Observed Signal · Jun 24, 2026 · Technical Release · Source: DEV Community · Impact: 2/5 · Sentiment: Positive
Invisible Latency: Instrument Handoffs in Voice Agents
An engineer recounts diagnosing a 1.4-second period of dead air on a live voice call that did not appear in APM traces because the waiting time occurred between spans. Traditional APM traces showed short, green spans (ASR, LLM, TTS), but the unattributed gap between turn-end and ASR-start (a handoff waiting on a lazily-created ASR connection and a contended connection pool) caused the UX issue. The author proposes adding explicit spans around handoffs (example: voice.handoff.vad_to_asr) using OpenTelemetry, alerting on that span's p95, and fixing pooling (pre-warm/resize/keepalive). After adding the span and pool fixes, handoff p95 fell from ~1400ms to ~70ms.
Practical observability best practice for voice/LLM-driven conversational systems: making handoff gaps visible enables targeted fixes to reduce UX-impacting latency. Relevant to teams building voice agents and conversational CX, but not a platform-level policy change.
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Key Takeaways & Evidence Grounding
- Author observed a live call on June 3 with 1.4 seconds of dead air after the user stopped speaking.
- APM/tracing showed all spans as short and green; the wait was in the unattributed gap between spans (turn-end to ASR-start).
- Root cause: ASR client lazily established streaming connections and contended on a connection pool under burst concurrency, delaying ASR-start by 1.4 seconds.
- Fix implemented: add an explicit OpenTelemetry span 'voice.handoff.vad_to_asr' around the handoff and change pool behavior (pre-warm, size for bursts, keep connections alive).
- Result reported: handoff p95 reduced from ~1400ms on the bad call to ~70ms steady-state after fixes; author provided runnable OpenTelemetry Python code and a trace query to surface handoff latency.
Connected Companies & Entities
3 Entities mapped“This is OpenTelemetry Python, `opentelemetry-api` and `opentelemetry-sdk`, the actual SDK calls, runnable....”
“The response streams to TTS (ElevenLabs) for the first audio byte, which is the moment the user hears anything....”
Ontology Mapping & Concepts
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