In _calculate_responses_token_usage (lines 291-295), when output_tokens == 0 and streaming_message_responses is None, there's no fallback to manually count output tokens from response.output. The analogous function _calculate_completions_token_usage has this fallback (lines 220-223) which counts from response.choices. For non-streaming Responses API calls where the API doesn't return usage data and tiktoken is configured, output tokens won't be recorded.