diff --git a/src/google/adk/cli/adk_web_server.py b/src/google/adk/cli/adk_web_server.py
index b97932d042..39be391e46 100644
--- a/src/google/adk/cli/adk_web_server.py
+++ b/src/google/adk/cli/adk_web_server.py
@@ -550,6 +550,28 @@ def _get_root_agent(self, agent_or_app: BaseAgent | App) -> BaseAgent:
       return agent_or_app.root_agent
     return agent_or_app
 
+  def _get_effective_modalities(
+      self, root_agent: BaseAgent, requested_modalities: List[str]
+  ) -> List[str]:
+    """Determines effective modalities, forcing AUDIO for native-audio models.
+
+    Native-audio models only support AUDIO modality. This method detects
+    native-audio models by checking if the model name contains "native-audio"
+    and forces AUDIO modality for those models.
+
+    Args:
+      root_agent: The root agent of the application.
+      requested_modalities: The modalities requested by the client.
+
+    Returns:
+      The effective modalities to use.
+    """
+    model = getattr(root_agent, "model", None)
+    model_name = model if isinstance(model, str) else ""
+    if "native-audio" in model_name:
+      return ["AUDIO"]
+    return requested_modalities
+
   def _create_runner(self, agentic_app: App) -> Runner:
     """Create a runner with common services."""
     return Runner(
@@ -1652,7 +1674,10 @@ async def run_agent_live(
 
       async def forward_events():
         runner = await self.get_runner_async(app_name)
-        run_config = RunConfig(response_modalities=modalities)
+        effective_modalities = self._get_effective_modalities(
+            runner.app.root_agent, modalities
+        )
+        run_config = RunConfig(response_modalities=effective_modalities)
         async with Aclosing(
             runner.run_live(
                 session=session,
diff --git a/tests/unittests/cli/test_fast_api.py b/tests/unittests/cli/test_fast_api.py
index 0c69605349..42b17f93ce 100755
--- a/tests/unittests/cli/test_fast_api.py
+++ b/tests/unittests/cli/test_fast_api.py
@@ -29,6 +29,7 @@
 
 from fastapi.testclient import TestClient
 from google.adk.agents.base_agent import BaseAgent
+from google.adk.agents.llm_agent import LlmAgent
 from google.adk.agents.run_config import RunConfig
 from google.adk.apps.app import App
 from google.adk.artifacts.base_artifact_service import ArtifactVersion
@@ -1411,5 +1412,91 @@ def test_builder_save_rejects_traversal(builder_test_client, tmp_path):
   assert not (tmp_path / "app" / "tmp" / "escape.yaml").exists()
 
 
+def test_native_audio_model_forces_audio_modality():
+  """Test that native-audio models force AUDIO modality."""
+  from google.adk.cli.adk_web_server import AdkWebServer
+
+  native_audio_agent = LlmAgent(
+      name="native_audio_agent",
+      model="gemini-live-2.5-flash-native-audio",
+  )
+
+  adk_web_server = AdkWebServer(
+      agent_loader=MagicMock(),
+      session_service=MagicMock(),
+      memory_service=MagicMock(),
+      artifact_service=MagicMock(),
+      credential_service=MagicMock(),
+      eval_sets_manager=MagicMock(),
+      eval_set_results_manager=MagicMock(),
+      agents_dir=".",
+  )
+
+  # Test: requesting TEXT should be forced to AUDIO
+  modalities = adk_web_server._get_effective_modalities(
+      native_audio_agent, ["TEXT"]
+  )
+  assert modalities == ["AUDIO"]
+
+  # Test: requesting AUDIO should stay AUDIO
+  modalities = adk_web_server._get_effective_modalities(
+      native_audio_agent, ["AUDIO"]
+  )
+  assert modalities == ["AUDIO"]
+
+
+def test_non_native_audio_model_keeps_requested_modality():
+  """Test that non-native-audio models keep the requested modality."""
+  from google.adk.cli.adk_web_server import AdkWebServer
+
+  regular_agent = LlmAgent(
+      name="regular_agent",
+      model="gemini-2.5-flash",
+  )
+
+  adk_web_server = AdkWebServer(
+      agent_loader=MagicMock(),
+      session_service=MagicMock(),
+      memory_service=MagicMock(),
+      artifact_service=MagicMock(),
+      credential_service=MagicMock(),
+      eval_sets_manager=MagicMock(),
+      eval_set_results_manager=MagicMock(),
+      agents_dir=".",
+  )
+
+  # Test: requesting TEXT should stay TEXT
+  modalities = adk_web_server._get_effective_modalities(regular_agent, ["TEXT"])
+  assert modalities == ["TEXT"]
+
+  # Test: requesting AUDIO should stay AUDIO
+  modalities = adk_web_server._get_effective_modalities(
+      regular_agent, ["AUDIO"]
+  )
+  assert modalities == ["AUDIO"]
+
+
+def test_agent_without_model_attribute():
+  """Test that agents without model attribute keep requested modality."""
+  from google.adk.cli.adk_web_server import AdkWebServer
+
+  base_agent = DummyAgent(name="base_agent")
+
+  adk_web_server = AdkWebServer(
+      agent_loader=MagicMock(),
+      session_service=MagicMock(),
+      memory_service=MagicMock(),
+      artifact_service=MagicMock(),
+      credential_service=MagicMock(),
+      eval_sets_manager=MagicMock(),
+      eval_set_results_manager=MagicMock(),
+      agents_dir=".",
+  )
+
+  # Test: BaseAgent without model attr should keep requested modality
+  modalities = adk_web_server._get_effective_modalities(base_agent, ["TEXT"])
+  assert modalities == ["TEXT"]
+
+
 if __name__ == "__main__":
   pytest.main(["-xvs", __file__])