fix: export thoughts_token_count to OpenTelemetry trace spans

src/google/adk/telemetry/tracing.py

@@ -744,6 +744,14 @@ def trace_generate_content_result(span: Span | None, llm_response: LlmResponse):
       span.set_attribute(
           GEN_AI_USAGE_OUTPUT_TOKENS, usage_metadata.candidates_token_count
       )
+    try:
+      if usage_metadata.thoughts_token_count is not None:
+        span.set_attribute(
+            'gen_ai.usage.experimental.reasoning_tokens',
+            usage_metadata.thoughts_token_count,
+        )
+    except AttributeError:
+      pass
 
   otel_logger.emit(
       LogRecord(
@@ -787,6 +795,14 @@ def trace_inference_result(
       span.set_attribute(
           GEN_AI_USAGE_OUTPUT_TOKENS, usage_metadata.candidates_token_count
       )
+    try:
+      if usage_metadata.thoughts_token_count is not None:
+        span.set_attribute(
+            'gen_ai.usage.experimental.reasoning_tokens',
+            usage_metadata.thoughts_token_count,
+        )
+    except AttributeError:
+      pass
 
   if is_experimental_semconv() and isinstance(gc_span, GenerateContentSpan):
     set_operation_details_attributes_from_response(

tests/unittests/telemetry/test_spans.py

@@ -1284,3 +1284,54 @@ def test_trace_tool_call_with_standard_error(
       mock.call('error.type', 'ValueError')
       in mock_span_fixture.set_attribute.call_args_list
   )
+
+
+def test_trace_inference_result_with_thinking_tokens(mock_span_fixture):
+  """Test trace_inference_result exports thoughts_token_count."""
+  llm_response = LlmResponse(
+      turn_complete=True,
+      finish_reason=types.FinishReason.STOP,
+      usage_metadata=types.GenerateContentResponseUsageMetadata(
+          total_token_count=110,
+          prompt_token_count=50,
+          candidates_token_count=10,
+          thoughts_token_count=50,
+      ),
+  )
+
+  trace_inference_result(mock_span_fixture, llm_response)
+
+  mock_span_fixture.set_attribute.assert_any_call(GEN_AI_USAGE_INPUT_TOKENS, 50)
+  mock_span_fixture.set_attribute.assert_any_call(
+      GEN_AI_USAGE_OUTPUT_TOKENS, 10
+  )
+  mock_span_fixture.set_attribute.assert_any_call(
+      'gen_ai.usage.experimental.reasoning_tokens', 50
+  )
+
+
+def test_trace_inference_result_without_thinking_tokens(mock_span_fixture):
+  """Test trace_inference_result works when thoughts_token_count is None."""
+  llm_response = LlmResponse(
+      turn_complete=True,
+      finish_reason=types.FinishReason.STOP,
+      usage_metadata=types.GenerateContentResponseUsageMetadata(
+          total_token_count=60,
+          prompt_token_count=50,
+          candidates_token_count=10,
+      ),
+  )
+
+  trace_inference_result(mock_span_fixture, llm_response)
+
+  mock_span_fixture.set_attribute.assert_any_call(GEN_AI_USAGE_INPUT_TOKENS, 50)
+  mock_span_fixture.set_attribute.assert_any_call(
+      GEN_AI_USAGE_OUTPUT_TOKENS, 10
+  )
+  # Verify reasoning_tokens is NOT set when thoughts_token_count is None
+  reasoning_calls = [
+      call
+      for call in mock_span_fixture.set_attribute.call_args_list
+      if call.args[0] == 'gen_ai.usage.experimental.reasoning_tokens'
+  ]
+  assert len(reasoning_calls) == 0