diff --git a/src/types.hpp b/src/types.hpp
index 3227a2b..58904d7 100644
--- a/src/types.hpp
+++ b/src/types.hpp
@@ -184,3 +184,111 @@ struct ActorGallery {
     bool     calib_valid{false};
     uint64_t calib_hash{0};
 };
+
+// ── Channel byte accounting ───────────────────────────────────────────────────
+/// TRACES: AR-004 | SR-002
+///
+/// KPN measures a channel's occupancy in *items* and its bandwidth in bytes,
+/// and gets the byte figure from `kpn::ChannelDataSize<T>`. That primary
+/// template returns `sizeof(T)` — right for a POD, badly wrong for every type
+/// below, each of which is a handful of vectors and a `cv::Mat` header owning
+/// megabytes on the heap.
+///
+/// Unspecialised, the diagnostics reported roughly 200 bytes for a message
+/// carrying a full decoded frame — off by four orders of magnitude at 1080p.
+/// That is not merely a cosmetic stat: it is the one instrument for choosing
+/// channel capacities against a memory ceiling, which is the open half of
+/// AR-004, and it was reading fiction.
+///
+/// **What the number means.** `cv::Mat` is reference-counted, so one decoded
+/// frame referenced from several messages is counted once per reference. The
+/// sum is therefore an upper bound on distinct bytes, and the right bound for
+/// the question being asked: how much would this channel keep alive if nothing
+/// else held it.
+///
+/// Declared against a forward declaration rather than including
+/// `<kpn/channel.hpp>` here, so the message definitions keep no dependency on
+/// the framework that carries them — and so any translation unit that can see
+/// these types also sees their sizes, which is what stops one channel being
+/// instantiated with the default and another with the specialisation.
+
+namespace kpn { template<typename T> struct ChannelDataSize; }
+
+namespace sae::bytes {
+
+inline std::size_t of(const cv::Mat& m) {
+    return m.empty() ? 0u : m.total() * m.elemSize();
+}
+inline std::size_t of(const std::vector<cv::Mat>& v) {
+    std::size_t n = 0;
+    for (const auto& m : v) n += of(m);
+    return n;
+}
+inline std::size_t of(const Frame& f) { return sizeof(Frame) + of(f.image); }
+
+inline std::size_t of(const std::vector<IdentifiedActor>& v) {
+    std::size_t n = v.size() * sizeof(IdentifiedActor);
+    for (const auto& a : v) {
+        n += of(a.crop);
+        // The id strings are short but there is one set per actor per frame,
+        // and a crowd frame carries dozens.
+        n += a.name.capacity() + a.imdb_id.capacity()
+           + a.tmdb_id.capacity() + a.jellyfin_id.capacity();
+    }
+    return n;
+}
+
+}  // namespace sae::bytes
+
+template<> struct kpn::ChannelDataSize<Frame> {
+    static std::size_t bytes(const Frame& f) { return sae::bytes::of(f); }
+};
+
+template<> struct kpn::ChannelDataSize<SceneFrame> {
+    static std::size_t bytes(const SceneFrame& v) {
+        return sizeof(SceneFrame) + sae::bytes::of(v.source)
+             + v.faces.size() * sizeof(DetectedFace);
+    }
+};
+
+template<> struct kpn::ChannelDataSize<AlignedSceneFrame> {
+    static std::size_t bytes(const AlignedSceneFrame& v) {
+        return sizeof(AlignedSceneFrame) + sae::bytes::of(v.source)
+             + v.faces.size() * sizeof(DetectedFace)
+             + sae::bytes::of(v.crops);
+    }
+};
+
+template<> struct kpn::ChannelDataSize<EmbeddedSceneFrame> {
+    static std::size_t bytes(const EmbeddedSceneFrame& v) {
+        return sizeof(EmbeddedSceneFrame) + sae::bytes::of(v.source)
+             + v.faces.size() * sizeof(DetectedFace)
+             + sae::bytes::of(v.crops)
+             + v.embeddings.size() * sizeof(Embedding);
+    }
+};
+
+template<> struct kpn::ChannelDataSize<TrackedSceneFrame> {
+    static std::size_t bytes(const TrackedSceneFrame& v) {
+        return sizeof(TrackedSceneFrame) + sae::bytes::of(v.source)
+             + v.faces.size() * sizeof(DetectedFace)
+             + sae::bytes::of(v.crops)
+             + v.track_ids.size() * sizeof(int)
+             + v.embeddings.size() * sizeof(Embedding);
+    }
+};
+
+template<> struct kpn::ChannelDataSize<MatchedSceneFrame> {
+    static std::size_t bytes(const MatchedSceneFrame& v) {
+        return sizeof(MatchedSceneFrame) + sae::bytes::of(v.source)
+             + sae::bytes::of(v.actors);
+    }
+};
+
+template<> struct kpn::ChannelDataSize<SceneAnnotation> {
+    static std::size_t bytes(const SceneAnnotation& v) {
+        return sizeof(SceneAnnotation) + sae::bytes::of(v.visible_actors);
+    }
+};
+
+// CutEvent owns nothing on the heap, so the default sizeof(T) is already right.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index d6e628c..ece8a25 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -28,6 +28,7 @@ add_executable(sae_tests
     test_embedding_dump.cpp
     test_audio_signature.cpp
     test_benchmark.cpp
+    test_channel_bytes.cpp
     ${CMAKE_SOURCE_DIR}/src/backends/gemm_backend.cpp
     ${CMAKE_SOURCE_DIR}/src/gallery/gallery_store.cpp
     ${CMAKE_SOURCE_DIR}/src/audio_signature.cpp
diff --git a/tests/test_channel_bytes.cpp b/tests/test_channel_bytes.cpp
new file mode 100644
index 0000000..c22d256
--- /dev/null
+++ b/tests/test_channel_bytes.cpp
@@ -0,0 +1,109 @@
+// Channel byte accounting for the pipeline message types.
+//
+// TRACES: AR-004 | SR-002
+//
+// kpn::ChannelDataSize<T> is what a channel reports as bytes pushed, and its
+// primary template returns sizeof(T). Every message type here is a handful of
+// vectors and a cv::Mat header owning megabytes on the heap, so unspecialised
+// the diagnostics reported ~200 bytes for a message carrying a full decoded
+// frame — off by four orders of magnitude at 1080p.
+//
+// That is the instrument for choosing channel capacities against a memory
+// ceiling, which is the open half of AR-004. These cases assert it measures the
+// payload rather than the header, because a stat that is quietly wrong is worse
+// than no stat: it was read as evidence.
+#include <catch2/catch_test_macros.hpp>
+
+#include <kpn/channel.hpp>
+
+#include "types.hpp"
+
+namespace {
+
+Frame frame_with_image(int w, int h) {
+    Frame f;
+    f.image = cv::Mat(h, w, CV_8UC3, cv::Scalar(0, 0, 0));
+    f.timestamp_sec = 1.0;
+    return f;
+}
+
+}  // namespace
+
+TEST_CASE("frame bytes count the decoded image, not the header", "[channel_bytes]") {
+    const Frame f = frame_with_image(1920, 1080);
+    const std::size_t got = kpn::ChannelDataSize<Frame>::bytes(f);
+
+    // 1920 * 1080 * 3 = 6,220,800 payload bytes.
+    REQUIRE(got >= 1920u * 1080u * 3u);
+    // The header is a rounding error next to it; this is the assertion that
+    // fails on the unspecialised default.
+    CHECK(got > 100u * sizeof(Frame));
+}
+
+TEST_CASE("an empty frame costs only its header", "[channel_bytes]") {
+    // The eof sentinel carries no image, and must not be charged for one.
+    Frame eof;
+    eof.eof = true;
+    CHECK(kpn::ChannelDataSize<Frame>::bytes(eof) == sizeof(Frame));
+}
+
+TEST_CASE("crops and embeddings are counted on top of the frame", "[channel_bytes]") {
+    // The case AR-003 created: a crowd frame occupies one slot exactly as an
+    // empty one does, and only the byte figure distinguishes them.
+    EmbeddedSceneFrame v;
+    v.source = frame_with_image(640, 360);
+    const std::size_t bare = kpn::ChannelDataSize<EmbeddedSceneFrame>::bytes(v);
+
+    constexpr int kFaces = 60;
+    for (int i = 0; i < kFaces; ++i) {
+        v.faces.push_back({});
+        v.crops.emplace_back(112, 112, CV_8UC3, cv::Scalar(0, 0, 0));
+        v.embeddings.emplace_back();
+    }
+    const std::size_t crowded = kpn::ChannelDataSize<EmbeddedSceneFrame>::bytes(v);
+
+    // 60 crops at 112*112*3 = 2,257,920 bytes, plus 60 * 2 KiB of embeddings.
+    CHECK(crowded - bare >= kFaces * (112u * 112u * 3u + sizeof(Embedding)));
+    // And the crowd frame really is the multiple of the empty one that the
+    // item-count capacity cannot see: 640x360x3 is ~691 KB, the crops ~2.26 MB.
+    CHECK(crowded > 3 * bare);
+}
+
+TEST_CASE("every message type on a channel measures its payload", "[channel_bytes]") {
+    // A specialisation missing for any one of these silently reverts that
+    // channel to sizeof(T), which is exactly how this went unnoticed.
+    const Frame f = frame_with_image(320, 240);
+    const std::size_t img = 320u * 240u * 3u;
+
+    SceneFrame sf;        sf.source = f;
+    AlignedSceneFrame af; af.source = f;
+    EmbeddedSceneFrame ef; ef.source = f;
+    TrackedSceneFrame tf; tf.source = f;
+    MatchedSceneFrame mf; mf.source = f;
+
+    CHECK(kpn::ChannelDataSize<SceneFrame>::bytes(sf)         >= img);
+    CHECK(kpn::ChannelDataSize<AlignedSceneFrame>::bytes(af)  >= img);
+    CHECK(kpn::ChannelDataSize<EmbeddedSceneFrame>::bytes(ef) >= img);
+    CHECK(kpn::ChannelDataSize<TrackedSceneFrame>::bytes(tf)  >= img);
+    CHECK(kpn::ChannelDataSize<MatchedSceneFrame>::bytes(mf)  >= img);
+
+    // SceneAnnotation carries no source frame — only the actors it identified,
+    // each with its own crop.
+    SceneAnnotation sa;
+    sa.visible_actors.push_back({});
+    sa.visible_actors.back().crop = cv::Mat(112, 112, CV_8UC3, cv::Scalar(0, 0, 0));
+    CHECK(kpn::ChannelDataSize<SceneAnnotation>::bytes(sa) >= 112u * 112u * 3u);
+}
+
+TEST_CASE("a shared image is charged to each message holding it", "[channel_bytes]") {
+    // cv::Mat is reference-counted, so a frame referenced from several messages
+    // is counted once per reference. The sum is an upper bound on distinct
+    // bytes, and the right bound for "what would this channel keep alive if
+    // nothing else held it" — which is the question a capacity answers.
+    const Frame f = frame_with_image(320, 240);
+    SceneFrame a; a.source = f;
+    SceneFrame b; b.source = f;   // shares the same pixel buffer
+
+    CHECK(kpn::ChannelDataSize<SceneFrame>::bytes(a)
+          == kpn::ChannelDataSize<SceneFrame>::bytes(b));
+}