fix: xref member-mark regex covers single-letter marks; recheck sheet diversity after cap

backend/agents/linker.py

@@ -38,7 +38,7 @@ def _xref_keys(assertion: Dict) -> List[str]:
     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, ...
+    if re.match(r"^[A-Z]+\d", tag):  # member marks: W12X26, HSS16X4X5/8, ...
         keys.append(f"tag:{tag}")
     return keys
 
@@ -76,9 +76,12 @@ def build_link_scopes(sheets: List[Dict]) -> List[AgentScope]:
         sheets_present = {a.get("sheet_number") for a in assertions}
         if len(assertions) < 2 or len(sheets_present) < 2:
             continue
+        chunk = assertions[:cap]
+        if len({a.get("sheet_number") for a in chunk}) < 2:
+            continue  # cap landed on a single sheet — xref adds nothing
         scopes.append(AgentScope(
             scope_id=f"xref:{key}",
-            payload={"assertions": assertions[:cap],
+            payload={"assertions": chunk,
                      "level": "xref", "family": key},
         ))
     return scopes

tests/agents/test_linker_xref.py

@@ -45,3 +45,30 @@ def test_xref_scope_joins_shared_member_tag():
     scopes = build_link_scopes(sheets)
     xref = [s for s in scopes if s.scope_id.startswith("xref:")]
     assert xref
+
+
+def test_xref_scope_joins_single_letter_member_mark():
+    # W-shapes (W12X26) are the most common steel marks and have one leading letter
+    sheets = [
+        _sheet("S102", 5, "roof", [_assertion("a1", tag="W12X26")]),
+        _sheet("S401", 30, "unknown", [_assertion("a2", tag="W12X26")]),
+    ]
+    scopes = build_link_scopes(sheets)
+    xref = [s for s in scopes if s.scope_id.startswith("xref:")]
+    assert xref, "single-letter member marks (W12X26) must join xref scopes"
+
+
+def test_xref_scope_rechecks_sheet_diversity_after_cap(monkeypatch):
+    from backend import config
+    monkeypatch.setattr(config, "AGENT_LINK_MAX_ASSERTIONS", 2)
+    sheets = [
+        _sheet("S401", 30, "roof", [_assertion("a1", ref="A/S205"),
+                                    _assertion("a2", ref="A/S205")]),
+        _sheet("S205", 20, "roof", [_assertion("a3", ref="A/S205")]),
+    ]
+    scopes = build_link_scopes(sheets)
+    xref = [s for s in scopes if s.scope_id.startswith("xref:")]
+    for scope in xref:
+        sheets_in_scope = {a["sheet_number"] for a in scope.payload["assertions"]}
+        assert len(sheets_in_scope) >= 2, \
+            "capped xref scope must still span two sheets"