feat: make LlmAgent.model optional with a default fallback

LlmAgent now resolves model from ancestors or a system default (gemini-2.5-flash) when unset. Added LlmAgent.set_default_model() to override the default globally

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 853006116

tests/unittests/agents/test_llm_agent_fields.py

@@ -52,11 +52,24 @@ async def _create_readonly_context(
   return ReadonlyContext(invocation_context)
 
 
-def test_canonical_model_empty():
-  agent = LlmAgent(name='test_agent')
-
-  with pytest.raises(ValueError):
-    _ = agent.canonical_model
+@pytest.mark.parametrize(
+    ('default_model', 'expected_model_name', 'expected_model_type'),
+    [
+        (LlmAgent.DEFAULT_MODEL, LlmAgent.DEFAULT_MODEL, Gemini),
+        ('gemini-2.0-flash', 'gemini-2.0-flash', Gemini),
+    ],
+)
+def test_canonical_model_default_fallback(
+    default_model, expected_model_name, expected_model_type
+):
+  original_default = LlmAgent._default_model
+  LlmAgent.set_default_model(default_model)
+  try:
+    agent = LlmAgent(name='test_agent')
+    assert isinstance(agent.canonical_model, expected_model_type)
+    assert agent.canonical_model.model == expected_model_name
+  finally:
+    LlmAgent.set_default_model(original_default)
 
 
 def test_canonical_model_str():

tests/unittests/flows/llm_flows/test_basic_processor.py

@@ -110,7 +110,9 @@ class TestBasicLlmRequestProcessor:
     assert llm_request.config.response_mime_type != 'application/json'
 
     # Should have checked if output schema can be used with tools
-    can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+    can_use_output_schema_with_tools.assert_called_once_with(
+        agent.canonical_model
+    )
 
   @pytest.mark.asyncio
   async def test_sets_output_schema_when_tools_present(self, mocker):
@@ -141,7 +143,9 @@ class TestBasicLlmRequestProcessor:
     assert llm_request.config.response_mime_type == 'application/json'
 
     # Should have checked if output schema can be used with tools
-    can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+    can_use_output_schema_with_tools.assert_called_once_with(
+        agent.canonical_model
+    )
 
   @pytest.mark.asyncio
   async def test_no_output_schema_no_tools(self):

tests/unittests/flows/llm_flows/test_output_schema_processor.py

@@ -191,7 +191,9 @@ async def test_output_schema_request_processor(
     assert not llm_request.config.system_instruction
 
   # Should have checked if output schema can be used with tools
-  can_use_output_schema_with_tools.assert_called_once_with(agent.model)
+  can_use_output_schema_with_tools.assert_called_once_with(
+      agent.canonical_model
+  )
 
 
 @pytest.mark.asyncio