From b0a4e2ea93fafc4d65d474f3ada616a0ad5e19d5 Mon Sep 17 00:00:00 2001
From: Ariver <ar@Arm1.local>
Date: Sun, 12 Jul 2026 11:39:35 +0800
Subject: [PATCH] feat: bind helper stream timing markers

---
 src/main.rs    |  452 +++++++++++++++++++++++++++++++++++--
 src/service.rs |  239 +++++++++++++++++--
 2 files changed, 638 insertions(+), 53 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 281351d..611993a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -34,6 +34,7 @@
 use reqwest::Client;
 use serde::{Deserialize, Serialize};
 use serde_json::json;
+use sha2::{Digest, Sha256};
 use tokio::time::{sleep, sleep_until, timeout};
 use tokio::{
     sync::{mpsc, mpsc::UnboundedReceiver, watch},
@@ -51,6 +52,11 @@
 const PCM_16K_SAMPLE_RATE_HZ: u32 = 16_000;
 const BOT_NUM_CHANNELS: u16 = 1;
 const TRACK_NAME: &str = "bot-main-audio";
+const STREAM_TIMING_VERSION: u32 = 1;
+const STREAM_TIMING_FIRST_SEGMENT: u64 = 1;
+const STREAM_TIMING_FIRST_CHUNK: u64 = 1;
+const STREAM_TIMING_MAX_ELAPSED_MS: u64 = 5_000;
+const STREAM_TIMING_SOURCE: &str = "stream_anchor_monotonic";
 
 #[tokio::main(flavor = "multi_thread")]
 async fn main() -> Result<()> {
@@ -660,6 +666,31 @@
         "reasonCode": reason_code,
         "retryable": retryable,
         "extension": extension,
+    });
+    println!("{payload}");
+}
+
+fn emit_anchored_activity(
+    call_id: &str,
+    trace_id: &str,
+    turn_id: &str,
+    event_name: &str,
+    marker: &RuntimeTurnStreamTimingMarker,
+    extension: serde_json::Value,
+) {
+    let payload = json!({
+        "type": "cv_activity",
+        "callId": call_id,
+        "traceId": trace_id,
+        "turnId": turn_id,
+        "eventName": event_name,
+        "eventWallTimeMs": current_time_millis(),
+        "serverDeltaMs": marker.server_delta_ms,
+        "serverDeltaSource": STREAM_TIMING_SOURCE,
+        "result": "ok",
+        "reasonCode": null,
+        "retryable": null,
+        "extension": marker.extension_with(extension),
     });
     println!("{payload}");
 }
@@ -1420,6 +1451,7 @@
             .await?;
         }
     }
+    state.close_timing();
     if !state.completed {
         warn!(
             call_id = %call_id,
@@ -1549,6 +1581,7 @@
                 call_id,
                 trace_id,
                 turn,
+                &event,
                 audio_chunk,
                 state,
                 turn_pipeline_started_at,
@@ -1567,6 +1600,7 @@
         }
         Some("turn_completed") => {
             state.completed = true;
+            state.close_timing();
             info!(
                 call_id = %call_id,
                 trace_id = %trace_id,
@@ -1577,6 +1611,7 @@
             );
         }
         Some("turn_failed") => {
+            state.close_timing();
             let error = event.error.as_ref();
             let reason_code = error
                 .and_then(|value| value.reason_code.as_deref())
@@ -1611,6 +1646,7 @@
         }
         Some("turn_cancelled") => {
             state.completed = true;
+            state.close_timing();
             info!(
                 call_id = %call_id,
                 trace_id = %trace_id,
@@ -1643,6 +1679,19 @@
         }
         Some("activity") => {
             if let Some(activity) = event.activity.as_ref() {
+                if activity.event_type.as_deref() == Some("tts_first_audio_chunk_ready") {
+                    if let Some(runtime_session_nonce) =
+                        bridge_config.runtime_session_nonce.as_deref()
+                    {
+                        let _ = state.arm_timing_anchor(
+                            call_id,
+                            trace_id,
+                            &turn.turn_id,
+                            runtime_session_nonce,
+                            &event,
+                        );
+                    }
+                }
                 info!(
                     call_id = %call_id,
                     trace_id = %trace_id,
@@ -1701,6 +1750,7 @@
     call_id: &str,
     trace_id: &str,
     turn: &FinishedSpeechTurn,
+    event: &RuntimeTurnStreamEvent,
     audio_chunk: &RuntimeTurnStreamAudioChunk,
     state: &mut RuntimeTurnStreamState,
     turn_pipeline_started_at: Instant,
@@ -1724,21 +1774,37 @@
         return Err(anyhow!("unsupported reply_audio_chunk format {format}"));
     }
     match state.reply_chunk_markers.observe(audio_chunk.segment_seq) {
-        ReplyChunkMarker::FirstReply => emit_activity(
-            call_id,
-            trace_id,
-            Some(&turn.turn_id),
-            "helper_first_reply_audio_chunk_received",
-            "ok",
-            None,
-            None,
-            json!({
+        ReplyChunkMarker::FirstReply => {
+            let extension = json!({
                 "segmentSeq": audio_chunk.segment_seq,
                 "chunkSeq": audio_chunk.chunk_seq,
                 "format": format.as_str(),
                 "bytes": payload.len(),
-            }),
-        ),
+            });
+            if let Some(marker) =
+                state.record_m6(call_id, trace_id, &turn.turn_id, event, audio_chunk)
+            {
+                emit_anchored_activity(
+                    call_id,
+                    trace_id,
+                    &turn.turn_id,
+                    "helper_first_reply_audio_chunk_received",
+                    &marker,
+                    extension,
+                );
+            } else {
+                emit_activity(
+                    call_id,
+                    trace_id,
+                    Some(&turn.turn_id),
+                    "helper_first_reply_audio_chunk_received",
+                    "ok",
+                    None,
+                    None,
+                    extension,
+                );
+            }
+        }
         ReplyChunkMarker::SegmentFirst => emit_activity(
             call_id,
             trace_id,
@@ -1875,21 +1941,33 @@
         sink.write_pcm_frame(frame).await?;
         if !state.first_audio_frame_written {
             state.first_audio_frame_written = true;
-            emit_activity(
-                call_id,
-                trace_id,
-                Some(&turn.turn_id),
-                "bot_reply_first_audio_frame_written",
-                "ok",
-                None,
-                None,
-                json!({
-                    "replyPlaybackMode": state.reply_playback_mode.as_str(),
-                    "format": format.as_str(),
-                    "chunkSeq": audio_chunk.chunk_seq,
-                    "replyTotalAfterVadEndMs": turn_pipeline_started_at.elapsed().as_millis() as u64,
-                }),
-            );
+            let extension = json!({
+                "replyPlaybackMode": state.reply_playback_mode.as_str(),
+                "format": format.as_str(),
+                "chunkSeq": audio_chunk.chunk_seq,
+                "replyTotalAfterVadEndMs": turn_pipeline_started_at.elapsed().as_millis() as u64,
+            });
+            if let Some(marker) = state.record_m7() {
+                emit_anchored_activity(
+                    call_id,
+                    trace_id,
+                    &turn.turn_id,
+                    "bot_reply_first_audio_frame_written",
+                    &marker,
+                    extension,
+                );
+            } else {
+                emit_activity(
+                    call_id,
+                    trace_id,
+                    Some(&turn.turn_id),
+                    "bot_reply_first_audio_frame_written",
+                    "ok",
+                    None,
+                    None,
+                    extension,
+                );
+            }
         }
         sleep_until(pacing_started_at + Duration::from_millis(((index + 1) as u64) * 20)).await;
     }
@@ -2347,6 +2425,7 @@
     pcm_stream_decoder: Option<audio::PcmS16leStreamDecoder>,
     pcm_stream_network_chunk_count: u64,
     reply_chunk_markers: ReplyChunkMarkerState,
+    timing: RuntimeTurnStreamTimingState,
 }
 
 impl Default for RuntimeTurnStreamState {
@@ -2363,8 +2442,233 @@
             pcm_stream_decoder: None,
             pcm_stream_network_chunk_count: 0,
             reply_chunk_markers: ReplyChunkMarkerState::default(),
+            timing: RuntimeTurnStreamTimingState::default(),
         }
     }
+}
+
+impl RuntimeTurnStreamState {
+    fn arm_timing_anchor(
+        &mut self,
+        call_id: &str,
+        trace_id: &str,
+        turn_id: &str,
+        runtime_session_nonce: &str,
+        event: &RuntimeTurnStreamEvent,
+    ) -> bool {
+        if self.timing.phase != RuntimeTurnStreamTimingPhase::Empty
+            || event.call_id.as_deref() != Some(call_id)
+            || event.trace_id.as_deref() != Some(trace_id)
+            || event.turn_id.as_deref() != Some(turn_id)
+        {
+            return false;
+        }
+        let Some(activity) = event.activity.as_ref() else {
+            return false;
+        };
+        if activity.event_type.as_deref() != Some("tts_first_audio_chunk_ready") {
+            return false;
+        }
+        let Some(extension) = activity.extension.as_ref() else {
+            return false;
+        };
+        let Some(anchor_id) = extension.stream_anchor_id.as_deref() else {
+            return false;
+        };
+        let valid_anchor_id = (16..=64).contains(&anchor_id.len()) && anchor_id.is_ascii();
+        let expected_nonce_hash = runtime_session_nonce_hash(runtime_session_nonce);
+        if extension.stream_timing_version != Some(STREAM_TIMING_VERSION)
+            || !valid_anchor_id
+            || extension.runtime_session_nonce_hash.as_deref() != Some(expected_nonce_hash.as_str())
+            || extension.segment_seq != Some(STREAM_TIMING_FIRST_SEGMENT)
+            || extension.stream_timing_validation.as_deref() != Some("bound")
+        {
+            return false;
+        }
+        let Some(server_delta_ms) = extension.stream_anchor_server_delta_ms else {
+            return false;
+        };
+        self.timing.anchor = Some(RuntimeTurnStreamTimingAnchor {
+            call_id: call_id.to_string(),
+            trace_id: trace_id.to_string(),
+            turn_id: turn_id.to_string(),
+            anchor_id: anchor_id.to_string(),
+            server_delta_ms,
+            runtime_session_nonce_hash: expected_nonce_hash,
+            segment_seq: STREAM_TIMING_FIRST_SEGMENT,
+            received_at: Instant::now(),
+        });
+        self.timing.phase = RuntimeTurnStreamTimingPhase::Armed;
+        true
+    }
+
+    fn record_m6(
+        &mut self,
+        call_id: &str,
+        trace_id: &str,
+        turn_id: &str,
+        event: &RuntimeTurnStreamEvent,
+        audio_chunk: &RuntimeTurnStreamAudioChunk,
+    ) -> Option<RuntimeTurnStreamTimingMarker> {
+        if self.timing.phase != RuntimeTurnStreamTimingPhase::Armed {
+            return None;
+        }
+        let anchor = self.timing.anchor.as_ref()?;
+        if anchor.call_id != call_id
+            || anchor.trace_id != trace_id
+            || anchor.turn_id != turn_id
+            || event.call_id.as_deref() != Some(call_id)
+            || event.trace_id.as_deref() != Some(trace_id)
+            || event.turn_id.as_deref() != Some(turn_id)
+            || audio_chunk.segment_seq != Some(anchor.segment_seq)
+            || audio_chunk.chunk_seq != Some(STREAM_TIMING_FIRST_CHUNK)
+            || audio_chunk.stream_timing_version != Some(STREAM_TIMING_VERSION)
+            || audio_chunk.stream_anchor_id.as_deref() != Some(anchor.anchor_id.as_str())
+        {
+            return None;
+        }
+        let Some(marker) = RuntimeTurnStreamTimingMarker::from_anchor(
+            anchor,
+            audio_chunk.chunk_seq.unwrap_or(STREAM_TIMING_FIRST_CHUNK),
+        ) else {
+            self.close_timing();
+            return None;
+        };
+        self.timing.chunk_seq = Some(marker.chunk_seq);
+        self.timing.phase = RuntimeTurnStreamTimingPhase::M6Recorded;
+        Some(marker)
+    }
+
+    fn record_m7(&mut self) -> Option<RuntimeTurnStreamTimingMarker> {
+        if self.timing.phase != RuntimeTurnStreamTimingPhase::M6Recorded {
+            return None;
+        }
+        let anchor = self.timing.anchor.as_ref()?;
+        let Some(marker) = RuntimeTurnStreamTimingMarker::from_anchor(
+            anchor,
+            self.timing.chunk_seq.unwrap_or(STREAM_TIMING_FIRST_CHUNK),
+        ) else {
+            self.close_timing();
+            return None;
+        };
+        self.timing.phase = RuntimeTurnStreamTimingPhase::M7Recorded;
+        Some(marker)
+    }
+
+    fn close_timing(&mut self) {
+        self.timing.close();
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum RuntimeTurnStreamTimingPhase {
+    Empty,
+    Armed,
+    M6Recorded,
+    M7Recorded,
+    Closed,
+}
+
+struct RuntimeTurnStreamTimingState {
+    phase: RuntimeTurnStreamTimingPhase,
+    anchor: Option<RuntimeTurnStreamTimingAnchor>,
+    chunk_seq: Option<u64>,
+}
+
+impl Default for RuntimeTurnStreamTimingState {
+    fn default() -> Self {
+        Self {
+            phase: RuntimeTurnStreamTimingPhase::Empty,
+            anchor: None,
+            chunk_seq: None,
+        }
+    }
+}
+
+impl RuntimeTurnStreamTimingState {
+    fn close(&mut self) {
+        self.anchor = None;
+        self.chunk_seq = None;
+        self.phase = RuntimeTurnStreamTimingPhase::Closed;
+    }
+}
+
+impl Drop for RuntimeTurnStreamTimingState {
+    fn drop(&mut self) {
+        self.anchor = None;
+        self.chunk_seq = None;
+    }
+}
+
+struct RuntimeTurnStreamTimingAnchor {
+    call_id: String,
+    trace_id: String,
+    turn_id: String,
+    anchor_id: String,
+    server_delta_ms: u64,
+    runtime_session_nonce_hash: String,
+    segment_seq: u64,
+    received_at: Instant,
+}
+
+struct RuntimeTurnStreamTimingMarker {
+    version: u32,
+    anchor_id: String,
+    anchor_server_delta_ms: u64,
+    anchor_elapsed_ms: u64,
+    server_delta_ms: u64,
+    runtime_session_nonce_hash: String,
+    segment_seq: u64,
+    chunk_seq: u64,
+}
+
+impl RuntimeTurnStreamTimingMarker {
+    fn from_anchor(anchor: &RuntimeTurnStreamTimingAnchor, chunk_seq: u64) -> Option<Self> {
+        let elapsed_ms = u64::try_from(anchor.received_at.elapsed().as_millis()).ok()?;
+        if elapsed_ms > STREAM_TIMING_MAX_ELAPSED_MS {
+            return None;
+        }
+        Some(Self {
+            version: STREAM_TIMING_VERSION,
+            anchor_id: anchor.anchor_id.clone(),
+            anchor_server_delta_ms: anchor.server_delta_ms,
+            anchor_elapsed_ms: elapsed_ms,
+            server_delta_ms: anchor.server_delta_ms.checked_add(elapsed_ms)?,
+            runtime_session_nonce_hash: anchor.runtime_session_nonce_hash.clone(),
+            segment_seq: anchor.segment_seq,
+            chunk_seq,
+        })
+    }
+
+    fn extension_with(&self, extra: serde_json::Value) -> serde_json::Value {
+        let mut extension = match extra {
+            serde_json::Value::Object(value) => value,
+            _ => serde_json::Map::new(),
+        };
+        extension.insert("streamTimingVersion".to_string(), json!(self.version));
+        extension.insert("streamAnchorId".to_string(), json!(self.anchor_id));
+        extension.insert(
+            "streamAnchorServerDeltaMs".to_string(),
+            json!(self.anchor_server_delta_ms),
+        );
+        extension.insert("anchorElapsedMs".to_string(), json!(self.anchor_elapsed_ms));
+        extension.insert(
+            "runtimeSessionNonceHash".to_string(),
+            json!(self.runtime_session_nonce_hash),
+        );
+        extension.insert("segmentSeq".to_string(), json!(self.segment_seq));
+        extension.insert("chunkSeq".to_string(), json!(self.chunk_seq));
+        extension.insert("streamTimingValidation".to_string(), json!("bound"));
+        serde_json::Value::Object(extension)
+    }
+}
+
+fn runtime_session_nonce_hash(value: &str) -> String {
+    let digest = Sha256::digest(value.as_bytes());
+    digest[..6]
+        .iter()
+        .map(|byte| format!("{byte:02x}"))
+        .collect()
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -2412,6 +2716,12 @@
 struct RuntimeTurnStreamEvent {
     #[serde(rename = "type", alias = "event")]
     event_type: Option<String>,
+    #[serde(rename = "callId")]
+    call_id: Option<String>,
+    #[serde(rename = "traceId")]
+    trace_id: Option<String>,
+    #[serde(rename = "turnId")]
+    turn_id: Option<String>,
     #[serde(rename = "seq")]
     seq: Option<u64>,
     #[serde(rename = "replyPlaybackMode")]
@@ -2449,6 +2759,10 @@
     chunk_seq: Option<u64>,
     #[serde(rename = "segmentSeq")]
     segment_seq: Option<u64>,
+    #[serde(rename = "streamTimingVersion")]
+    stream_timing_version: Option<u32>,
+    #[serde(rename = "streamAnchorId")]
+    stream_anchor_id: Option<String>,
     format: Option<String>,
     #[serde(rename = "sampleRate")]
     sample_rate: Option<u32>,
@@ -2465,6 +2779,23 @@
     stage: Option<String>,
     #[serde(rename = "reasonCode")]
     reason_code: Option<String>,
+    extension: Option<RuntimeTurnStreamTimingExtension>,
+}
+
+#[derive(Clone, Deserialize)]
+struct RuntimeTurnStreamTimingExtension {
+    #[serde(rename = "streamTimingVersion")]
+    stream_timing_version: Option<u32>,
+    #[serde(rename = "streamAnchorId")]
+    stream_anchor_id: Option<String>,
+    #[serde(rename = "streamAnchorServerDeltaMs")]
+    stream_anchor_server_delta_ms: Option<u64>,
+    #[serde(rename = "runtimeSessionNonceHash")]
+    runtime_session_nonce_hash: Option<String>,
+    #[serde(rename = "segmentSeq")]
+    segment_seq: Option<u64>,
+    #[serde(rename = "streamTimingValidation")]
+    stream_timing_validation: Option<String>,
 }
 
 #[derive(Deserialize)]
@@ -3562,7 +3893,10 @@
 
 #[cfg(test)]
 mod tests {
-    use super::{ReplyChunkMarker, ReplyChunkMarkerState, RuntimeTurnStreamEvent};
+    use super::{
+        ReplyChunkMarker, ReplyChunkMarkerState, RuntimeTurnStreamEvent, RuntimeTurnStreamState,
+        RuntimeTurnStreamTimingPhase, runtime_session_nonce_hash,
+    };
 
     #[test]
     fn reply_chunk_marker_state_emits_turn_first_once_and_later_segment_first_once() {
@@ -3595,4 +3929,68 @@
             event.audio_chunk.and_then(|chunk| chunk.segment_seq)
         );
     }
+
+    #[test]
+    fn runtime_turn_stream_reads_frozen_m5_and_audio_chunk_timing_contract() {
+        let activity: RuntimeTurnStreamEvent = serde_json::from_str(
+            r#"{"type":"activity","callId":"call-1","traceId":"trace-1","turnId":"turn-1","activity":{"eventType":"tts_first_audio_chunk_ready","extension":{"streamTimingVersion":1,"streamAnchorId":"0123456789abcdef","streamAnchorServerDeltaMs":1200,"runtimeSessionNonceHash":"abcdef012345","segmentSeq":1,"streamTimingValidation":"bound"}}}"#,
+        )
+        .expect("m5 activity event");
+        let audio: RuntimeTurnStreamEvent = serde_json::from_str(
+            r#"{"type":"reply_audio_chunk","callId":"call-1","traceId":"trace-1","turnId":"turn-1","audioChunk":{"chunkSeq":1,"segmentSeq":1,"streamTimingVersion":1,"streamAnchorId":"0123456789abcdef","format":"pcm_s16le","payloadBase64":"AA==","last":false}}"#,
+        )
+        .expect("timed audio chunk event");
+
+        assert_eq!(Some("call-1"), activity.call_id.as_deref());
+        assert_eq!(Some("trace-1"), activity.trace_id.as_deref());
+        assert_eq!(Some("turn-1"), activity.turn_id.as_deref());
+        let extension = activity.activity.unwrap().extension.unwrap();
+        assert_eq!(Some(1), extension.stream_timing_version);
+        assert_eq!(Some(1200), extension.stream_anchor_server_delta_ms);
+        assert_eq!(
+            Some("0123456789abcdef"),
+            audio.audio_chunk.unwrap().stream_anchor_id.as_deref()
+        );
+    }
+
+    #[test]
+    fn runtime_turn_stream_timing_records_m6_and_m7_once_then_rejects_terminal_late_events() {
+        let nonce = "runtime-nonce";
+        let nonce_hash = runtime_session_nonce_hash(nonce);
+        let activity: RuntimeTurnStreamEvent = serde_json::from_str(&format!(
+            r#"{{"type":"activity","callId":"call-1","traceId":"trace-1","turnId":"turn-1","activity":{{"eventType":"tts_first_audio_chunk_ready","extension":{{"streamTimingVersion":1,"streamAnchorId":"0123456789abcdef","streamAnchorServerDeltaMs":1200,"runtimeSessionNonceHash":"{nonce_hash}","segmentSeq":1,"streamTimingValidation":"bound"}}}}}}"#,
+        ))
+        .expect("m5 activity event");
+        let audio: RuntimeTurnStreamEvent = serde_json::from_str(
+            r#"{"type":"reply_audio_chunk","callId":"call-1","traceId":"trace-1","turnId":"turn-1","audioChunk":{"chunkSeq":1,"segmentSeq":1,"streamTimingVersion":1,"streamAnchorId":"0123456789abcdef","format":"pcm_s16le","payloadBase64":"AA==","last":false}}"#,
+        )
+        .expect("timed audio chunk event");
+        let mut state = RuntimeTurnStreamState::default();
+
+        assert!(state.arm_timing_anchor("call-1", "trace-1", "turn-1", nonce, &activity));
+        let chunk = audio.audio_chunk.as_ref().expect("audio chunk");
+        assert!(
+            state
+                .record_m6("call-1", "trace-1", "turn-1", &audio, chunk)
+                .is_some()
+        );
+        assert!(
+            state
+                .record_m6("call-1", "trace-1", "turn-1", &audio, chunk)
+                .is_none()
+        );
+        assert!(state.record_m7().is_some());
+        assert!(state.record_m7().is_none());
+
+        state.close_timing();
+        assert_eq!(RuntimeTurnStreamTimingPhase::Closed, state.timing.phase);
+        assert!(state.timing.anchor.is_none());
+        assert!(!state.arm_timing_anchor("call-1", "trace-1", "turn-1", nonce, &activity));
+        assert!(
+            state
+                .record_m6("call-1", "trace-1", "turn-1", &audio, chunk)
+                .is_none()
+        );
+        assert!(state.record_m7().is_none());
+    }
 }
diff --git a/src/service.rs b/src/service.rs
index bb64683..1e3e0c2 100644
--- a/src/service.rs
+++ b/src/service.rs
@@ -4,7 +4,7 @@
     io::{BufRead, BufReader, Read, Write},
     net::{TcpListener, TcpStream},
     process::{Child, Command, Stdio},
-    sync::{Arc, Mutex},
+    sync::{Arc, Mutex, mpsc},
     thread,
     time::{Duration, SystemTime, UNIX_EPOCH},
 };
@@ -17,6 +17,8 @@
 const DEFAULT_BIND_ADDR: &str = "127.0.0.1:18080";
 const HTTP_READ_LIMIT_BYTES: usize = 1024 * 1024;
 const DEFAULT_START_READY_TIMEOUT_MS: u64 = 10_000;
+const ANCHORED_CALLBACK_MAX_ATTEMPTS: u32 = 3;
+const ANCHORED_CALLBACK_RETRY_DELAY: Duration = Duration::from_millis(50);
 
 pub fn service_mode_enabled() -> bool {
     env::args().any(|arg| arg == "service" || arg == "--service")
@@ -192,7 +194,7 @@
             trace_id,
         );
     }
-    let child = match spawn_worker(&start_req, &profile, &state.config) {
+    let child = match spawn_worker(&start_req, &profile, &state.config, trace_id.as_deref()) {
         Ok(value) => value,
         Err(spawn_error) => {
             warn!(
@@ -214,17 +216,27 @@
     let runtime_session_id = format!("rt_{}", start_req.call_id);
     let mut child = child;
     let child_stdout = child.stdout.take();
+    let event_callback_url = start_req
+        .event_callback
+        .as_ref()
+        .and_then(|value| value.url.clone())
+        .filter(|value| !value.trim().is_empty());
+    let event_callback_token = Some(profile.event_callback_token.clone());
+    let event_callback_sender = if event_callback_url.is_some() {
+        let (sender, receiver) = mpsc::channel();
+        spawn_event_callback_worker(receiver);
+        Some(sender)
+    } else {
+        None
+    };
     let session = HelperSession {
         call_id: start_req.call_id.clone(),
         trace_id: trace_id.clone(),
         runtime_session_nonce: start_req.runtime_session_nonce.clone(),
         runtime_session_id,
-        event_callback_url: start_req
-            .event_callback
-            .as_ref()
-            .and_then(|value| value.url.clone())
-            .filter(|value| !value.trim().is_empty()),
-        event_callback_token: Some(profile.event_callback_token.clone()),
+        event_callback_url,
+        event_callback_token,
+        event_callback_sender,
         status: "STARTING".to_string(),
         bot_participant_joined: false,
         bot_track_ready: false,
@@ -340,6 +352,7 @@
     req: &SessionStartRequest,
     profile: &AuthProfile,
     config: &ServiceConfig,
+    trace_id: Option<&str>,
 ) -> Result<Child> {
     let exe = env::current_exe().context("failed to resolve helper executable")?;
     let mut command = Command::new(exe);
@@ -348,15 +361,15 @@
         .env("CV_CALL_ID", &req.call_id)
         .env(
             "CV_TRACE_ID",
-            req.trace_id
-                .clone()
+            trace_id
+                .map(str::to_string)
                 .unwrap_or_else(|| format!("trace_{}", req.call_id)),
         )
         .env("CV_RUNTIME_CALL_ID", &req.call_id)
         .env(
             "CV_RUNTIME_TRACE_ID",
-            req.trace_id
-                .clone()
+            trace_id
+                .map(str::to_string)
                 .unwrap_or_else(|| format!("trace_{}", req.call_id)),
         )
         .env("CV_RUNTIME_SESSION_NONCE", &req.runtime_session_nonce)
@@ -482,16 +495,35 @@
         .and_then(Value::as_str)
         .unwrap_or("worker_event");
     let result = value.get("result").and_then(Value::as_str).unwrap_or("ok");
-    let callback = {
+    let queued_callback = {
         let Ok(mut sessions) = state.sessions.lock() else {
             return;
         };
         let Some(session) = sessions.get_mut(call_id) else {
             return;
         };
+        let turn_id = value.get("turnId").and_then(Value::as_str);
+        let terminal = matches!(
+            event_name,
+            "turn_completed" | "turn_failed" | "turn_cancelled"
+        );
+        if event_name == "turn_bridge_requested" {
+            session.active_turn_id = turn_id.map(str::to_string);
+        }
         session.last_event_type = Some(event_name.to_string());
         session.last_event_at = now_millis();
-        let callback = build_event_callback_dispatch(session, value);
+        let callback = if is_anchored_callback(value) && !anchored_callback_bound(session, value) {
+            warn!(
+                call_id = %call_id,
+                trace_id = ?session.trace_id,
+                event_name = %event_name,
+                "helper rejected unbound anchored callback"
+            );
+            None
+        } else {
+            build_event_callback_dispatch(session, value)
+        };
+        let callback_sender = session.event_callback_sender.clone();
         if result != "ok" {
             session.status = "FAILED".to_string();
             session.bot_participant_joined = false;
@@ -512,10 +544,19 @@
                 _ => {}
             }
         }
-        callback
+        if terminal && turn_id == session.active_turn_id.as_deref() {
+            session.active_turn_id = None;
+        }
+        callback.zip(callback_sender)
     };
-    if let Some(callback) = callback {
-        dispatch_event_callback(callback);
+    if let Some((callback, sender)) = queued_callback {
+        if sender.send(callback).is_err() {
+            warn!(
+                call_id = %call_id,
+                event_name = %event_name,
+                "helper event callback queue closed"
+            );
+        }
     }
 }
 
@@ -543,23 +584,55 @@
     })
 }
 
-fn dispatch_event_callback(callback: EventCallbackDispatch) {
+fn spawn_event_callback_worker(receiver: mpsc::Receiver<EventCallbackDispatch>) {
     thread::spawn(move || {
-        let event_name = callback
-            .payload
-            .get("eventName")
-            .and_then(Value::as_str)
-            .unwrap_or("worker_event")
-            .to_string();
-        match post_event_callback(&callback) {
+        run_event_callback_worker(receiver, deliver_event_callback);
+    });
+}
+
+fn run_event_callback_worker<F>(receiver: mpsc::Receiver<EventCallbackDispatch>, mut deliver: F)
+where
+    F: FnMut(EventCallbackDispatch),
+{
+    while let Ok(callback) = receiver.recv() {
+        deliver(callback);
+    }
+}
+
+fn deliver_event_callback(callback: EventCallbackDispatch) {
+    deliver_event_callback_with(callback, post_event_callback);
+}
+
+fn deliver_event_callback_with<F>(callback: EventCallbackDispatch, mut post: F)
+where
+    F: FnMut(&EventCallbackDispatch) -> Result<u16>,
+{
+    let event_name = callback
+        .payload
+        .get("eventName")
+        .and_then(Value::as_str)
+        .unwrap_or("worker_event")
+        .to_string();
+    let max_attempts = if is_anchored_callback(&callback.payload) {
+        ANCHORED_CALLBACK_MAX_ATTEMPTS
+    } else {
+        1
+    };
+    for attempt in 1..=max_attempts {
+        let result = post(&callback);
+        let retry =
+            attempt < max_attempts && !matches!(&result, Ok(status) if (200..300).contains(status));
+        match result {
             Ok(status) if (200..300).contains(&status) => {
                 info!(
                     call_id = %callback.call_id,
                     trace_id = ?callback.trace_id,
                     event_name = %event_name,
                     status = status,
+                    attempt = attempt,
                     "helper event callback delivered"
                 );
+                return;
             }
             Ok(status) => {
                 warn!(
@@ -567,6 +640,7 @@
                     trace_id = ?callback.trace_id,
                     event_name = %event_name,
                     status = status,
+                    attempt = attempt,
                     "helper event callback rejected"
                 );
             }
@@ -575,12 +649,62 @@
                     call_id = %callback.call_id,
                     trace_id = ?callback.trace_id,
                     event_name = %event_name,
+                    attempt = attempt,
                     error = %error,
                     "helper event callback failed"
                 );
             }
         }
-    });
+        if retry {
+            thread::sleep(ANCHORED_CALLBACK_RETRY_DELAY);
+        }
+    }
+}
+
+fn is_anchored_callback(payload: &Value) -> bool {
+    matches!(
+        payload.get("eventName").and_then(Value::as_str),
+        Some("helper_first_reply_audio_chunk_received" | "bot_reply_first_audio_frame_written")
+    ) && payload.get("serverDeltaSource").and_then(Value::as_str) == Some("stream_anchor_monotonic")
+}
+
+fn anchored_callback_bound(session: &HelperSession, payload: &Value) -> bool {
+    let extension = payload.get("extension");
+    let anchor_id = extension
+        .and_then(|value| value.get("streamAnchorId"))
+        .and_then(Value::as_str);
+    let anchor_base = extension
+        .and_then(|value| value.get("streamAnchorServerDeltaMs"))
+        .and_then(Value::as_u64);
+    let elapsed = extension
+        .and_then(|value| value.get("anchorElapsedMs"))
+        .and_then(Value::as_u64);
+    let delta = payload.get("serverDeltaMs").and_then(Value::as_u64);
+    let expected_delta =
+        anchor_base.and_then(|base| elapsed.and_then(|value| base.checked_add(value)));
+    let expected_nonce_hash = crate::runtime_session_nonce_hash(&session.runtime_session_nonce);
+    payload.get("callId").and_then(Value::as_str) == Some(session.call_id.as_str())
+        && payload.get("traceId").and_then(Value::as_str) == session.trace_id.as_deref()
+        && payload.get("turnId").and_then(Value::as_str) == session.active_turn_id.as_deref()
+        && extension
+            .and_then(|value| value.get("streamTimingVersion"))
+            .and_then(Value::as_u64)
+            == Some(1)
+        && anchor_id.is_some_and(|value| (16..=64).contains(&value.len()) && value.is_ascii())
+        && extension
+            .and_then(|value| value.get("runtimeSessionNonceHash"))
+            .and_then(Value::as_str)
+            == Some(expected_nonce_hash.as_str())
+        && extension
+            .and_then(|value| value.get("segmentSeq"))
+            .and_then(Value::as_u64)
+            == Some(1)
+        && extension
+            .and_then(|value| value.get("streamTimingValidation"))
+            .and_then(Value::as_str)
+            == Some("bound")
+        && elapsed.is_some_and(|value| value <= 5_000)
+        && delta == expected_delta
 }
 
 fn post_event_callback(callback: &EventCallbackDispatch) -> Result<u16> {
@@ -913,6 +1037,7 @@
     runtime_session_id: String,
     event_callback_url: Option<String>,
     event_callback_token: Option<String>,
+    event_callback_sender: Option<mpsc::Sender<EventCallbackDispatch>>,
     status: String,
     bot_participant_joined: bool,
     bot_track_ready: bool,
@@ -1115,3 +1240,65 @@
         Err(_) => default_value,
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{EventCallbackDispatch, deliver_event_callback_with, run_event_callback_worker};
+    use anyhow::Result;
+    use serde_json::{Value, json};
+    use std::{cell::RefCell, rc::Rc, sync::mpsc};
+
+    #[test]
+    fn same_session_callback_retries_m7_before_terminal() {
+        let (sender, receiver) = mpsc::channel();
+        let dispatch = |payload| EventCallbackDispatch {
+            url: "http://callback.test/endpoint".to_string(),
+            token: "test-token".to_string(),
+            call_id: "call-1".to_string(),
+            trace_id: Some("trace-1".to_string()),
+            runtime_session_nonce: "test-nonce".to_string(),
+            payload,
+        };
+        sender
+            .send(dispatch(json!({
+                "eventName": "bot_reply_first_audio_frame_written",
+                "serverDeltaSource": "stream_anchor_monotonic"
+            })))
+            .expect("queue m7");
+        sender
+            .send(dispatch(json!({"eventName": "turn_completed"})))
+            .expect("queue terminal");
+        drop(sender);
+
+        let observed = Rc::new(RefCell::new(Vec::new()));
+        let observed_for_worker = observed.clone();
+        run_event_callback_worker(receiver, move |callback| {
+            let observed_for_post = observed_for_worker.clone();
+            let mut m7_attempt = 0;
+            deliver_event_callback_with(callback, move |dispatch| -> Result<u16> {
+                let event_name = dispatch
+                    .payload
+                    .get("eventName")
+                    .and_then(Value::as_str)
+                    .expect("event name")
+                    .to_string();
+                observed_for_post.borrow_mut().push(event_name.clone());
+                if event_name == "bot_reply_first_audio_frame_written" && m7_attempt == 0 {
+                    m7_attempt += 1;
+                    Ok(500)
+                } else {
+                    Ok(200)
+                }
+            });
+        });
+
+        assert_eq!(
+            vec![
+                "bot_reply_first_audio_frame_written",
+                "bot_reply_first_audio_frame_written",
+                "turn_completed"
+            ],
+            *observed.borrow()
+        );
+    }
+}

--
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