diff --git a/tests/tools/test_credential_pool_env_fallback.py b/tests/tools/test_credential_pool_env_fallback.py
index 938484f0..b157bb87 100644
--- a/tests/tools/test_credential_pool_env_fallback.py
+++ b/tests/tools/test_credential_pool_env_fallback.py
@@ -106,10 +106,20 @@ class TestCredentialPoolSeedsFromDotEnv:
         assert active_sources == set()
         assert entries == []
 
-    def test_os_environ_still_wins_over_dotenv(self, isolated_hermes_home, monkeypatch):
-        """get_env_value checks os.environ first — verify seeding picks that up."""
-        _write_env_file(isolated_hermes_home, DEEPSEEK_API_KEY="sk-dotenv-stale")
-        monkeypatch.setenv("DEEPSEEK_API_KEY", "sk-env-fresh-xyz")
+    def test_dotenv_wins_over_stale_os_environ(self, isolated_hermes_home, monkeypatch):
+        """.env should win over a stale os.environ value.
+
+        Inverted from the pre-#18254 behaviour. Stale env vars inherited
+        from parent shells (Codex CLI, test harnesses) used to shadow
+        deliberate updates to ~/.hermes/.env, causing auth.json to cache
+        an outdated key and silent 401 errors. The invariant now is:
+        when a key appears in both sources, .env wins.
+
+        Sister coverage in tests/agent/test_credential_pool.py exercises
+        the load_pool path; this case exercises _seed_from_env directly.
+        """
+        _write_env_file(isolated_hermes_home, DEEPSEEK_API_KEY="sk-dotenv-fresh")
+        monkeypatch.setenv("DEEPSEEK_API_KEY", "sk-env-stale-xyz")
 
         from agent.credential_pool import _seed_from_env
         entries = []
@@ -118,7 +128,7 @@ class TestCredentialPoolSeedsFromDotEnv:
         assert changed is True
         seeded = [e for e in entries if e.source == "env:DEEPSEEK_API_KEY"]
         assert len(seeded) == 1
-        assert seeded[0].access_token == "sk-env-fresh-xyz"
+        assert seeded[0].access_token == "sk-dotenv-fresh"
 
 
 class TestAuthResolvesFromDotEnv: