feat(a2a): Infer agent cards from ADK agents

Allow A2A setup in get_fast_api_app to serve ADK agent directories without a checked-in agent.json by building the AgentCard from the loaded ADK agent.

Fixes #2237

Tests:

uv run python -m pytest tests/unittests/cli/test_fast_api_a2a.py tests/unittests/cli/test_fast_api.py -k a2a tests/unittests/a2a/utils/test_agent_card_builder.py tests/unittests/a2a/utils/test_agent_to_a2a.py -q

uv run pre-commit run --files src/google/adk/cli/fast_api.py tests/unittests/cli/test_fast_api_a2a.py

Author: jordanchendev

src/google/adk/cli/fast_api.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+import asyncio
 from contextlib import asynccontextmanager
 import importlib
 import json
@@ -49,6 +50,8 @@
 from starlette.types import Lifespan
 from watchdog.observers import Observer
 
+from ..a2a.utils.agent_card_builder import AgentCardBuilder
+from ..apps.app import App
 from ..auth.credential_service.in_memory_credential_service import InMemoryCredentialService
 from ..runners import Runner
 from ..telemetry._agent_engine import get_propagated_context
@@ -691,12 +694,14 @@ async def _get_a2a_runner_async() -> Runner:
         return _get_a2a_runner_async
 
       for p in base_path.iterdir():
-        # only folders with an agent.json file representing agent card are valid
-        # a2a agents
+        has_agent_card = (p / "agent.json").is_file()
+        has_agent_definition = (
+            is_single_agent_directory(p) or (p / "__init__.py").is_file()
+        )
         if (
             p.is_file()
             or p.name.startswith((".", "__pycache__"))
-            or not (p / "agent.json").is_file()
+            or not (has_agent_card or has_agent_definition)
         ):
           continue
 
@@ -716,9 +721,23 @@ async def _get_a2a_runner_async() -> Runner:
               push_config_store=push_config_store,
           )
 
-          with (p / "agent.json").open("r", encoding="utf-8") as f:
-            data = json.load(f)
-            agent_card = AgentCard(**data)
+          if has_agent_card:
+            with (p / "agent.json").open("r", encoding="utf-8") as f:
+              data = json.load(f)
+              agent_card = AgentCard(**data)
+          else:
+            loaded_agent = agent_loader.load_agent(app_name)
+            agent = (
+                loaded_agent.root_agent
+                if isinstance(loaded_agent, App)
+                else loaded_agent
+            )
+            agent_card = asyncio.run(
+                AgentCardBuilder(
+                    agent=agent,
+                    rpc_url=f"http://{host}:{port}/a2a/{app_name}",
+                ).build()
+            )
 
           a2a_app = A2AStarletteApplication(
               agent_card=agent_card,

tests/unittests/cli/test_fast_api_a2a.py

@@ -0,0 +1,93 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest.mock import MagicMock
+from unittest.mock import patch
+
+from google.adk.agents.base_agent import BaseAgent
+from google.adk.cli.fast_api import get_fast_api_app
+
+
+def test_a2a_infers_agent_card_without_agent_json(tmp_path, monkeypatch):
+  """A2A setup builds an agent card from agent.py when agent.json is absent."""
+
+  class _TestAgent(BaseAgent):
+    pass
+
+  agent_dir = tmp_path / "test_a2a_agent"
+  agent_dir.mkdir()
+  (agent_dir / "agent.py").write_text("root_agent = None\n")
+  agent = _TestAgent(
+      name="test_a2a_agent",
+      description="Generated card from ADK agent",
+  )
+  agent_loader = MagicMock()
+  agent_loader.load_agent.return_value = agent
+
+  with (
+      patch(
+          "google.adk.cli.fast_api.create_session_service_from_options",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.cli.fast_api.create_artifact_service_from_options",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.cli.fast_api.create_memory_service_from_options",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.cli.fast_api.LocalEvalSetsManager",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.cli.fast_api.LocalEvalSetResultsManager",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.cli.fast_api._create_task_store_from_options",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "google.adk.a2a.executor.a2a_agent_executor.A2aAgentExecutor",
+          return_value=MagicMock(),
+      ),
+      patch(
+          "a2a.server.request_handlers.DefaultRequestHandler",
+          return_value=MagicMock(),
+      ),
+      patch("a2a.server.apps.A2AStarletteApplication") as mock_a2a_app,
+  ):
+    mock_a2a_app.return_value.routes.return_value = []
+    monkeypatch.chdir(tmp_path)
+
+    get_fast_api_app(
+        agents_dir=".",
+        agent_loader=agent_loader,
+        web=False,
+        session_service_uri="",
+        artifact_service_uri="",
+        memory_service_uri="",
+        a2a=True,
+        host="127.0.0.1",
+        port=8000,
+    )
+
+  agent_loader.load_agent.assert_called_once_with("test_a2a_agent")
+  mock_a2a_app.assert_called_once()
+  agent_card = mock_a2a_app.call_args.kwargs["agent_card"]
+  assert agent_card.name == "test_a2a_agent"
+  assert agent_card.description == "Generated card from ADK agent"
+  assert agent_card.url == "http://127.0.0.1:8000/a2a/test_a2a_agent"