Prompt Versioning and the Eval Harness

Prompts ARE production behavior. A two-line prompt change can shift the model's output distribution as much as a feature flag flip. Without versioning and an eval harness, you cannot answer the simplest production questions: did quality regress this week? did this prompt change cause yesterday's user complaints? can we roll back? Teams that skip this discipline ship rapidly until the first quality incident, then spend weeks rebuilding trust.

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