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1. IL-0409 AssemblyAI's async and LLM Gateway APIs returned 5xx after its auth service could not get ENIs and scaled to zero

   AssemblyAI2026

   Symptoms

   On 29 January 2026 AssemblyAI's Async Transcription API and LLM Gateway on the NA API endpoints went down for 25 minutes, returning 500 errors, and then ran degraded for two hours. The status page first reported the investigation at 17:54 UTC.

   Service outageHigh error rateHigh latencyCloud API rate limit exhausted

   Trigger

   An influx of batch transcription jobs higher than usual made autoscaling add workers until spot instances ran out, on-demand instances were used, and the AWS Elastic Network Interface limit was reached.

   Root cause

   Capacity exhaustion

   Component

   Amazon ECS

   Contributing factor

   Excessive blast radiusMonitoring blind spotAutomation amplificationIgnored early signal

   Remediation

   • Worked through the backlog of requests until turnaround times were back to normal

   Action items

   • Add alerts on ENI utilisation
   • Run the authentication service on dedicated capacity, separate from batch workers
   • Fix the Lambda function that rate-limited the AWS API during recovery
   • Run everything in multiple regions

   Lessons

   AssemblyAI calls the outage preventable: ENI utilisation was not monitored, and because the authentication service shared resources with batch workers, a traffic spike in one starved the other. The warning signs were there but were not connected, and a tool built for recovery ended up blocking recovery.

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