diff --git a/docs/internal/upstream/pr13-shaded-polyline-terminator.md b/docs/internal/upstream/pr13-shaded-polyline-terminator.md
new file mode 100644
index 000000000..54ea49a80
--- /dev/null
+++ b/docs/internal/upstream/pr13-shaded-polyline-terminator.md
@@ -0,0 +1,9 @@
+# PR #13 shaded-polyline terminator provenance
+
+Parked from NyperYuhgard's PSXrecomp PR #13, commit
+[`2ba52e9754d672fb5efdb6f8cba49c82f27a8734`](https://github.com/mstan/psxrecomp/commit/2ba52e9754d672fb5efdb6f8cba49c82f27a8734).
+
+The GP0 shaded-polyline terminator is checked before alternating color/vertex
+decoding, so the hardware terminator is accepted in either stream position. All
+other GPU, DMA, CD-ROM, MDEC, and SPU changes from the mixed upstream commit are
+intentionally excluded from this focused branch.
diff --git a/runtime/src/gpu.c b/runtime/src/gpu.c
index f9bcacaca..88e119ef1 100644
--- a/runtime/src/gpu.c
+++ b/runtime/src/gpu.c
@@ -3584,6 +3584,14 @@ static void gpu_write_gp0_body(uint32_t val) {
 
     /* State: shaded polyline — alternating color, vertex words */
     if (gp0_state == GP0_POLYLINE_SHADED) {
+        /* The terminator can arrive in either the color or vertex position.
+         * Check it before interpreting the alternating shaded-polyline stream;
+         * otherwise a vertex-position terminator is consumed as coordinates and
+         * de-phases all following GP0 commands. */
+        if ((val & 0xF000F000u) == 0x50005000u) {
+            gp0_state = GP0_IDLE;
+            return;
+        }
         /* Even words (after cmd) are colors, odd words are vertices.
          * Sequence: [cmd+C0] [V0] [C1] [V1] [C2] [V2] ...
          * polyline_has_prev tracks: 0=need V0, 1=need C_next, 2=need V_next */
@@ -3598,12 +3606,7 @@ static void gpu_write_gp0_body(uint32_t val) {
             return;
         }
         if (polyline_has_prev == 1) {
-            /* Expecting color word (or terminator). Same hardware rule as the
-             * mono case: ONLY the masked 0x50005000 pattern terminates. */
-            if ((val & 0xF000F000u) == 0x50005000u) {
-                gp0_state = GP0_IDLE;
-                return;
-            }
+            /* Expecting color word. */
             polyline_color = rgb888_to_rgb555(val & 0xFFFFFFu);
             polyline_has_prev = 2;
             return;