advisory secondary) live as independent, separately-testable decorators rather than one
  module's commingled state. Keeping them as wrappers is what makes that separation free.

### 6.5 Why `barge_in_flush` on `AudioPipe` (not just `clear_playout_ring`)

- `clear_playout_ring` (slice-2) clears the *ring*. `barge_in_flush` clears the ring AND
  drains the *inbound brain queue* (`rx_audio_out`). The distinction matters: on a brain
  disconnect (slice-2's case), the brain is gone — `rx_audio_out` will drain itself on the
  next `Disconnected` `try_recv`. On a barge-in, the brain is alive and may have queued
  frames pre-barge that would un-mute immediately if not drained here. Two different
  "clear the playout path" semantics, two methods.

---

## 7. Done-criteria

1. `cargo test --all` passes (stable + 1.85, the CI matrix).
2. `cargo fmt --check` + `cargo clippy -- -D warnings` clean.
3. `loop_driver.rs` + `rtc_session.rs` **byte-identical** to slice-3 — CI-asserted via
   `git diff --exit-code main -- crates/rutster-media/src/loop_driver.rs
   crates/rutster-media/src/rtc_session.rs` (the §8.5 #6 seam gate, restated for slice-4).
4. Dedicated media thread drives sessions off the tokio pool; `MediaThread` integration test
   passes (AcceptOffer / Delete / Shutdown).
5. `Reflex` state-machine unit tests all pass:
   - `SpeechStarted` → next `next_pcm_frame` returns None even if ring has frames.
   - `SpeechStarted` then `inner.next_pcm_frame()=Some` → un-mutes, returns the frame.
   - `SpeechStopped` during Muted → stays Muted.
   - `SpeechStopped` during Playing → no-op.
   - Duplicate `SpeechStarted` re-flushes + stays Muted.
   - Metrics counters (`barge_in_count`, `frames_suppressed`) increment correctly.
   - `advisory_rx` full → `advisory_dropped` increments, no panic.
6. `LocalVadReflex` unit tests pass:
   - RMS computation verified against a known-loud + known-quiet frame.
   - Debounce: N-1 above-threshold frames do NOT trip; the Nth does.
   - Re-arm: above-threshold → trip; below-threshold → re-arm; next streak trips again.
   - `on_pcm_frame` ALWAYS delegates to inner (caller audio reaches the brain even during barge).
7. `barge_in_flush` unit tests pass (ring + `rx_audio_out` drain).
8. Barge-in e2e (PRIMARY PATH, proves wedge #1): synthetic loud caller audio → playout killed
   within ≤4 ticks (≤80 ms wallclock: 3 debounce + 1 drain+apply) **WITHOUT any brain advisory**.
   The brain's `speech_started` advisory arrives later + is a no-op (mute already applied).
9. Barge-in e2e (SECONDARY PATH, exercises slice-3 advisory plumbing): `MockRealtimeBrain`
   emits `speech_started` on schedule; local VAD is NOT tripped (quiet synthetic caller audio);
   advisory → kill → fresh `audio_out` → resume. Proves the advisory→reflex→kill path still works.
10. S4 turn-ownership lock preserved: `MockRealtimeBrain` still asserts
    `turn_detection: null` on `session.update` (slice-3's #7, unchanged).
11. `MockRealtimeBrain` extended to emit `speech_started`/`speech_stopped` on schedule.
12. `cargo doc --no-deps` renders the new `reflex.rs` + `media_thread.rs` module/item docs
    cleanly (learner-facing comments present per AGENTS.md code style).

---

## 8. Open decisions

- ~~Trigger source~~ — decided: **both, local VAD primary + advisory secondary** (revised after
  2026-07-01 adversarial review; initial brainstorming landed on advisory-only, the review
  surfaced that advisory-only contradicts wedge #1's "VAD killing TTS the instant the caller
  speaks, without the brain"). Local VAD in `on_pcm_frame` is the primary trigger; the brain's
  `speech_started`/`speech_stopped` advisory is the secondary/confirmation.
- ~~Resume semantics~~ — decided: first fresh `audio_out` post-barge; `SpeechStopped`
  observational only.
- ~~Thread model~~ — decided: single dedicated `std::thread`; per-session/threadpool deferred.
- **`MockRealtimeBrain` advisory schedule API shape** — landed in §4.5 as a programmable
  "after N audio_in frames" schedule. Could alternatively be a free-form
  `Vec<(trigger_frame_count, AdvisoryEvent)>` queue. The plan will pin the concrete API.
- **Thread shutdown ordering vs TapEngine teardown** — `Delete` command handler fires
  `close_tx` + bounded-await the engine task (750 ms cap via
  `tokio::runtime::Handle::block_on(timeout(...))`); the reply oneshot returns after
  teardown. Cold-path, std thread briefly enters the tokio runtime to await. Documented as
  an acceptable deviation (not the 20 ms tick).

---

## 9. Cross-references

- [slice-1 spec](2026-06-28-slice-1-webrtc-loopback-design.md) — the media loop + the seam
  (`AudioSource`/`AudioSink` traits in `rutster-media`); slice-1 §8.5 #6 is the seam gate
  this slice re-affirms.
- [slice-2 spec](2026-06-28-slice-2-agent-tap-design.md) — the tap interface, the
  `TapAudioPipe`, the core-authoritative playout buffer (§4.1), the `flush_tx` side-channel
  pattern that the `advisory_rx` mirrors.
- slice-3 (merged `c30a452`) — `MockRealtimeBrain`, the translator, the
  `speech_started`/`speech_stopped` protocol events, the S4 turn-ownership lock.
- [ADR-0002](../../adr/0002-north-star-and-fused-core.md) — fused vertical; the hot-path
  hop invariant this slice re-affirms (§2.3 audit).
- [ADR-0008](../../adr/0008-fob-and-green-zone.md) — FOB/green-zone doctrine; the reflex is
  a FOB member (hot-path, security-constitutive for turn-taking, differentiating).
- [ARCHITECTURE.md](../../ARCHITECTURE.md) — §"Media plane" ("Dedicated timing threads
  for the 20ms loop, never the shared tokio pool" — this slice lands it); §"Biggest
  technical risk" (the reflex loop *is* the remaining long pole).
- [PORT_PLAN.md](../../PORT_PLAN.md) — §Phasing, step 4 = barge-in.
