diff --git a/tradein-mvp/backend/app/services/matching/houses.py b/tradein-mvp/backend/app/services/matching/houses.py
index 87d78c84..64a1fa67 100644
--- a/tradein-mvp/backend/app/services/matching/houses.py
+++ b/tradein-mvp/backend/app/services/matching/houses.py
@@ -34,11 +34,11 @@ def match_or_create_house(
 ) -> tuple[int, float, str]:
     """Match existing house or create new canonical record.
 
-    ⚠️ KNOWN LIMITATION: Concurrent calls for the same address can race —
-    two scrapers calling simultaneously for an unknown address can both
-    miss Tier 0-3 and each INSERT a new house row. Mitigation (Stage 8.x):
-    gate scraper concurrency to 1 per fingerprint OR add
-    pg_advisory_xact_lock(hashtext(normalized_address)) at function entry.
+    Concurrency-safe: serializes concurrent calls for the same address fingerprint
+    via pg_advisory_xact_lock(42, hashtext(fp)). The lock is transaction-scoped,
+    released on the caller's COMMIT/ROLLBACK. Without this, two scrapers calling
+    for an unknown address could both miss Tier 0-3 and each INSERT a duplicate
+    house row. Closes finding #1 from 2026-05-24 audit.
 
     Returns:
         (house_id, confidence ∈ [0.0, 1.0], method ∈ {
@@ -54,6 +54,18 @@ def match_or_create_house(
     """
     cad = building_cadastral_number or cadastral_number
 
+    # Compute fingerprint early so we can acquire the advisory lock before any tier reads.
+    fp = address_fingerprint(address, lat, lon)
+
+    # Serialize concurrent calls for the same address fingerprint to prevent
+    # racing INSERTs into houses (finding #1 from 2026-05-24 audit).
+    # pg_advisory_xact_lock takes (int4, int4) — namespace 42 = "tradein house matching".
+    # Lock is released automatically on caller's COMMIT/ROLLBACK.
+    db.execute(
+        text('SELECT pg_advisory_xact_lock(42, hashtext(:fp))'),
+        {'fp': fp},
+    )
+
     # Tier 0: cadastral number exact match
     if cad:
         row = db.execute(
@@ -83,9 +95,6 @@ def match_or_create_house(
                     ext_id)
         return (house_id, 1.0, 'source_exact')
 
-    # Compute fingerprint once for Tier 2 and reuse for new house registration
-    fp = address_fingerprint(address, lat, lon)
-
     # Tier 2: address fingerprint lookup
     row = db.execute(
         text(
diff --git a/tradein-mvp/backend/tests/test_matching.py b/tradein-mvp/backend/tests/test_matching.py
index 1f9c7b0c..51c38bd9 100644
--- a/tradein-mvp/backend/tests/test_matching.py
+++ b/tradein-mvp/backend/tests/test_matching.py
@@ -148,6 +148,7 @@ def test_match_house_tier0_cadastr():
     from app.services.matching.houses import match_or_create_house
 
     db = _make_db([
+        None,         # pg_advisory_xact_lock
         {'id': 42},   # cadastral match
         None,         # _upsert_house_source (INSERT ... ON CONFLICT)
     ])
@@ -171,7 +172,8 @@ def test_match_house_tier1_source_exact():
     from app.services.matching.houses import match_or_create_house
 
     db = _make_db([
-        {'house_id': 7},  # house_sources hit (first and only execute call)
+        None,             # pg_advisory_xact_lock
+        {'house_id': 7},  # house_sources hit
     ])
     house_id, conf, method = match_or_create_house(
         db, 'avito', 'ext-999',
@@ -186,6 +188,7 @@ def test_match_house_tier2_fingerprint():
     from app.services.matching.houses import match_or_create_house
 
     db = _make_db([
+        None,              # pg_advisory_xact_lock
         None,              # house_sources miss
         {'house_id': 15},  # fingerprint hit
         None,              # _upsert_house_source
@@ -204,6 +207,7 @@ def test_match_house_tier3_geo():
     from app.services.matching.houses import match_or_create_house
 
     db = _make_db([
+        None,              # pg_advisory_xact_lock
         None,              # house_sources miss
         None,              # fingerprint miss
         {'id': 22, 'dist': 15.5},  # geo hit
@@ -224,6 +228,7 @@ def test_match_house_new():
     from app.services.matching.houses import match_or_create_house
 
     db = _make_db([
+        None,          # pg_advisory_xact_lock
         None,          # house_sources miss
         None,          # fingerprint miss
         None,          # geo miss
@@ -240,6 +245,35 @@ def test_match_house_new():
     assert method == 'new'
 
 
+def test_match_house_advisory_lock_called_first():
+    """Regression: pg_advisory_xact_lock must be the first execute() call
+    on match_or_create_house. Re-ordering would re-introduce the race
+    (finding #1 from 2026-05-24 audit). PR #501."""
+    from app.services.matching.houses import match_or_create_house
+
+    db = _make_db([
+        None,           # pg_advisory_xact_lock
+        {'house_id': 7},  # house_sources Tier 1 hit (short path)
+    ])
+    match_or_create_house(
+        db, 'avito', 'regress-001',
+        address='Тестовая 1', lat=56.8, lon=60.5,
+    )
+
+    # First execute MUST be the lock acquisition, with bind {'fp': <hex>}.
+    first_call = db.execute.call_args_list[0]
+    sql_obj = first_call[0][0]   # TextClause
+    bind = first_call[0][1]      # dict
+
+    assert 'pg_advisory_xact_lock' in str(sql_obj), (
+        f'first execute must be advisory lock, got: {sql_obj}'
+    )
+    assert 'fp' in bind, f"lock bind must include fp, got: {bind}"
+    assert isinstance(bind['fp'], str) and len(bind['fp']) == 32, (
+        f"fp must be 32-char sha256 hex, got: {bind.get('fp')!r}"
+    )
+
+
 # ---------------------------------------------------------------------------
 # match_or_create_listing — mock DB tier routing
 # ---------------------------------------------------------------------------