Claude's 2-hour outage exposed critical prompt compatibility issues between AI models, forcing one team to overhaul their fallback strategy.

When Claude went down, failover to GPT-4o failed unexpectedly: ~30% of tool calls returned in wrong format despite identical logic and tools

Root cause was model-specific behavior—prompts tuned over months for Claude's instruction-following didn't transfer to GPT-4o without modification

Team implemented separate prompt variants per provider, regular testing on eval sets, and automatic traffic failover without code changes

A second outage occurred the day after, but went unnoticed at application layer thanks to improved multi-provider failover setup