If you think deterministic code doesn't need a refusal path - you're watching two different systems fail in identical ways without realizing it.
Cloudflare's bot detection failure illustrates the trap perfectly. When input validation failed, the newer proxy crashed. The older one returned a bot score of zero - a decision it had no basis for. Both were failures. But they were failures of the same underlying architecture: a path that always answers, whether it should or not.
Here's what most teams get wrong about the Level 0 floor:
๐๐ญ'๐ฌ ๐ง๐จ๐ญ ๐๐๐จ๐ฎ๐ญ ๐ญ๐ก๐ ๐๐๐ฌ๐๐ง๐๐ ๐จ๐ ๐ ๐ฆ๐จ๐๐๐ฅ. A default branch that always fires is not simpler than an LLM fallback - it's the same architecture wearing different clothes. Both invent answers where no rule applied. Both hand the caller a result indistinguishable from a decided answer. Both have coverage of 1.0 by construction, which means coverage tells you nothing.
The real floor is explicit refusal. A system with an intentional "I cannot answer this" path - one that declares which inputs it can handle and routes the rest somewhere else - that's Level 0. A rules router with a catch-all default? You're already off the floor.
๐๐ก๐ ๐ฆ๐ข๐ฌ๐ฌ๐ข๐ง๐ ๐ฆ๐๐๐ฌ๐ฎ๐ซ๐๐ฆ๐๐ง๐ญ: Anthropic tells you to use the simplest thing and add complexity only when it demonstrably improves outcomes. That's directionally correct and operationally useless. You need a runtime measurement, not a design-time judgement. The deterministic implementation itself must measure coverage on real traffic and tell you when it hits its limit.
The question isn't whether to use an LLM. The question is whether you can close the domain and prove it. If you can't, your router needs to say so - not crash, not guess, but refuse explicitly and let something downstream decide what comes next.
Read the full breakdown:
https://ranjankumar.in/genai-architectures-level-0-deterministic-floor-refusal-path
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