diff --git a/src/google/adk/agents/llm_agent.py b/src/google/adk/agents/llm_agent.py
index cd14cefb3b..c8133f8ba0 100644
--- a/src/google/adk/agents/llm_agent.py
+++ b/src/google/adk/agents/llm_agent.py
@@ -943,6 +943,24 @@ def __maybe_save_output_to_state(self, event: Event):
 
   @model_validator(mode='after')
   def __model_validator_after(self) -> LlmAgent:
+    generate_content_config = self.generate_content_config
+    if (
+        self.tools
+        and generate_content_config
+        and generate_content_config.response_mime_type == 'application/json'
+    ):
+      tool_config = generate_content_config.tool_config
+      function_calling_config = (
+          tool_config.function_calling_config if tool_config else None
+      )
+      mode = function_calling_config.mode if function_calling_config else None
+      mode_value = getattr(mode, 'value', mode)
+      if mode_value is None or str(mode_value).upper() != 'NONE':
+        raise ValueError(
+            '`response_mime_type="application/json"` is incompatible with '
+            'function calling. Set `function_calling_config.mode` to "NONE" '
+            'or remove tools from the agent.'
+        )
     return self
 
   @field_validator('generate_content_config', mode='after')
diff --git a/tests/unittests/agents/test_llm_agent_fields.py b/tests/unittests/agents/test_llm_agent_fields.py
index 92f3c34e49..9fcb2b5be7 100644
--- a/tests/unittests/agents/test_llm_agent_fields.py
+++ b/tests/unittests/agents/test_llm_agent_fields.py
@@ -250,6 +250,51 @@ def _a_tool():
   )
 
 
+@pytest.mark.parametrize('mode', ['AUTO', 'ANY', None])
+def test_generate_content_config_json_response_with_function_calling_throws(
+    mode,
+):
+  def _a_tool():
+    pass
+
+  function_calling_config = (
+      types.FunctionCallingConfig(mode=mode) if mode else None
+  )
+  tool_config = (
+      types.ToolConfig(function_calling_config=function_calling_config)
+      if function_calling_config
+      else None
+  )
+
+  with pytest.raises(ValueError, match='response_mime_type="application/json"'):
+    LlmAgent(
+        name='test_agent',
+        tools=[_a_tool],
+        generate_content_config=types.GenerateContentConfig(
+            response_mime_type='application/json',
+            tool_config=tool_config,
+        ),
+    )
+
+
+def test_generate_content_config_json_response_with_function_calling_none():
+  def _a_tool():
+    pass
+
+  agent = LlmAgent(
+      name='test_agent',
+      tools=[_a_tool],
+      generate_content_config=types.GenerateContentConfig(
+          response_mime_type='application/json',
+          tool_config=types.ToolConfig(
+              function_calling_config=types.FunctionCallingConfig(mode='NONE')
+          ),
+      ),
+  )
+
+  assert agent.generate_content_config.response_mime_type == 'application/json'
+
+
 def test_before_model_callback():
   def _before_model_callback(
       callback_context: CallbackContext,