chore: update default model to gemini-3.5-flash

gemini-3-flash-preview never reached GA; the Gemini 3 Flash line went
stable as gemini-3.5-flash, so move the built-in default off the preview
alias.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 930895010

Author: GWeale

src/google/adk/agents/config_schemas/AgentConfig.json

@@ -3159,7 +3159,7 @@
             }
           ],
           "default": null,
-          "description": "Optional. LlmAgent.model. Provide a model name string (e.g. \"gemini-3-flash-preview\"). If not set, the model will be inherited from the ancestor or fall back to the system default (gemini-3-flash-preview unless overridden via LlmAgent.set_default_model). To construct a model instance from code, use model_code.",
+          "description": "Optional. LlmAgent.model. Provide a model name string (e.g. \"gemini-3.5-flash\"). If not set, the model will be inherited from the ancestor or fall back to the system default (gemini-3.5-flash unless overridden via LlmAgent.set_default_model). To construct a model instance from code, use model_code.",
           "title": "Model"
         },
         "model_code": {

src/google/adk/agents/llm_agent.py

@@ -198,7 +198,7 @@ async def _convert_tool_union_to_tools(
 class LlmAgent(BaseAgent, abc.ABC):
   """LLM-based Agent."""
 
-  DEFAULT_MODEL: ClassVar[str] = 'gemini-3-flash-preview'
+  DEFAULT_MODEL: ClassVar[str] = 'gemini-3.5-flash'
   """System default model used when no model is set on an agent."""
 
   DEFAULT_LIVE_MODEL: ClassVar[str] = 'gemini-live-2.5-flash-native-audio'
@@ -215,7 +215,7 @@ class LlmAgent(BaseAgent, abc.ABC):
 
   When not set, the agent will inherit the model from its ancestor. If no
   ancestor provides a model, the agent uses the default model configured via
-  LlmAgent.set_default_model. The built-in default is gemini-3-flash-preview.
+  LlmAgent.set_default_model. The built-in default is gemini-3.5-flash.
   """
 
   config_type: ClassVar[Type[BaseAgentConfig]] = LlmAgentConfig

src/google/adk/agents/llm_agent_config.py

@@ -55,9 +55,9 @@ class LlmAgentConfig(BaseAgentConfig):
       default=None,
       description=(
           'Optional. LlmAgent.model. Provide a model name string (e.g.'
-          ' "gemini-3-flash-preview"). If not set, the model will be inherited'
+          ' "gemini-3.5-flash"). If not set, the model will be inherited'
           ' from the ancestor or fall back to the system default'
-          ' (gemini-3-flash-preview unless overridden via'
+          ' (gemini-3.5-flash unless overridden via'
           ' LlmAgent.set_default_model). To construct a model instance from'
           ' code, use model_code.'
       ),

src/google/adk/cli/cli_create.py

@@ -128,13 +128,13 @@ def _prompt_for_model() -> str:
   model_choice = click.prompt(
       """\
 Choose a model for the root agent:
-1. gemini-2.5-flash
+1. gemini-3.5-flash
 2. Other models (fill later)
 Choose model""",
       type=click.Choice(["1", "2"]),
   )
   if model_choice == "1":
-    return "gemini-2.5-flash"
+    return "gemini-3.5-flash"
   else:
     click.secho(_OTHER_MODEL_MSG, fg="green")
     return "<FILL_IN_MODEL>"

tests/unittests/cli/utils/test_cli_create.py

@@ -257,7 +257,7 @@ def test_prompt_for_google_api_key(monkeypatch: pytest.MonkeyPatch) -> None:
 def test_prompt_for_model_gemini(monkeypatch: pytest.MonkeyPatch) -> None:
   """Selecting option '1' should return the default Gemini model string."""
   monkeypatch.setattr(click, "prompt", lambda *a, **k: "1")
-  assert cli_create._prompt_for_model() == "gemini-2.5-flash"
+  assert cli_create._prompt_for_model() == "gemini-3.5-flash"
 
 
 def test_prompt_for_model_other(monkeypatch: pytest.MonkeyPatch) -> None: