diff --git a/tests/README.md b/tests/README.md
index 381a95582..4fb909294 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -74,7 +74,14 @@ python3 tests/run_focus.py --area services --fast --durations 25 --durations-min
 
 The `slow` marker is opt-in. Mark a test `slow` only with duration evidence
 (from `--durations`), not by guessing - see the fast-lane policy in
-`TESTING_STANDARD.md`.
+`TESTING_STANDARD.md`. `--fast` is for quick reviewer feedback and must not
+replace the full suite before merge. A `slow` mark only excludes a test from the
+fast lane; the test stays runnable directly, e.g.:
+
+```bash
+python3 -m pytest tests/test_auth_config_lock_concurrency.py
+python3 -m pytest -m slow
+```
 
 ## Core principles
 
diff --git a/tests/test_auth_config_lock_concurrency.py b/tests/test_auth_config_lock_concurrency.py
index 62d75a17a..f5cc8a18c 100644
--- a/tests/test_auth_config_lock_concurrency.py
+++ b/tests/test_auth_config_lock_concurrency.py
@@ -25,6 +25,7 @@ def _fresh_auth_manager(tmp_path):
 class TestConcurrentCreateUser:
     """Concurrent create_user calls must not lose accounts."""
 
+    @pytest.mark.slow
     def test_parallel_creates_no_lost_users(self, tmp_path):
         mgr = _fresh_auth_manager(tmp_path)
         num_users = 50
@@ -63,6 +64,7 @@ class TestConcurrentCreateUser:
 class TestConcurrentDeleteUser:
     """Concurrent deletes must not corrupt state."""
 
+    @pytest.mark.slow
     def test_parallel_deletes_no_corruption(self, tmp_path):
         mgr = _fresh_auth_manager(tmp_path)
         mgr.create_user("admin", "adminpw", is_admin=True)
@@ -90,6 +92,7 @@ class TestConcurrentDeleteUser:
 class TestConcurrentRenameUser:
     """Concurrent renames must not lose or duplicate users."""
 
+    @pytest.mark.slow
     def test_parallel_renames_no_lost_users(self, tmp_path):
         mgr = _fresh_auth_manager(tmp_path)
         mgr.create_user("admin", "adminpw", is_admin=True)
@@ -115,6 +118,7 @@ class TestConcurrentRenameUser:
 class TestConcurrentMixedOperations:
     """Mixed create/delete/rename at the same time."""
 
+    @pytest.mark.slow
     def test_mixed_operations_no_corruption(self, tmp_path):
         mgr = _fresh_auth_manager(tmp_path)
         mgr.create_user("admin", "adminpw", is_admin=True)
@@ -161,6 +165,7 @@ class TestConcurrentMixedOperations:
 class TestDiskConsistency:
     """Verify auth.json is never in a corrupt state during concurrent writes."""
 
+    @pytest.mark.slow
     def test_file_always_valid_json_during_concurrent_ops(self, tmp_path):
         mgr = _fresh_auth_manager(tmp_path)
         mgr.create_user("admin", "adminpw", is_admin=True)
diff --git a/tests/test_run_focus.py b/tests/test_run_focus.py
index a19a9cf5b..696999605 100644
--- a/tests/test_run_focus.py
+++ b/tests/test_run_focus.py
@@ -7,7 +7,9 @@ injected fake executor so no pytest subprocess is ever spawned.
 from __future__ import annotations
 
 import argparse
+import subprocess
 import sys
+from pathlib import Path
 
 import pytest
 
@@ -351,3 +353,47 @@ def test_durations_min_with_durations_is_allowed():
         "--durations=25",
         "--durations-min=0.05",
     ]]
+
+
+# --- fast lane deselects evidence-backed slow tests (real collection) -------
+
+# Node names in tests/test_auth_config_lock_concurrency.py: the single unmarked
+# fast test, and the five @pytest.mark.slow tests the fast lane must exclude.
+_FAST_AUTH_CONCURRENCY_TEST = "test_parallel_creates_same_username_only_one_wins"
+_SLOW_AUTH_CONCURRENCY_TESTS = (
+    "test_parallel_creates_no_lost_users",
+    "test_parallel_deletes_no_corruption",
+    "test_parallel_renames_no_lost_users",
+    "test_mixed_operations_no_corruption",
+    "test_file_always_valid_json_during_concurrent_ops",
+)
+
+
+def test_fast_lane_collects_only_unmarked_auth_concurrency_test():
+    """`--fast` collection drops the marked slow tests but keeps the fast one.
+
+    Unlike the other tests here, this runs a real `--collect-only` so it proves
+    the `slow` markers actually deselect during collection, not just that the
+    command is built with `not slow`.
+    """
+    repo_root = Path(__file__).resolve().parents[1]
+    result = subprocess.run(
+        [
+            sys.executable,
+            "tests/run_focus.py",
+            "--fast",
+            "--",
+            "--collect-only",
+            "-q",
+            "tests/test_auth_config_lock_concurrency.py",
+        ],
+        cwd=repo_root,
+        capture_output=True,
+        text=True,
+    )
+    assert result.returncode == 0, result.stderr or result.stdout
+    collected = result.stdout
+
+    assert _FAST_AUTH_CONCURRENCY_TEST in collected
+    for slow_test in _SLOW_AUTH_CONCURRENCY_TESTS:
+        assert slow_test not in collected, f"slow test was not deselected: {slow_test}"