The Probability Pixie
It worked yesterday, I swear.
The Probability Pixie is the monster of non-deterministic output. The same prompt produces different results each time, and the developer who got a great answer on Tuesday can't reproduce it on Wednesday. Consistency becomes a matter of luck.
Symptom
Brittle prompts checked into the repo, demos that work once and never again, and bug reports that read 'the AI used to do this.'
Why It Matters
Treating a probabilistic component as a deterministic one is how reliable systems quietly become superstitious ones. The failure mode is subtle — usually similar enough to slip past review.
What the Chapter Gives You
Where in your stack the Pixie is most dangerous, how to constrain non-determinism without losing the value, and the contracts to put around AI-touching code.
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