diff --git a/src/google/adk/cli/fast_api.py b/src/google/adk/cli/fast_api.py
index 7ff10c5414..d2cf83e366 100644
--- a/src/google/adk/cli/fast_api.py
+++ b/src/google/adk/cli/fast_api.py
@@ -317,7 +317,7 @@ async def get_agent_builder(
 
   if a2a:
     try:
-      from a2a.server.apps import A2AStarletteApplication
+      from a2a.server.apps import A2AFastAPIApplication
       from a2a.server.request_handlers import DefaultRequestHandler
       from a2a.server.tasks import InMemoryTaskStore
       from a2a.types import AgentCard
@@ -375,18 +375,12 @@ async def _get_a2a_runner_async() -> Runner:
             data = json.load(f)
             agent_card = AgentCard(**data)
 
-          a2a_app = A2AStarletteApplication(
+          a2a_app = A2AFastAPIApplication(
               agent_card=agent_card,
               http_handler=request_handler,
-          )
-
-          routes = a2a_app.routes(
-              rpc_url=f"/a2a/{app_name}",
-              agent_card_url=f"/a2a/{app_name}{AGENT_CARD_WELL_KNOWN_PATH}",
-          )
+          ).build()
 
-          for new_route in routes:
-            app.router.routes.append(new_route)
+          app.mount(f"/a2a/{app_name}", a2a_app)
 
           logger.info("Successfully configured A2A agent: %s", app_name)
 
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index f0b6c89e7e..4b5381b466 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -26,6 +26,8 @@
 from unittest.mock import MagicMock
 from unittest.mock import patch
 
+from a2a.utils import AGENT_CARD_WELL_KNOWN_PATH
+from a2a.utils import PREV_AGENT_CARD_WELL_KNOWN_PATH
 from fastapi.testclient import TestClient
 from google.adk.agents.base_agent import BaseAgent
 from google.adk.agents.run_config import RunConfig
@@ -422,11 +424,22 @@ def temp_agents_dir_with_a2a():
 
     # Create agent.json file
     agent_card = {
+        "capabilities": {"pushNotifications": True, "streaming": True},
+        "defaultInputModes": ["text", "text/plain"],
+        "defaultOutputModes": ["text", "text/plain"],
         "name": "test_a2a_agent",
         "description": "Test A2A agent",
         "version": "1.0.0",
         "author": "test",
-        "capabilities": ["text"],
+        "protocolVersion": "0.2.6",
+        "skills": [{
+            "description": "Makes the tests pass",
+            "examples": ["Fix the tests."],
+            "id": "test_a2a_agent",
+            "name": "Test A2A agent",
+            "tags": ["testing"],
+        }],
+        "url": "",
     }
 
     with open(agent_dir / "agent.json", "w") as f:
@@ -496,20 +509,12 @@ def test_app_with_a2a(
       patch(
           "a2a.server.request_handlers.DefaultRequestHandler"
       ) as mock_handler,
-      patch("a2a.server.apps.A2AStarletteApplication") as mock_a2a_app,
   ):
     # Configure mocks
     mock_task_store.return_value = MagicMock()
     mock_executor.return_value = MagicMock()
     mock_handler.return_value = MagicMock()
 
-    # Mock A2AStarletteApplication
-    mock_app_instance = MagicMock()
-    mock_app_instance.routes.return_value = (
-        []
-    )  # Return empty routes for testing
-    mock_a2a_app.return_value = mock_app_instance
-
     # Change to temp directory
     original_cwd = os.getcwd()
     os.chdir(temp_agents_dir_with_a2a)
@@ -959,9 +964,21 @@ def list_agents(self):
 )
 def test_a2a_agent_discovery(test_app_with_a2a):
   """Test that A2A agents are properly discovered and configured."""
-  # This test mainly verifies that the A2A setup doesn't break the app
+  # This test verifies that the A2A setup doesn't break the app
+  # and that the well known card works
   response = test_app_with_a2a.get("/list-apps")
   assert response.status_code == 200
+  response2 = test_app_with_a2a.get(
+      f"/a2a/test_a2a_agent{AGENT_CARD_WELL_KNOWN_PATH}"
+  )
+  assert response2.status_code == 200
+  # testing backward compatibility
+  response3 = test_app_with_a2a.get(
+      f"/a2a/test_a2a_agent{PREV_AGENT_CARD_WELL_KNOWN_PATH}"
+  )
+  assert response3.status_code == 200
+  response4 = test_app_with_a2a.get(f"/a2a/test_a2a_agent/openapi.json")
+  assert response4.status_code == 200
   logger.info("A2A agent discovery test passed")