diff --git a/klausur-service/backend/grid_editor_api.py b/klausur-service/backend/grid_editor_api.py
index 384d56c..c7f25f3 100644
--- a/klausur-service/backend/grid_editor_api.py
+++ b/klausur-service/backend/grid_editor_api.py
@@ -23,7 +23,7 @@ from fastapi import APIRouter, HTTPException, Request
 from cv_box_detect import detect_boxes, split_page_into_zones
 from cv_vocab_types import PageZone
 from cv_color_detect import detect_word_colors, recover_colored_text
-from cv_ocr_engines import fix_cell_phonetics, fix_ipa_continuation_cell, _text_has_garbled_ipa, _lookup_ipa, _words_to_reading_order_text
+from cv_ocr_engines import fix_cell_phonetics, fix_ipa_continuation_cell, _text_has_garbled_ipa, _lookup_ipa, _words_to_reading_order_text, _group_words_into_lines
 from cv_words_first import _cluster_rows, _build_cells
 from ocr_pipeline_session_store import (
     get_session_db,
@@ -2317,6 +2317,24 @@ async def _build_grid_core(session_id: str, session: dict) -> dict:
                           if (c.get("word_boxes") or c.get("text", "").strip())]
         logger.info("Step 5i: removed %d bullet/artifact word_boxes", bullet_removed)
 
+    # 5j. Normalise word_box order to reading order (group by Y, sort by X).
+    # The frontend renders colored cells from word_boxes array order
+    # (GridTable.tsx), so they MUST be in left-to-right reading order.
+    wb_reordered = 0
+    for z in zones_data:
+        for cell in z.get("cells", []):
+            wbs = cell.get("word_boxes") or []
+            if len(wbs) < 2:
+                continue
+            lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+            sorted_wbs = [w for line in lines for w in line]
+            # Check if order actually changed
+            if [id(w) for w in sorted_wbs] != [id(w) for w in wbs]:
+                cell["word_boxes"] = sorted_wbs
+                wb_reordered += 1
+    if wb_reordered:
+        logger.info("Step 5j: re-ordered word_boxes in %d cells to reading order", wb_reordered)
+
     duration = time.time() - t0
 
     # 6. Build result
diff --git a/klausur-service/backend/tests/test_grid_editor_api.py b/klausur-service/backend/tests/test_grid_editor_api.py
index ff03ab5..f32a8d2 100644
--- a/klausur-service/backend/tests/test_grid_editor_api.py
+++ b/klausur-service/backend/tests/test_grid_editor_api.py
@@ -1046,3 +1046,50 @@ class TestBlueBulletFilter:
         # so it doesn't matter which one is removed — the text stays correct.
         # The key thing is ONE of the duplicates is removed.
         assert True  # Removing either duplicate is correct
+
+
+# ---------------------------------------------------------------------------
+# Word_box reading order normalisation (Step 5j)
+# ---------------------------------------------------------------------------
+
+class TestWordBoxReadingOrder:
+    """Verify word_boxes are sorted into reading order for frontend rendering."""
+
+    def test_single_line_sorted_by_left(self):
+        """Words on same Y line sorted by X (left) position."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "up",        "left": 376, "top": 264, "width": 22, "height": 19},
+            {"text": "tie",       "left": 284, "top": 264, "width": 23, "height": 14},
+            {"text": "sb/sth",    "left": 309, "top": 264, "width": 57, "height": 20},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["tie", "sb/sth", "up"]
+
+    def test_two_lines_preserves_line_order(self):
+        """Words on two Y lines: first line first, then second line."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "b)", "left": 100, "top": 290, "width": 20, "height": 15},
+            {"text": "cat", "left": 50, "top": 264, "width": 30, "height": 15},
+            {"text": "dog", "left": 100, "top": 264, "width": 30, "height": 15},
+            {"text": "a)", "left": 50, "top": 290, "width": 20, "height": 15},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=10)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["cat", "dog", "a)", "b)"]
+
+    def test_already_sorted_unchanged(self):
+        """Already-sorted word_boxes stay in same order."""
+        from cv_ocr_engines import _group_words_into_lines
+        wbs = [
+            {"text": "tie",    "left": 284, "top": 264, "width": 23, "height": 14},
+            {"text": "sb/sth", "left": 309, "top": 264, "width": 57, "height": 20},
+            {"text": "up",     "left": 376, "top": 264, "width": 22, "height": 19},
+        ]
+        lines = _group_words_into_lines(wbs, y_tolerance_px=15)
+        sorted_wbs = [w for line in lines for w in line]
+        assert [w["text"] for w in sorted_wbs] == ["tie", "sb/sth", "up"]
+        # Same objects, same order
+        assert [id(w) for w in sorted_wbs] == [id(w) for w in wbs]