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
 # ---------------------------------------------------------------------------