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

Merge #5235

`_determine_agent_language()` handled only 3 of the 4 agent structures that
`load_agent()` supports. When an agent is a flat module
(`agents_dir/my_agent.py`, with no subdirectory), `load_agent()` loads it
successfully but `_determine_agent_language()` raised `ValueError`.

This adds the missing flat-module branch so language detection is consistent
with all four supported load patterns:
- `{name}/agent.py`
- `{name}/__init__.py`
- `{name}/root_agent.yaml`
- `{name}.py` (flat module)

It also adds `__init__.py` to the recognized agent marker files listed in the
`adk web` / `adk api_server` AGENTS_DIR docstrings, and adds a
`TestDetermineAgentLanguage` suite covering all four patterns plus the
unrecognized-structure error case.

Co-authored-by: Haran Rajkumar <haranrk@google.com>
PiperOrigin-RevId: 933831825

Author: 2 people

src/google/adk/cli/cli_tools_click.py

@@ -1806,8 +1806,8 @@ 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 `agent.py` or `root_agent.yaml` files) or a path pointing
-  directly to a single agent folder.
+  agent containing `agent.py`, `__init__.py`, or `root_agent.yaml`) or a path
+  pointing directly to a single agent folder.
 
   Example:
 
@@ -1947,8 +1947,8 @@ def cli_api_server(
   """Starts a FastAPI server for agents.
 
   AGENTS_DIR: The directory of agents (where each subdirectory is a single
-  agent containing `agent.py` or `root_agent.yaml` files) or a path pointing
-  directly to a single agent folder.
+  agent containing `agent.py`, `__init__.py`, or `root_agent.yaml`) or a path
+  pointing directly to a single agent folder.
 
   Example:
 

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

@@ -471,6 +471,8 @@ 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

@@ -1039,3 +1039,56 @@ 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")