fix: distinguish callable Anthropic bearer sources from OAuth text

agent/client_lifecycle.py

@@ -78,7 +78,10 @@ def _swap_fallback_clients(agent, fb_client, fb_provider: str, fb_model: str, fb
         agent.api_key = agent._anthropic_api_key = effective_key
         agent._anthropic_base_url = fb_base_url
         agent._anthropic_client = build_anthropic_client(effective_key, fb_base_url, timeout=timeout)
-        agent._is_anthropic_oauth = _is_oauth_token(effective_key) if is_anthropic else False
+        agent._is_anthropic_oauth = (
+            _is_oauth_token(effective_key)
+            if is_anthropic and isinstance(effective_key, str) else False
+        )
         agent.client, agent._client_kwargs = None, {}
         return
     agent.api_key = credential

tests/agent/test_fallback_dynamic_credential.py

@@ -33,3 +33,17 @@ def test_fallback_rebuild_preserves_rotating_and_static_credentials(mode, dynami
     finally:
         client.close()
         rebuilt.close()
+
+
+def test_anthropic_fallback_keeps_callable_without_treating_it_as_oauth_text():
+    source = lambda: "fixture-token"  # noqa: E731
+    client = OpenAI(api_key=source, base_url="http://localhost:1234/v1")
+    agent = SimpleNamespace()
+    try:
+        _swap_fallback_clients(agent, client, "anthropic", "fixture", str(client.base_url), "anthropic_messages")
+        assert agent.api_key is source
+        assert agent._anthropic_api_key is source
+        assert agent._is_anthropic_oauth is False
+        agent._anthropic_client.close()
+    finally:
+        client.close()