Vapi SDK Silent Failure: start() Resolves on Mic Block
skywave84 · reddit · 2026-09-01
The author discovered a hidden bug while integrating the Vapi Web Voice Agent: when the browser blocks microphone access, vapiSDK.start() still resolves successfully without throwing an error or emitting an event. This led to a massive discrepancy between button clicks (81) and actual calls (0).
Solution:
- Do not rely on the Promise resolution; instead, listen for the call-start event.
- Implement a timeout check; if call-start isn't fired within a set period, treat it as a failure, log it, and show a fallback UI.
- Check navigator.permissions for microphone status before starting to distinguish failure reasons.
Lesson:
If your instrumentation looks identical when the feature is completely broken, you are not measuring the feature. Additionally, data center traffic (e.g., email security scanners) can skew analytics, so check your city breakdowns.
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