Seer fixes Seer: How Seer pointed us toward a bug and helped fix an outage

Seer, an AI agent used by Sentry to identify bugs and suggest fixes, played a crucial role in diagnosing and resolving an outage that it inadvertently caused. On February 21, 2026, Seer's AI-powered issue summarization failed in the EU region due to an upstream issue with Google's Vertex AI platform, compounded by a flaw in Seer's own code that incorrectly blocklisted its provisioned throughput (PT) region. This led to a cascade of blocklisting across all EU regions, resulting in a total outage. The root cause was a missing code exception for the PT region in the blocklist logic, which was present in the US deployment but not in the EU deployment. Seer was instrumental in quickly identifying the issue by analyzing error patterns, which helped engineers realize the oversight and implement a six-line code fix to prevent PT regions from being blocklisted, ultimately restoring service. The incident highlighted the critical importance of ensuring that application code is fully aware of infrastructure provisioning to avoid turning minor upstream issues into major outages.

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