fix: make generate_session_id independent of the global random seed

fenn/reproducibility.py

@@ -105,9 +105,11 @@ def generate_session_id() -> str:
         "grass",
     ]
 
-    # Select words
-    adj = random.choice(adjectives)
-    noun = random.choice(nouns)
+    # Select words with `secrets`, not `random`: a session id must identify a
+    # run, and `set_seed()` reseeds the global `random` state, which would
+    # otherwise make every run under the same seed pick the same adjective/noun.
+    adj = secrets.choice(adjectives)
+    noun = secrets.choice(nouns)
 
     # Add a secure hex suffix (2 bytes = 4 hex chars) to ensure uniqueness
     hex_suffix = secrets.token_hex(2)

tests/unit/test_reproducibility.py

@@ -123,6 +123,21 @@ def test_uniqueness(self):
         # With 2-byte hex suffix, collisions should be extremely rare
         assert len(ids) > 45
 
+    def test_word_choice_is_independent_of_the_global_seed(self):
+        # `set_seed()` reseeds `random`; a session id must still vary run to run.
+        set_seed(1234)
+        first = generate_session_id().rsplit("_", 1)[0]
+        set_seed(1234)
+        second = generate_session_id().rsplit("_", 1)[0]
+
+        seen = set()
+        for _ in range(30):
+            set_seed(1234)
+            seen.add(generate_session_id().rsplit("_", 1)[0])
+        # first/second are the timestamp+words; more than one distinct value
+        # means the words are not pinned by the seed
+        assert len(seen) > 1 or first != second
+
     def test_uses_secrets_for_hex(self):
         with patch(
             "fenn.reproducibility.secrets.token_hex", return_value="abcd"