diff --git a/docs/SPEC.md b/docs/SPEC.md
index 1ab5a20..31b7efd 100644
--- a/docs/SPEC.md
+++ b/docs/SPEC.md
@@ -110,8 +110,19 @@ Two consequences worth stating:
   depends on timing. The same command run twice can produce different dumps, and
   a golden fixture cannot be built on that.
 
-**Gap:** entire requirement, and it is **cross-repo** — the drop-versus-block
-decision lives in KPN. Removing `max_faces` (AR-003) remains gated on it.
+**Current:** fixed in KPN — node data outputs use `push_blocking`, sentinels
+remain out-of-band so EOF can always overtake a stalled data path. Verified on
+the same clip: 385 of 385 sampled frames written, zero drops, and two
+consecutive runs byte-identical where previously they were not.
+
+It also ran *faster* (29 s → 17 s). A dropped frame has already cost its decode,
+and the overflow exception cost more — so the lossy path was paying for work it
+then discarded.
+
+**Gap:** the remaining half — bounding by **bytes in flight** rather than item
+count. Channel capacity is still a count of items, and a face carries a 112×112
+crop plus a 512-float embedding, so a crowded frame occupies far more memory per
+slot than a sparse one. That matters once `max_faces` is removed (AR-003).
 
 ## AR-005 — Face alignment and crop
 
diff --git a/docs/requirements.md b/docs/requirements.md
index cbae3d6..44d3efd 100644
--- a/docs/requirements.md
+++ b/docs/requirements.md
@@ -31,7 +31,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | AR-001 | Detect faces in sampled frames; emit bbox, confidence, 5-point landmarks in original pixel space | SR-002 | High | Done |
 | AR-002 | Minimum face size **32×32 px** (VR-005 measured), expressed in **original** resolution (decoupled from `dense_scale`) | SR-002 | High | Planned |
 | AR-003 | No fixed per-frame face cap — crowd scenes must not lose background cast | SR-002 | Medium | Planned |
-| AR-004 | Backpressure: unbounded faces/frame absorbed by slowing, never by dropping or throwing | SR-002 | High | **Blocked — cross-repo.** Every KPN node output uses the dropping `push()`; `push_blocking()` exists but is never called. Needs a KPN change |
+| AR-004 | Backpressure: unbounded faces/frame absorbed by slowing, never by dropping or throwing | SR-002 | High | **Done** — KPN node outputs use `push_blocking`; sentinels stay out-of-band. Verified: 385/385 frames, 0 drops, byte-identical across runs |
 | AR-005 | Align to 112×112 via ArcFace 5-point similarity transform | SR-002 | High | Done |
 | AR-006 | 512-d L2-normalised embeddings, batched | SR-002 | High | Done |
 | AR-007 | Associate detections by IoU + embedding, with **frame-dependent** weighting | SR-002 | High | **Done** — `track_alpha` is the base for ordinary frames; drops to embedding-only on cut/boundary and for dormant tracks |
diff --git a/external/KPN b/external/KPN
index 4b6e498..be6e922 160000
--- a/external/KPN
+++ b/external/KPN
@@ -1 +1 @@
-Subproject commit 4b6e498ba7e70a34cc0b57638f9e56a43b7f41ae
+Subproject commit be6e92268c2e9c7684d9247de851c02bb62334bd