diff --git a/scripts/corpus_1000/mapper_audit.py b/scripts/corpus_1000/mapper_audit.py
new file mode 100644
index 000000000..6d82c5f1d
--- /dev/null
+++ b/scripts/corpus_1000/mapper_audit.py
@@ -0,0 +1,56 @@
+"""Mapper faithfulness audit: for identity pass-through fields (dims, counts,
+thicknesses) compare the RAW cert value to what EpcPropertyDataMapper produced.
+Any mismatch is a mapper bug silently feeding the SAP engine wrong inputs."""
+from __future__ import annotations
+import json
+from collections import defaultdict
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+CORPUS="backend/epc_api/json_samples/RdSAP-Schema-21.0.1/corpus.jsonl"
+def gv(x): return x.get('value') if isinstance(x,dict) else x
+def num(x):
+    x=gv(x)
+    if x is None: return None
+    s=str(x).lower().replace('mm','').replace('m2','').strip()
+    try: return float(s)
+    except: return s  # keep strings like 'measured'/'NI' for equality
+mism=defaultdict(list)
+n=0
+for line in open(CORPUS):
+    d=json.loads(line)
+    try: epc=EpcPropertyDataMapper.from_api_response(d)
+    except Exception: continue
+    n+=1; u=d.get('uprn')
+    # top-level counts
+    checks=[('door_count', d.get('door_count'), epc.door_count),
+            ('habitable', d.get('habitable_room_count'), getattr(epc,'habitable_room_count',None)),
+            ('led', d.get('led_fixed_lighting_bulbs_count'), epc.led_fixed_lighting_bulbs_count),
+            ('cfl', d.get('cfl_fixed_lighting_bulbs_count'), epc.cfl_fixed_lighting_bulbs_count),
+            ('incand', d.get('incandescent_fixed_lighting_bulbs_count'), epc.incandescent_fixed_lighting_bulbs_count)]
+    for name,rv,ev in checks:
+        if rv is not None and ev is not None and num(rv)!=num(ev):
+            mism[name].append((u,rv,ev))
+    # per-building-part fabric
+    rbps=d.get('sap_building_parts') or []; ebps=epc.sap_building_parts or []
+    for i,rbp in enumerate(rbps):
+        if i>=len(ebps): 
+            mism['DROPPED_building_part'].append((u,len(rbps),len(ebps))); break
+        ebp=ebps[i]
+        for name,rv,ev in [('wall_thick',rbp.get('wall_thickness'),getattr(ebp,'wall_thickness_mm',None)),
+                           ('wall_ins_thick',rbp.get('wall_insulation_thickness'),getattr(ebp,'wall_insulation_thickness',None)),
+                           ('roof_ins_thick',rbp.get('roof_insulation_thickness'),getattr(ebp,'roof_insulation_thickness',None)),
+                           ('wall_con',rbp.get('wall_construction'),getattr(ebp,'wall_construction',None)),
+                           ('wall_ins_type',rbp.get('wall_insulation_type'),getattr(ebp,'wall_insulation_type',None))]:
+            if rv is not None and ev is not None and num(rv)!=num(ev):
+                mism[name].append((u,rv,ev))
+    # windows: count + total area
+    rw=d.get('sap_windows') or []; ew=epc.sap_windows or []
+    if len(rw)!=len(ew): mism['window_count'].append((u,len(rw),len(ew)))
+    ra=sum((gv(w.get('window_width')) or 0)*(gv(w.get('window_height')) or 0) for w in rw)
+    ea=sum((float(w.window_width) if w.window_width else 0)*(float(w.window_height) if w.window_height else 0) for w in ew)
+    if abs(ra-ea)>0.02: mism['window_area'].append((u,round(ra,2),round(ea,2)))
+print(f"audited {n} certs")
+print("=== field mismatches (raw cert vs EpcPropertyData) ===")
+for k,v in sorted(mism.items(), key=lambda x:-len(x[1])):
+    print(f"\n[{len(v):4d}] {k}")
+    for u,rv,ev in v[:5]: print(f"      {u}: raw={rv} epc={ev}")
+if not mism: print("  (no mismatches — mapper is identity-faithful on all checked fields)")