diff --git a/tests/agent/test_anthropic_adapter.py b/tests/agent/test_anthropic_adapter.py
index 0ba2ba29f51..259e9c1c523 100644
--- a/tests/agent/test_anthropic_adapter.py
+++ b/tests/agent/test_anthropic_adapter.py
@@ -157,6 +157,13 @@ class TestBuildAnthropicClient:
 
 
 class TestReadClaudeCodeCredentials:
+    @pytest.fixture(autouse=True)
+    def no_keychain(self, monkeypatch):
+        monkeypatch.setattr(
+            "agent.anthropic_adapter._read_claude_code_credentials_from_keychain",
+            lambda: None,
+        )
+
     def test_reads_valid_credentials(self, tmp_path, monkeypatch):
         cred_file = tmp_path / ".claude" / ".credentials.json"
         cred_file.parent.mkdir(parents=True)