diff --git a/src/daedalus_decoder.c b/src/daedalus_decoder.c
index 8394df6..90cacad 100644
--- a/src/daedalus_decoder.c
+++ b/src/daedalus_decoder.c
@@ -298,21 +298,27 @@ static int dispatch_deblock_pass(
         uint8_t *, size_t, size_t,
         const daedalus_h264_deblock_meta *);
 
+    /* daedalus-fourier kernel naming convention:
+     *   _v = "v_loop_filter" — filter applied VERTICALLY across a
+     *        HORIZONTAL edge.  Use for our orient=1 (H edge).
+     *   _h = "h_loop_filter" — filter applied HORIZONTALLY across a
+     *        VERTICAL edge.  Use for our orient=0 (V edge).
+     * The names refer to the FILTER DIRECTION, not the edge direction. */
     deblock_dispatch_fn fn;
     if (target_plane == 0) {
-        if (target_orient == 0)
-            fn = target_bS_intra ? daedalus_dispatch_h264_deblock_luma_v_intra
-                                 : daedalus_dispatch_h264_deblock_luma_v;
-        else
+        if (target_orient == 0)  /* V edge → h_loop_filter */
             fn = target_bS_intra ? daedalus_dispatch_h264_deblock_luma_h_intra
                                  : daedalus_dispatch_h264_deblock_luma_h;
+        else                      /* H edge → v_loop_filter */
+            fn = target_bS_intra ? daedalus_dispatch_h264_deblock_luma_v_intra
+                                 : daedalus_dispatch_h264_deblock_luma_v;
     } else {
         if (target_orient == 0)
-            fn = target_bS_intra ? daedalus_dispatch_h264_deblock_chroma_v_intra
-                                 : daedalus_dispatch_h264_deblock_chroma_v;
-        else
             fn = target_bS_intra ? daedalus_dispatch_h264_deblock_chroma_h_intra
                                  : daedalus_dispatch_h264_deblock_chroma_h;
+        else
+            fn = target_bS_intra ? daedalus_dispatch_h264_deblock_chroma_v_intra
+                                 : daedalus_dispatch_h264_deblock_chroma_v;
     }
 
     return fn(dec->dctx, sub, scratch, stride, n, meta_scratch);
diff --git a/tests/test_deblock_smoke.c b/tests/test_deblock_smoke.c
index 16d5524..7a686a8 100644
--- a/tests/test_deblock_smoke.c
+++ b/tests/test_deblock_smoke.c
@@ -105,28 +105,43 @@ static int build_mb_edges(int mb_x, int mb_y, int last_mb_x, int last_mb_y,
              (e == 0) ? 4 : (int)(1 + xs64() % 3),
              /*boundary?*/ (e == 0 && mb_y == 0));
 
-    /* V chroma Cb: 2 edges. */
+    /* V chroma Cb: 2 edges.  Read DEBLOCK_CHROMA_MODE env at runtime:
+     *   unset / "all"  → both edges (current default test).
+     *   "intra_only"   → only bS=4 boundary edge.
+     *   "none"         → all chroma edges bS=0 (luma-only test).
+     * Lets us bisect substrate divergence without rebuilding. */
+    int chroma_intra_only = 0, chroma_none = 0;
+    const char *cm = getenv("DEBLOCK_CHROMA_MODE");
+    if (cm) {
+        if (!strcmp(cm, "intra_only"))   chroma_intra_only = 1;
+        else if (!strcmp(cm, "none"))    chroma_none = 1;
+    }
+
     for (int e = 0; e < 2; e++)
         EDGE(0, /*Cb*/1, e,
              (e == 0) ? 4 : (int)(1 + xs64() % 3),
+             (chroma_none) || (chroma_intra_only && e != 0) ||
              (e == 0 && mb_x == 0));
 
     /* H chroma Cb. */
     for (int e = 0; e < 2; e++)
         EDGE(1, 1, e,
              (e == 0) ? 4 : (int)(1 + xs64() % 3),
+             (chroma_none) || (chroma_intra_only && e != 0) ||
              (e == 0 && mb_y == 0));
 
     /* V chroma Cr. */
     for (int e = 0; e < 2; e++)
         EDGE(0, /*Cr*/2, e,
              (e == 0) ? 4 : (int)(1 + xs64() % 3),
+             (chroma_none) || (chroma_intra_only && e != 0) ||
              (e == 0 && mb_x == 0));
 
     /* H chroma Cr. */
     for (int e = 0; e < 2; e++)
         EDGE(1, 2, e,
              (e == 0) ? 4 : (int)(1 + xs64() % 3),
+             (chroma_none) || (chroma_intra_only && e != 0) ||
              (e == 0 && mb_y == 0));
 
 #undef EDGE