fix(agent_loader): support flat-module agents in _determine_agent_language

src/google/adk/cli/cli_tools_click.py

@@ -1632,8 +1632,9 @@ def cli_web(
 ):
   """Starts a FastAPI server with Web UI for agents.
 
-  AGENTS_DIR: The directory of agents, where each subdirectory is a single
-  agent, containing at least `__init__.py` and `agent.py` files.
+  AGENTS_DIR: The directory of agents, where each entry is a single agent —
+  either a subdirectory (containing `agent.py`, `__init__.py`, or
+  `root_agent.yaml`) or a flat `<agent_name>.py` module file.
 
   Example:
 
@@ -1745,8 +1746,9 @@ def cli_api_server(
 ):
   """Starts a FastAPI server for agents.
 
-  AGENTS_DIR: The directory of agents, where each subdirectory is a single
-  agent, containing at least `__init__.py` and `agent.py` files.
+  AGENTS_DIR: The directory of agents, where each entry is a single agent —
+  either a subdirectory (containing `agent.py`, `__init__.py`, or
+  `root_agent.yaml`) or a flat `<agent_name>.py` module file.
 
   Example:
 

src/google/adk/cli/utils/agent_loader.py

@@ -420,6 +420,8 @@ def _determine_agent_language(
       return "python"
     elif (base_path / "__init__.py").exists():
       return "python"
+    elif (base_path.parent / f"{agent_name}.py").exists():
+      return "python"
 
     raise ValueError(f"Could not determine agent type for '{agent_name}'.")
 

tests/unittests/cli/utils/test_agent_loader.py

@@ -1006,3 +1006,56 @@ def test_validate_agent_name_rejects_nonexistent_agent(self):
       # 'subprocess' is a valid identifier but shouldn't be importable as an agent
       with pytest.raises(ValueError, match="Agent not found"):
         loader.load_agent("subprocess")
+
+
+class TestDetermineAgentLanguage:
+  """Tests for AgentLoader._determine_agent_language covering all 4 load patterns."""
+
+  def test_flat_module_returns_python(self):
+    """Flat-module agent (agents_dir/agent_name.py) is detected as python."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+      (temp_path / "my_agent.py").write_text("root_agent = None\n")
+      loader = AgentLoader(temp_dir)
+      assert loader._determine_agent_language("my_agent") == "python"
+
+  def test_agent_py_subdirectory_returns_python(self):
+    """Subdirectory with agent.py is detected as python."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+      agent_dir = temp_path / "my_agent"
+      agent_dir.mkdir()
+      (agent_dir / "agent.py").write_text("root_agent = None\n")
+      loader = AgentLoader(temp_dir)
+      assert loader._determine_agent_language("my_agent") == "python"
+
+  def test_init_py_subdirectory_returns_python(self):
+    """Subdirectory with __init__.py is detected as python."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+      agent_dir = temp_path / "my_agent"
+      agent_dir.mkdir()
+      (agent_dir / "__init__.py").write_text("root_agent = None\n")
+      loader = AgentLoader(temp_dir)
+      assert loader._determine_agent_language("my_agent") == "python"
+
+  def test_root_agent_yaml_returns_yaml(self):
+    """Subdirectory with root_agent.yaml is detected as yaml."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+      agent_dir = temp_path / "my_agent"
+      agent_dir.mkdir()
+      (agent_dir / "root_agent.yaml").write_text("root_agent: {}\n")
+      loader = AgentLoader(temp_dir)
+      assert loader._determine_agent_language("my_agent") == "yaml"
+
+  def test_unrecognized_structure_raises_value_error(self):
+    """A directory with no recognized structure raises ValueError."""
+    with tempfile.TemporaryDirectory() as temp_dir:
+      temp_path = Path(temp_dir)
+      agent_dir = temp_path / "my_agent"
+      agent_dir.mkdir()
+      (agent_dir / "main.py").write_text("root_agent = None\n")
+      loader = AgentLoader(temp_dir)
+      with pytest.raises(ValueError, match="Could not determine agent type"):
+        loader._determine_agent_language("my_agent")