fix: allow LlmAgent model to be provided via CodeConfig

LlmAgentConfig.model now accepts either a plain model string or a CodeConfig. This lets YAML configs pass a LiteLLM instance with managed API settings (e.g., api_base and fallbacks) so agents can hit KimiK2’s managed endpoint instead of only the default modelID.

Close #3579

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 838978654

tests/unittests/agents/test_agent_config.py

@@ -29,6 +29,7 @@ from google.adk.agents.llm_agent import LlmAgent
 from google.adk.agents.loop_agent import LoopAgent
 from google.adk.agents.parallel_agent import ParallelAgent
 from google.adk.agents.sequential_agent import SequentialAgent
+from google.adk.models.lite_llm import LiteLlm
 import pytest
 import yaml
 
@@ -259,6 +260,53 @@ sub_agents:
   assert config.root.agent_class == agent_class_value
 
 
+def test_agent_config_litellm_model_with_custom_args(tmp_path: Path):
+  yaml_content = """\
+name: managed_api_agent
+description: Agent using LiteLLM managed endpoint
+instruction: Respond concisely.
+model_code:
+  name: google.adk.models.lite_llm.LiteLlm
+  args:
+    - name: model
+      value: kimi/k2
+    - name: api_base
+      value: https://proxy.litellm.ai/v1
+"""
+  config_file = tmp_path / "litellm_agent.yaml"
+  config_file.write_text(yaml_content)
+
+  agent = config_agent_utils.from_config(str(config_file))
+
+  assert isinstance(agent, LlmAgent)
+  assert isinstance(agent.model, LiteLlm)
+  assert agent.model.model == "kimi/k2"
+  assert agent.model._additional_args.get("api_base") == (
+      "https://proxy.litellm.ai/v1"
+  )
+
+
+def test_agent_config_legacy_model_mapping_still_supported(tmp_path: Path):
+  yaml_content = """\
+name: managed_api_agent
+description: Agent using LiteLLM managed endpoint
+instruction: Respond concisely.
+model:
+  name: google.adk.models.lite_llm.LiteLlm
+  args:
+    - name: model
+      value: kimi/k2
+"""
+  config_file = tmp_path / "legacy_litellm_agent.yaml"
+  config_file.write_text(yaml_content)
+
+  agent = config_agent_utils.from_config(str(config_file))
+
+  assert isinstance(agent, LlmAgent)
+  assert isinstance(agent.model, LiteLlm)
+  assert agent.model.model == "kimi/k2"
+
+
 def test_agent_config_discriminator_custom_agent():
   class MyCustomAgentConfig(BaseAgentConfig):
     agent_class: Literal["mylib.agents.MyCustomAgent"] = (