diff --git a/app/contracts/frames.py b/app/contracts/frames.py
index 65fe884..b7781ed 100644
--- a/app/contracts/frames.py
+++ b/app/contracts/frames.py
@@ -17,2 +17,6 @@ class FrameTime:
     time_source: str | None = None
+    # Private identity metadata; frame_snapshot does not export these fields to Redis.
+    invalidation_reason: str | None = None
+    invalidated_at_wall_ms: int | None = None
+    invalidated_frames: int = 0
 
diff --git a/app/event_artifact_contract.py b/app/event_artifact_contract.py
index aabdc29..92f510c 100644
--- a/app/event_artifact_contract.py
+++ b/app/event_artifact_contract.py
@@ -96,3 +96,6 @@ def build_body(event, media, error, base, relative):
     if type(wall) is not int or wall <= 0:
-        return None
+        from app.event_time_fallback import fallback_time
+        wall = fallback_time(event)
+        if wall is None:
+            return None
     detail = dict(aiRecordedAtWallMs=event['aiRecordedAtWallMs'], aiRecordedStage=STAGE,
@@ -103,2 +106,4 @@ def build_body(event, media, error, base, relative):
                   artifact=media, artifactError=error)
+    if timing.get('detectedAtWallMs') is None and event.get('eventTimeFallback'):
+        detail['eventTime'] = dict(event['eventTimeFallback'], detectedAtWallMs=wall)
     detail['roiLabel'] = (event.get('scenarioConfig') or {}).get(event['scenario'], {}).get('roi_name', event['roiId'])
diff --git a/app/event_capture.py b/app/event_capture.py
index a18ab97..586cbef 100644
--- a/app/event_capture.py
+++ b/app/event_capture.py
@@ -99,2 +99,3 @@ class EventCapture:
                     queued=self.results.qsize(), native_drop=self.native.lib.contextra_encoded_dropped(),
+                    event_time_fallback=getattr(self, 'time_fallback_diagnostic', None),
                     training=self.training.status() if self.training else None)
@@ -135,2 +136,11 @@ class EventCapture:
                 payload['scenarioConfig'] = snapshot.get('scenarioConfig')
+                from app.event_time_fallback import event_diagnostic, fallback_time
+                diagnostic = event_diagnostic(event.frame, getattr(self, 'provider_key', None))
+                if diagnostic is not None:
+                    payload['eventTimeFallback'] = diagnostic
+                    if fallback_time(payload) is not None:
+                        self.metrics['event_time_fallback_candidates'] += 1
+                        self.time_fallback_diagnostic = diagnostic
+                        if self.metrics['event_time_fallback_candidates'] == 1:
+                            print('[EVENT_TIME] ' + json.dumps(diagnostic, ensure_ascii=True), flush=True)
                 accepted, reason = self.store.accept(payload)
diff --git a/app/frame_time.py b/app/frame_time.py
index fa3deca..6b51d76 100644
--- a/app/frame_time.py
+++ b/app/frame_time.py
@@ -2,2 +2,3 @@
 from dataclasses import replace
+import time
 from pydantic import BaseModel, ConfigDict, Field
@@ -28,3 +29,3 @@ def packet_values(packet):
 class RtpFrameClock:
-    def __init__(self, anchor):
+    def __init__(self, anchor, *, clock_ns=time.time_ns):
         self.anchor = anchor
@@ -32,2 +33,14 @@ class RtpFrameClock:
         self.invalidated = False
+        self.clock_ns = clock_ns
+        self.invalidation_reason = None
+        self.invalidated_at_wall_ms = None
+        self.invalidated_frames = 0
+
+    def _carry_fault(self, value):
+        if not self.invalidated or value is None:
+            return value
+        self.invalidated_frames += 1
+        return replace(value, invalidation_reason=self.invalidation_reason,
+                       invalidated_at_wall_ms=self.invalidated_at_wall_ms,
+                       invalidated_frames=self.invalidated_frames)
 
@@ -35,3 +48,3 @@ class RtpFrameClock:
         if packet is None:
-            return self.anchor
+            return self._carry_fault(self.anchor)
         raw, rate, ssrc = packet_values(packet)
@@ -40,4 +53,10 @@ class RtpFrameClock:
             # Wrap/restart cannot silently reuse the previous cycle's zero Anchor.
-            if raw < old or rate != old_rate or ssrc != old_ssrc:
+            reasons = [reason for changed, reason in (
+                (raw < old, 'RTP_TIMESTAMP_BACKWARD'),
+                (ssrc != old_ssrc, 'SSRC_CHANGED'),
+                (rate != old_rate, 'RTP_CLOCK_RATE_CHANGED')) if changed]
+            if reasons and not self.invalidated:
                 self.invalidated = True
+                self.invalidation_reason = ','.join(reasons)
+                self.invalidated_at_wall_ms = self.clock_ns() // 1_000_000
         self.previous = raw, rate, ssrc
@@ -50,6 +69,6 @@ class RtpFrameClock:
         base = self.anchor or FrameTime()
-        return replace(base, anchor_rtp_timestamp=0 if self.anchor else None,
+        return self._carry_fault(replace(base, anchor_rtp_timestamp=0 if self.anchor else None,
                        source_rtp_timestamp=raw, source_rtp_timestamp_ext=None,
                        rtp_clock_rate=rate, source_time_wall_ms=wall,
-                       sync_status='ESTIMATED' if valid else 'RAW_ONLY')
+                       sync_status='ESTIMATED' if valid else 'RAW_ONLY'))
 
@@ -64,4 +83,8 @@ def validate_carried_time(value, anchor):
     time = FrameTime(**value)
+    from app.event_time_fallback import validate_clock_fault
+    validate_clock_fault(time)
     if time.source_rtp_timestamp is None:
-        if time != anchor:
+        original = replace(time, invalidation_reason=None, invalidated_at_wall_ms=None,
+                           invalidated_frames=0)
+        if original != anchor:
             raise ValueError('Unexpected unobserved Frame time.')