diff --git a/backend/agents/linker.py b/backend/agents/linker.py
index 184c8e0..0eb6635 100644
--- a/backend/agents/linker.py
+++ b/backend/agents/linker.py
@@ -30,9 +30,23 @@ def _family(assertion: Dict) -> str:
     )
 
 
+def _xref_keys(assertion: Dict) -> List[str]:
+    """Cross-level join keys: detail references and member tags."""
+    location = assertion.get("location_key") or {}
+    keys = []
+    ref = re.sub(r"\s+", "", str(location.get("detail_reference") or "")).upper()
+    if ref:
+        keys.append(f"detail:{ref}")
+    tag = re.sub(r"\s+", "", str(location.get("tag") or "")).upper()
+    if re.match(r"^[A-Z]{2,}\d", tag):  # member marks: HSS16X4X5/8, W12X26, ...
+        keys.append(f"tag:{tag}")
+    return keys
+
+
 def build_link_scopes(sheets: List[Dict]) -> List[AgentScope]:
     """Partition facts by level and object/tag family, then enforce a hard cap."""
     buckets: Dict[Tuple[str, str], List[Dict]] = defaultdict(list)
+    xref: Dict[str, List[Dict]] = defaultdict(list)
     for sheet in sheets:
         for assertion in sheet.get("assertions", []):
             enriched = {
@@ -44,6 +58,8 @@ def build_link_scopes(sheets: List[Dict]) -> List[AgentScope]:
             level = str((assertion.get("location_key") or {}).get("level")
                         or sheet.get("level") or "unknown").lower()
             buckets[(level, _family(assertion))].append(enriched)
+            for key in _xref_keys(assertion):
+                xref[key].append(enriched)
 
     scopes: List[AgentScope] = []
     cap = max(2, config.AGENT_LINK_MAX_ASSERTIONS)
@@ -56,6 +72,15 @@ def build_link_scopes(sheets: List[Dict]) -> List[AgentScope]:
                 scope_id=f"{level}:{family}:{offset // cap + 1}",
                 payload={"assertions": chunk, "level": level, "family": family},
             ))
+    for key, assertions in sorted(xref.items()):
+        sheets_present = {a.get("sheet_number") for a in assertions}
+        if len(assertions) < 2 or len(sheets_present) < 2:
+            continue
+        scopes.append(AgentScope(
+            scope_id=f"xref:{key}",
+            payload={"assertions": assertions[:cap],
+                     "level": "xref", "family": key},
+        ))
     return scopes
 
 
diff --git a/tests/agents/test_linker_xref.py b/tests/agents/test_linker_xref.py
new file mode 100644
index 0000000..a236934
--- /dev/null
+++ b/tests/agents/test_linker_xref.py
@@ -0,0 +1,47 @@
+from backend.agents.base import AgentScope
+from backend.agents.linker import build_link_scopes
+
+
+def _sheet(number, page, level, assertions):
+    return {"sheet_number": number, "page_number": page,
+            "discipline": "Structural", "level": level,
+            "assertions": assertions}
+
+
+def _assertion(id_, ref=None, tag=None, level=None):
+    return {"id": id_, "attribute": "stud_pack_size", "value": "(5) 2x6",
+            "source_text": "(5) 2x6 STUD PACK",
+            "location_key": {"detail_reference": ref, "tag": tag,
+                             "level": level}}
+
+
+def test_xref_scope_joins_same_detail_reference_across_levels():
+    sheets = [
+        _sheet("S101", 10, "foundation", [_assertion("a1", ref="A/S205")]),
+        _sheet("S205", 20, "roof", [_assertion("a2", ref="A/S205")]),
+        _sheet("S401", 30, "roof", [_assertion("a3", ref="A/S205")]),
+    ]
+    scopes = build_link_scopes(sheets)
+    xref = [s for s in scopes if s.scope_id.startswith("xref:")]
+    assert xref, "expected a cross-level detail-reference scope"
+    ids = {a["id"] for s in xref for a in s.payload["assertions"]}
+    assert ids == {"a1", "a2", "a3"}
+
+
+def test_xref_scope_requires_two_distinct_sheets():
+    sheets = [
+        _sheet("S401", 30, "roof", [_assertion("a1", ref="A/S205"),
+                                    _assertion("a2", ref="A/S205")]),
+    ]
+    scopes = build_link_scopes(sheets)
+    assert not [s for s in scopes if s.scope_id.startswith("xref:")]
+
+
+def test_xref_scope_joins_shared_member_tag():
+    sheets = [
+        _sheet("S102", 5, "roof", [_assertion("a1", tag="HSS16X4X5/8")]),
+        _sheet("S401", 30, "unknown", [_assertion("a2", tag="HSS16X4X5/8")]),
+    ]
+    scopes = build_link_scopes(sheets)
+    xref = [s for s in scopes if s.scope_id.startswith("xref:")]
+    assert xref