fix(otel): emit trace_output as span attribute instead of event#9255
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trace_output was emitted via tracing::info!() which becomes an OTEL event (log line on the span), not a span attribute. Observability backends like MLflow and Langfuse (via OTEL ingestion) look for span attributes, so the output was invisible to them. Declare trace_output as a field on reply_stream_span and use Span::record() so it becomes a proper OTEL span attribute. The Langfuse custom layer handles both paths identically (on_record and on_event both call handle_record), so this is a no-op for that path. Fixes #8586 Signed-off-by: Douwe Osinga <douwe@squareup.com>
alexhancock
approved these changes
May 18, 2026
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Summary
trace_outputwas emitted viatracing::info!()which becomes an OTEL event (a log line attached to the span), not a span attribute. Observability backends like MLflow and Langfuse (via OTEL ingestion) look for span attributes to populate input/output fields, so the assistant output was invisible to them.Changes
trace_outputas a field onreply_stream_spanso it can accept a recorded valuetracing::info!(trace_output = ...)withtracing::Span::current().record("trace_output", ...)to set it as a proper span attributeThe Langfuse custom layer's
on_recordandon_eventboth callhandle_recordwith the same metadata, so this change is transparent to the Langfuse-specific path.Fixes #8586