The mental model for LLM guardrails: a separate layer that distrusts the model
Careless_Sabfey_4906 · reddit · 2026-09-10
The author shares a mental-model shift on LLM guardrails: they're not the model refusing requests, but a separate layer that trusts the model not at all.
Their structure:
- Inbound: every prompt is checked before reaching the model — injection attempts, policy violations, PII get blocked or flagged
- Outbound: responses are checked before users see them — catching leaks, fabricated claims, toxic output
Key insight: guardrails must be their own layer, not a system prompt, since prompts are suggestions a model can be talked into skipping; external checks actually enforce. Keyword matching alone misses politely phrased violations.
Open question: latency — every check adds delay, and the author asks how others balance that tradeoff.
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