chore(config): Reimplements AgentConfig with pydantic v2 convention and allow all possible values for agent_class field in all Agent Configs

All below are valid values now.

```
agent_class: LlmAgent
agent_class: google.adk.agents.LlmAgent
agent_class: google.adk.agents.llm_agent.LlmAgent
```
PiperOrigin-RevId: 800228114

src/google/adk/agents/agent_config.py

@@ -14,53 +14,60 @@
 
 from __future__ import annotations
 
+from typing import Annotated
 from typing import Any
+from typing import get_args
 from typing import Union
 
 from pydantic import Discriminator
 from pydantic import RootModel
+from pydantic import Tag
 
 from ..utils.feature_decorator import experimental
-from .base_agent import BaseAgentConfig
+from .base_agent_config import BaseAgentConfig
 from .llm_agent_config import LlmAgentConfig
 from .loop_agent_config import LoopAgentConfig
-from .parallel_agent import ParallelAgentConfig
-from .sequential_agent import SequentialAgentConfig
+from .parallel_agent_config import ParallelAgentConfig
+from .sequential_agent_config import SequentialAgentConfig
 
-# A discriminated union of all possible agent configurations.
-ConfigsUnion = Union[
-    LlmAgentConfig,
-    LoopAgentConfig,
-    ParallelAgentConfig,
-    SequentialAgentConfig,
-    BaseAgentConfig,
-]
+_ADK_AGENT_CLASSES: set[str] = {
+    "LlmAgent",
+    "LoopAgent",
+    "ParallelAgent",
+    "SequentialAgent",
+}
 
 
-def agent_config_discriminator(v: Any):
+def agent_config_discriminator(v: Any) -> str:
+  """Discriminator function that returns the tag name for Pydantic."""
   if isinstance(v, dict):
-    agent_class = v.get("agent_class", "LlmAgent")
-    if agent_class in [
-        "LlmAgent",
-        "LoopAgent",
-        "ParallelAgent",
-        "SequentialAgent",
-    ]:
+    agent_class: str = v.get("agent_class", "LlmAgent")
+
+    # Look up the agent_class in our dynamically built mapping
+    if agent_class in _ADK_AGENT_CLASSES:
       return agent_class
-    else:
-      return "BaseAgent"
+
+    # For non ADK agent classes, use BaseAgent to handle it.
+    return "BaseAgent"
 
   raise ValueError(f"Invalid agent config: {v}")
 
 
+# A discriminated union of all possible agent configurations.
+ConfigsUnion = Annotated[
+    Union[
+        Annotated[LlmAgentConfig, Tag("LlmAgent")],
+        Annotated[LoopAgentConfig, Tag("LoopAgent")],
+        Annotated[ParallelAgentConfig, Tag("ParallelAgent")],
+        Annotated[SequentialAgentConfig, Tag("SequentialAgent")],
+        Annotated[BaseAgentConfig, Tag("BaseAgent")],
+    ],
+    Discriminator(agent_config_discriminator),
+]
+
+
 # Use a RootModel to represent the agent directly at the top level.
 # The `discriminator` is applied to the union within the RootModel.
 @experimental
 class AgentConfig(RootModel[ConfigsUnion]):
   """The config for the YAML schema to create an agent."""
-
-  class Config:
-    # Pydantic v2 requires this for discriminated unions on RootModel
-    # This tells the model to look at the 'agent_class' field of the input
-    # data to decide which model from the `ConfigsUnion` to use.
-    discriminator = Discriminator(agent_config_discriminator)

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

@@ -681,12 +681,29 @@
     "EnterpriseWebSearch": {
       "additionalProperties": false,
       "description": "Tool to search public web data, powered by Vertex AI Search and Sec4 compliance.",
-      "properties": {},
+      "properties": {
+        "excludeDomains": {
+          "anyOf": [
+            {
+              "items": {
+                "type": "string"
+              },
+              "type": "array"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "description": "Optional. List of domains to be excluded from the search results. The default limit is 2000 domains.",
+          "title": "Excludedomains"
+        }
+      },
       "title": "EnterpriseWebSearch",
       "type": "object"
     },
     "Environment": {
-      "description": "Required. The environment being operated.",
+      "description": "The environment being operated.",
       "enum": [
         "ENVIRONMENT_UNSPECIFIED",
         "ENVIRONMENT_BROWSER"
@@ -1476,20 +1493,7 @@
               "$ref": "#/$defs/Content"
             },
             {
-              "items": {
-                "anyOf": [
-                  {
-                    "$ref": "#/$defs/File"
-                  },
-                  {
-                    "$ref": "#/$defs/Part"
-                  },
-                  {
-                    "type": "string"
-                  }
-                ]
-              },
-              "type": "array"
+              "type": "string"
             },
             {
               "$ref": "#/$defs/File"
@@ -1498,7 +1502,20 @@
               "$ref": "#/$defs/Part"
             },
             {
-              "type": "string"
+              "items": {
+                "anyOf": [
+                  {
+                    "type": "string"
+                  },
+                  {
+                    "$ref": "#/$defs/File"
+                  },
+                  {
+                    "$ref": "#/$defs/Part"
+                  }
+                ]
+              },
+              "type": "array"
             },
             {
               "type": "null"
@@ -1830,10 +1847,10 @@
         "speechConfig": {
           "anyOf": [
             {
-              "$ref": "#/$defs/SpeechConfig"
+              "type": "string"
             },
             {
-              "type": "string"
+              "$ref": "#/$defs/SpeechConfig"
             },
             {
               "type": "null"
@@ -1999,6 +2016,22 @@
           ],
           "default": null,
           "description": "Optional. Filter search results to a specific time range.\n      If customers set a start time, they must set an end time (and vice versa).\n      "
+        },
+        "excludeDomains": {
+          "anyOf": [
+            {
+              "items": {
+                "type": "string"
+              },
+              "type": "array"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "description": "Optional. List of domains to be excluded from the search results.\n      The default limit is 2000 domains.",
+          "title": "Excludedomains"
         }
       },
       "title": "GoogleSearch",
@@ -2356,10 +2389,6 @@
         "agent_class": {
           "default": "LlmAgent",
           "description": "The value is used to uniquely identify the LlmAgent class. If it is empty, it is by default an LlmAgent.",
-          "enum": [
-            "LlmAgent",
-            ""
-          ],
           "title": "Agent Class",
           "type": "string"
         },
@@ -2618,7 +2647,6 @@
       "description": "The config for the YAML schema of a LoopAgent.",
       "properties": {
         "agent_class": {
-          "const": "LoopAgent",
           "default": "LoopAgent",
           "description": "The value is used to uniquely identify the LoopAgent class.",
           "title": "Agent Class",
@@ -2774,7 +2802,6 @@
       "description": "The config for the YAML schema of a ParallelAgent.",
       "properties": {
         "agent_class": {
-          "const": "ParallelAgent",
           "default": "ParallelAgent",
           "description": "The value is used to uniquely identify the ParallelAgent class.",
           "title": "Agent Class",
@@ -3680,7 +3707,6 @@
       "description": "The config for the YAML schema of a SequentialAgent.",
       "properties": {
         "agent_class": {
-          "const": "SequentialAgent",
           "default": "SequentialAgent",
           "description": "The value is used to uniquely identify the SequentialAgent class.",
           "title": "Agent Class",
@@ -4413,6 +4439,18 @@
           "default": null,
           "description": "Optional. Tool to support URL context retrieval."
         },
+        "computerUse": {
+          "anyOf": [
+            {
+              "$ref": "#/$defs/ToolComputerUse"
+            },
+            {
+              "type": "null"
+            }
+          ],
+          "default": null,
+          "description": "Optional. Tool to support the model interacting directly with the\n      computer. If enabled, it automatically populates computer-use specific\n      Function Declarations."
+        },
         "codeExecution": {
           "anyOf": [
             {
@@ -4424,18 +4462,6 @@
           ],
           "default": null,
           "description": "Optional. CodeExecution tool type. Enables the model to execute code as part of generation."
-        },
-        "computerUse": {
-          "anyOf": [
-            {
-              "$ref": "#/$defs/ToolComputerUse"
-            },
-            {
-              "type": "null"
-            }
-          ],
-          "default": null,
-          "description": "Optional. Tool to support the model interacting directly with the computer. If enabled, it automatically populates computer-use specific Function Declarations."
         }
       },
       "title": "Tool",
@@ -4556,7 +4582,8 @@
       "type": "object"
     }
   },
-  "anyOf": [
+  "description": "The config for the YAML schema to create an agent.",
+  "oneOf": [
     {
       "$ref": "#/$defs/LlmAgentConfig"
     },
@@ -4573,6 +4600,5 @@
       "$ref": "#/$defs/BaseAgentConfig"
     }
   ],
-  "description": "The config for the YAML schema to create an agent.",
   "title": "AgentConfig"
 }

src/google/adk/agents/llm_agent_config.py

@@ -37,7 +37,7 @@ class LlmAgentConfig(BaseAgentConfig):
       extra='forbid',
   )
 
-  agent_class: Literal['LlmAgent', ''] = Field(
+  agent_class: str = Field(
       default='LlmAgent',
       description=(
           'The value is used to uniquely identify the LlmAgent class. If it is'

src/google/adk/agents/loop_agent_config.py

@@ -16,7 +16,6 @@
 
 from __future__ import annotations
 
-from typing import Literal
 from typing import Optional
 
 from pydantic import ConfigDict
@@ -34,7 +33,7 @@ class LoopAgentConfig(BaseAgentConfig):
       extra='forbid',
   )
 
-  agent_class: Literal['LoopAgent'] = Field(
+  agent_class: str = Field(
       default='LoopAgent',
       description='The value is used to uniquely identify the LoopAgent class.',
   )

src/google/adk/agents/parallel_agent_config.py

@@ -16,8 +16,6 @@
 
 from __future__ import annotations
 
-from typing import Literal
-
 from pydantic import ConfigDict
 from pydantic import Field
 
@@ -30,12 +28,12 @@ class ParallelAgentConfig(BaseAgentConfig):
   """The config for the YAML schema of a ParallelAgent."""
 
   model_config = ConfigDict(
-      extra='forbid',
+      extra="forbid",
   )
 
-  agent_class: Literal['ParallelAgent'] = Field(
-      default='ParallelAgent',
+  agent_class: str = Field(
+      default="ParallelAgent",
       description=(
-          'The value is used to uniquely identify the ParallelAgent class.'
+          "The value is used to uniquely identify the ParallelAgent class."
       ),
   )

src/google/adk/agents/sequential_agent_config.py

@@ -16,8 +16,6 @@
 
 from __future__ import annotations
 
-from typing import Literal
-
 from pydantic import ConfigDict
 from pydantic import Field
 
@@ -30,12 +28,12 @@ class SequentialAgentConfig(BaseAgentConfig):
   """The config for the YAML schema of a SequentialAgent."""
 
   model_config = ConfigDict(
-      extra='forbid',
+      extra="forbid",
   )
 
-  agent_class: Literal['SequentialAgent'] = Field(
-      default='SequentialAgent',
+  agent_class: str = Field(
+      default="SequentialAgent",
       description=(
-          'The value is used to uniquely identify the SequentialAgent class.'
+          "The value is used to uniquely identify the SequentialAgent class."
       ),
   )

tests/unittests/agents/test_agent_config.py

@@ -12,18 +12,23 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from pathlib import Path
 from typing import Literal
+from typing import Type
 
+from google.adk.agents import config_agent_utils
 from google.adk.agents.agent_config import AgentConfig
+from google.adk.agents.base_agent import BaseAgent
 from google.adk.agents.base_agent_config import BaseAgentConfig
-from google.adk.agents.llm_agent_config import LlmAgentConfig
-from google.adk.agents.loop_agent_config import LoopAgentConfig
-from google.adk.agents.parallel_agent_config import ParallelAgentConfig
-from google.adk.agents.sequential_agent_config import SequentialAgentConfig
+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
+import pytest
 import yaml
 
 
-def test_agent_config_discriminator_default_is_llm_agent():
+def test_agent_config_discriminator_default_is_llm_agent(tmp_path: Path):
   yaml_content = """\
 name: search_agent
 model: gemini-2.0-flash
@@ -32,17 +37,29 @@ instruction: a fake instruction
 tools:
   - name: google_search
 """
-  config_data = yaml.safe_load(yaml_content)
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
 
-  config = AgentConfig.model_validate(config_data)
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
 
-  assert isinstance(config.root, LlmAgentConfig)
+  assert isinstance(agent, LlmAgent)
   assert config.root.agent_class == "LlmAgent"
 
 
-def test_agent_config_discriminator_llm_agent():
-  yaml_content = """\
-agent_class: LlmAgent
+@pytest.mark.parametrize(
+    "agent_class_value",
+    [
+        "LlmAgent",
+        "google.adk.agents.LlmAgent",
+        "google.adk.agents.llm_agent.LlmAgent",
+    ],
+)
+def test_agent_config_discriminator_llm_agent(
+    agent_class_value: str, tmp_path: Path
+):
+  yaml_content = f"""\
+agent_class: {agent_class_value}
 name: search_agent
 model: gemini-2.0-flash
 description: a sample description
@@ -50,66 +67,191 @@ instruction: a fake instruction
 tools:
   - name: google_search
 """
-  config_data = yaml.safe_load(yaml_content)
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
 
-  config = AgentConfig.model_validate(config_data)
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
 
-  assert isinstance(config.root, LlmAgentConfig)
-  assert config.root.agent_class == "LlmAgent"
+  assert isinstance(agent, LlmAgent)
+  assert config.root.agent_class == agent_class_value
 
 
-def test_agent_config_discriminator_loop_agent():
-  yaml_content = """\
-agent_class: LoopAgent
+@pytest.mark.parametrize(
+    "agent_class_value",
+    [
+        "LoopAgent",
+        "google.adk.agents.LoopAgent",
+        "google.adk.agents.loop_agent.LoopAgent",
+    ],
+)
+def test_agent_config_discriminator_loop_agent(
+    agent_class_value: str, tmp_path: Path
+):
+  yaml_content = f"""\
+agent_class: {agent_class_value}
 name: CodePipelineAgent
 description: Executes a sequence of code writing, reviewing, and refactoring.
-sub_agents:
-  - config_path: sub_agents/code_writer_agent.yaml
-  - config_path: sub_agents/code_reviewer_agent.yaml
-  - config_path: sub_agents/code_refactorer_agent.yaml
+sub_agents: []
 """
-  config_data = yaml.safe_load(yaml_content)
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
 
-  config = AgentConfig.model_validate(config_data)
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
 
-  assert isinstance(config.root, LoopAgentConfig)
-  assert config.root.agent_class == "LoopAgent"
+  assert isinstance(agent, LoopAgent)
+  assert config.root.agent_class == agent_class_value
 
 
-def test_agent_config_discriminator_parallel_agent():
-  yaml_content = """\
-agent_class: ParallelAgent
+@pytest.mark.parametrize(
+    "agent_class_value",
+    [
+        "ParallelAgent",
+        "google.adk.agents.ParallelAgent",
+        "google.adk.agents.parallel_agent.ParallelAgent",
+    ],
+)
+def test_agent_config_discriminator_parallel_agent(
+    agent_class_value: str, tmp_path: Path
+):
+  yaml_content = f"""\
+agent_class: {agent_class_value}
 name: CodePipelineAgent
 description: Executes a sequence of code writing, reviewing, and refactoring.
-sub_agents:
-  - config_path: sub_agents/code_writer_agent.yaml
-  - config_path: sub_agents/code_reviewer_agent.yaml
-  - config_path: sub_agents/code_refactorer_agent.yaml
+sub_agents: []
 """
-  config_data = yaml.safe_load(yaml_content)
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
 
-  config = AgentConfig.model_validate(config_data)
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
 
-  assert isinstance(config.root, ParallelAgentConfig)
-  assert config.root.agent_class == "ParallelAgent"
+  assert isinstance(agent, ParallelAgent)
+  assert config.root.agent_class == agent_class_value
 
 
-def test_agent_config_discriminator_sequential_agent():
-  yaml_content = """\
-agent_class: SequentialAgent
+@pytest.mark.parametrize(
+    "agent_class_value",
+    [
+        "SequentialAgent",
+        "google.adk.agents.SequentialAgent",
+        "google.adk.agents.sequential_agent.SequentialAgent",
+    ],
+)
+def test_agent_config_discriminator_sequential_agent(
+    agent_class_value: str, tmp_path: Path
+):
+  yaml_content = f"""\
+agent_class: {agent_class_value}
 name: CodePipelineAgent
 description: Executes a sequence of code writing, reviewing, and refactoring.
-sub_agents:
-  - config_path: sub_agents/code_writer_agent.yaml
-  - config_path: sub_agents/code_reviewer_agent.yaml
-  - config_path: sub_agents/code_refactorer_agent.yaml
+sub_agents: []
 """
-  config_data = yaml.safe_load(yaml_content)
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
 
-  config = AgentConfig.model_validate(config_data)
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
 
-  assert isinstance(config.root, SequentialAgentConfig)
-  assert config.root.agent_class == "SequentialAgent"
+  assert isinstance(agent, SequentialAgent)
+  assert config.root.agent_class == agent_class_value
+
+
+@pytest.mark.parametrize(
+    ("agent_class_value", "expected_agent_type"),
+    [
+        ("LoopAgent", LoopAgent),
+        ("google.adk.agents.LoopAgent", LoopAgent),
+        ("google.adk.agents.loop_agent.LoopAgent", LoopAgent),
+        ("ParallelAgent", ParallelAgent),
+        ("google.adk.agents.ParallelAgent", ParallelAgent),
+        ("google.adk.agents.parallel_agent.ParallelAgent", ParallelAgent),
+        ("SequentialAgent", SequentialAgent),
+        ("google.adk.agents.SequentialAgent", SequentialAgent),
+        ("google.adk.agents.sequential_agent.SequentialAgent", SequentialAgent),
+    ],
+)
+def test_agent_config_discriminator_with_sub_agents(
+    agent_class_value: str, expected_agent_type: Type[BaseAgent], tmp_path: Path
+):
+  # Create sub-agent config files
+  sub_agent_dir = tmp_path / "sub_agents"
+  sub_agent_dir.mkdir()
+  sub_agent_config = """\
+name: sub_agent_{index}
+model: gemini-2.0-flash
+description: a sub agent
+instruction: sub agent instruction
+"""
+  (sub_agent_dir / "sub_agent1.yaml").write_text(
+      sub_agent_config.format(index=1)
+  )
+  (sub_agent_dir / "sub_agent2.yaml").write_text(
+      sub_agent_config.format(index=2)
+  )
+  yaml_content = f"""\
+agent_class: {agent_class_value}
+name: main_agent
+description: main agent with sub agents
+sub_agents:
+  - config_path: sub_agents/sub_agent1.yaml
+  - config_path: sub_agents/sub_agent2.yaml
+"""
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
+
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
+
+  assert isinstance(agent, expected_agent_type)
+  assert config.root.agent_class == agent_class_value
+
+
+@pytest.mark.parametrize(
+    ("agent_class_value", "expected_agent_type"),
+    [
+        ("LlmAgent", LlmAgent),
+        ("google.adk.agents.LlmAgent", LlmAgent),
+        ("google.adk.agents.llm_agent.LlmAgent", LlmAgent),
+    ],
+)
+def test_agent_config_discriminator_llm_agent_with_sub_agents(
+    agent_class_value: str, expected_agent_type: Type[BaseAgent], tmp_path: Path
+):
+  # Create sub-agent config files
+  sub_agent_dir = tmp_path / "sub_agents"
+  sub_agent_dir.mkdir()
+  sub_agent_config = """\
+name: sub_agent_{index}
+model: gemini-2.0-flash
+description: a sub agent
+instruction: sub agent instruction
+"""
+  (sub_agent_dir / "sub_agent1.yaml").write_text(
+      sub_agent_config.format(index=1)
+  )
+  (sub_agent_dir / "sub_agent2.yaml").write_text(
+      sub_agent_config.format(index=2)
+  )
+  yaml_content = f"""\
+agent_class: {agent_class_value}
+name: main_agent
+model: gemini-2.0-flash
+description: main agent with sub agents
+instruction: main agent instruction
+sub_agents:
+  - config_path: sub_agents/sub_agent1.yaml
+  - config_path: sub_agents/sub_agent2.yaml
+"""
+  config_file = tmp_path / "test_config.yaml"
+  config_file.write_text(yaml_content)
+
+  config = AgentConfig.model_validate(yaml.safe_load(yaml_content))
+  agent = config_agent_utils.from_config(str(config_file))
+
+  assert isinstance(agent, expected_agent_type)
+  assert config.root.agent_class == agent_class_value
 
 
 def test_agent_config_discriminator_custom_agent():