diff --git a/src/google/adk/agents/llm_agent.py b/src/google/adk/agents/llm_agent.py
index 2eeb82f90c2..3785be27856 100644
--- a/src/google/adk/agents/llm_agent.py
+++ b/src/google/adk/agents/llm_agent.py
@@ -391,7 +391,9 @@ class LlmAgent(BaseAgent, abc.ABC):
   """The additional content generation configurations.
 
   NOTE: not all fields are usable, e.g. tools must be configured via `tools`,
-  thinking_config can be configured here or via the `planner`. If both are set, the planner's configuration takes precedence.
+  candidate_count must be 1 or unset, and thinking_config can be configured
+  here or via the `planner`. If both are set, the planner's configuration
+  takes precedence.
 
   For example: use this config to adjust model temperature, configure safety
   settings, etc.
@@ -1252,6 +1254,16 @@ def validate_generate_content_config(
           ' or its client, not via'
           ' LlmAgent.generate_content_config.http_options.base_url.'
       )
+    if (
+        generate_content_config.candidate_count is not None
+        and generate_content_config.candidate_count > 1
+    ):
+      raise ValueError(
+          'candidate_count must be 1 or unset. LlmResponse keeps one'
+          ' candidate, so extra values are requested then discarded. Pass'
+          ' generate_content_config=types.GenerateContentConfig() without'
+          ' candidate_count, or set candidate_count=1.'
+      )
     return generate_content_config
 
   @override
diff --git a/tests/unittests/agents/test_llm_agent_error_messages.py b/tests/unittests/agents/test_llm_agent_error_messages.py
index ae23d219829..8d2a9aec9d7 100644
--- a/tests/unittests/agents/test_llm_agent_error_messages.py
+++ b/tests/unittests/agents/test_llm_agent_error_messages.py
@@ -147,6 +147,15 @@ def test_response_schema_error_includes_move_guidance(self):
     with pytest.raises(ValueError, match=r'Move your schema'):
       LlmAgent.validate_generate_content_config(config)
 
+  def test_candidate_count_error_names_generate_content_config(self):
+    """candidate_count > 1 should name generate_content_config in the error."""
+    config = types.GenerateContentConfig(candidate_count=2)
+    with pytest.raises(
+        ValueError, match=r'generate_content_config'
+    ) as exc_info:
+      LlmAgent.validate_generate_content_config(config)
+    assert 'candidate_count=1' in str(exc_info.value)
+
 
 class TestGenerateContentKwargErrors:
   """Tests for misplaced GenerateContentConfig kwargs on LlmAgent."""
diff --git a/tests/unittests/agents/test_llm_agent_fields.py b/tests/unittests/agents/test_llm_agent_fields.py
index 77beb5d8608..53ce69d5fa2 100644
--- a/tests/unittests/agents/test_llm_agent_fields.py
+++ b/tests/unittests/agents/test_llm_agent_fields.py
@@ -449,6 +449,24 @@ def test_validate_generate_content_config_http_options_base_url_throw():
     )
 
 
+def test_validate_generate_content_config_candidate_count_throw():
+  """candidate_count greater than 1 is rejected at LlmAgent construction."""
+  with pytest.raises(ValueError, match=r'candidate_count must be 1 or unset'):
+    LlmAgent(
+        name='test_agent',
+        generate_content_config=types.GenerateContentConfig(candidate_count=2),
+    )
+
+
+def test_validate_generate_content_config_candidate_count_one_allowed():
+  """candidate_count=1 remains settable on generate_content_config."""
+  agent = LlmAgent(
+      name='test_agent',
+      generate_content_config=types.GenerateContentConfig(candidate_count=1),
+  )
+  assert agent.generate_content_config.candidate_count == 1
+
+
 def test_validate_generate_content_config_http_options_allowed():
   """Tests that request-time http options remain settable in config."""
   extra_body = {'tool_config': {'function_calling_config': {'mode': 'AUTO'}}}