diff --git a/api/services/docling/template.py b/api/services/docling/template.py
index b2b3929..51307ef 100644
--- a/api/services/docling/template.py
+++ b/api/services/docling/template.py
@@ -44,6 +44,34 @@ def _furn_kind(it):
     return "chrome_text"
 
 
+def synthesize_part_box(part_band, content_x_band):
+    """Return the one authoritative S5 part-box projection.
+
+    Parts remain boxes in S5, but the box is a projection rather than intrinsic
+    geometry: document content margins provide the x-extent and the part band
+    provides y. The band end is already bounded by the next part in bands.py;
+    the original label box remains a separate anchor for rendering/review.
+
+    Coordinates stay in the first-pass PDF-point BOTTOMLEFT bbox shape.
+    """
+    if not content_x_band:
+        return None
+    try:
+        x_left = content_x_band["x_left"]
+        x_right = content_x_band["x_right"]
+        y_start = part_band["y_start"]
+        y_end = part_band["y_end"]
+    except KeyError:
+        return None
+    return {
+        "l": round(x_left, 1),
+        "t": round(y_start, 1),
+        "r": round(x_right, 1),
+        "b": round(y_end, 1),
+        "coord_origin": "BOTTOMLEFT",
+    }
+
+
 def build(structured, bands, furniture, pdf=None, page_roles=None):
     page_roles = page_roles or {}
     part_bbox = {p["label"]: p.get("bbox")
@@ -124,10 +152,15 @@ def build(structured, bands, furniture, pdf=None, page_roles=None):
         main = [{"question": m["question"], "y_start": m["y_start"], "y_end": m["y_end"],
                  "is_start": m.get("is_start", True),
                  "source": "auto", "confirmed": False} for m in pb["main"]]
-        part = [{"label": p["label"], "question": p["question"],
-                 "y_start": p["y_start"], "y_end": p["y_end"],
-                 "label_box": part_bbox.get(p["label"]),     # app may render a box instead of lines
-                 "source": "auto", "confirmed": False} for p in pb["part"]]
+        part = []
+        for p in pb["part"]:
+            part.append({
+                "label": p["label"], "question": p["question"],
+                "y_start": p["y_start"], "y_end": p["y_end"],
+                "label_box": part_bbox.get(p["label"]),  # anchor, not the part extent
+                "box": synthesize_part_box(p, xband),
+                "source": "auto", "confirmed": False,
+            })
         pr = page_roles.get(pgs) or page_roles.get(pg) or {}
         pages[pgs] = {
             "role": pr.get("role", "question"),
diff --git a/tests/test_docling_auto_map.py b/tests/test_docling_auto_map.py
index a36e641..f4d7d8d 100644
--- a/tests/test_docling_auto_map.py
+++ b/tests/test_docling_auto_map.py
@@ -26,7 +26,9 @@ def test_auto_map_matches_spike_physics_template_shape():
     assert len(result["margins"]) == len(expected["margins"])
     assert set(result["pages"].keys()) == set(expected["pages"].keys())
     assert result["pages"]["2"]["role"] == expected["pages"]["2"]["role"]
-    assert result["pages"]["2"]["part_bands"][0].keys() == expected["pages"]["2"]["part_bands"][0].keys()
+    part_band = result["pages"]["2"]["part_bands"][0]
+    assert set(expected["pages"]["2"]["part_bands"][0].keys()).issubset(part_band.keys())
+    assert part_band["box"]
 
 
 @pytest.mark.skipif(not BORN_DIGITAL_PDF.exists(), reason="born-digital spike PDF not present")
diff --git a/tests/test_docling_template.py b/tests/test_docling_template.py
new file mode 100644
index 0000000..652f6f9
--- /dev/null
+++ b/tests/test_docling_template.py
@@ -0,0 +1,55 @@
+from api.services.docling.template import build, synthesize_part_box
+
+
+def test_synthesize_part_box_uses_content_margins_and_band_y():
+    part_band = {"label": "01.1", "y_start": 712.34, "y_end": 601.27}
+    content_x_band = {"x_left": 54.04, "x_right": 521.96}
+
+    assert synthesize_part_box(part_band, content_x_band) == {
+        "l": 54.0,
+        "t": 712.3,
+        "r": 522.0,
+        "b": 601.3,
+        "coord_origin": "BOTTOMLEFT",
+    }
+
+
+def test_synthesize_part_box_returns_none_without_margin_contract():
+    assert synthesize_part_box({"y_start": 700, "y_end": 650}, {}) is None
+    assert synthesize_part_box({"y_start": 700}, {"x_left": 50, "x_right": 520}) is None
+
+
+def test_build_carries_label_box_as_anchor_and_single_synthesized_part_box():
+    structured = {
+        "board": "aqa",
+        "paper_code": "8463/1",
+        "questions": [{
+            "question": "01",
+            "parts": [{
+                "label": "01.1",
+                "page": 2,
+                "bbox": {"l": 40, "t": 720, "r": 70, "b": 705},
+            }],
+        }],
+    }
+    bands = {
+        "pages": {
+            "2": {
+                "main": [{"question": "01", "y_start": 730, "y_end": 0, "is_start": True}],
+                "part": [{"label": "01.1", "question": "01", "y_start": 720, "y_end": 610}],
+            }
+        }
+    }
+    furniture = {
+        "n_pages": 2,
+        "content_margins": {
+            "content_x_band": {"x_left": 55, "x_right": 515},
+            "per_page": {"2": {"top": 760, "bottom": 40, "left": 55, "right": 515}},
+        },
+        "items": [],
+    }
+
+    part = build(structured, bands, furniture)["pages"]["2"]["part_bands"][0]
+
+    assert part["label_box"] == {"l": 40, "t": 720, "r": 70, "b": 705}
+    assert part["box"] == {"l": 55, "t": 720, "r": 515, "b": 610, "coord_origin": "BOTTOMLEFT"}