---
title: Nondeterminism
description: 'The property that identical-looking requests can produce different behavior because of sampling, concurrency, infrastructure, model updates, or hidden state.'
canonical_url: 'https://darkfactory.dev/glossary/nondeterminism'
markdown_url: 'https://darkfactory.dev/glossary/nondeterminism.md'
collection: glossary
date_published: '2026-08-03T00:00:00-04:00'
date_modified: '2026-08-26T00:00:00-04:00'
---

# Nondeterminism


## Definition

Nondeterminism is the property that identical-looking requests can produce different behavior. Sampling is one source, but concurrency, numerical execution, changing infrastructure, model updates, caches, tool state, and hidden context can also alter a run.

Setting temperature to zero does not guarantee reproducibility across a distributed AI system. A provider may route to a new model snapshot, a tool may return current data, or concurrent actions may complete in another order.

Evaluate a nondeterministic system with repeated trials and distributions, not one lucky run. Record seeds where supported, model and harness versions, infrastructure metadata, tool responses, and the variance of the metric that informs the decision.

## Distinguish it from nearby terms

Nondeterminism means variation across nominally equivalent runs. Unreliability means failure to meet requirements. A variable system can still meet a statistical service level, while a perfectly deterministic system can fail every time.

## Check your understanding

A test passes once and the agent is promoted. Across 100 identical runs, it causes the prohibited action in six. The single pass measured neither reliability nor the tail risk.

## Also called

non-determinism

## Related factory areas

- [Verification, evaluation & quality truth](https://darkfactory.dev/factory/verification)

## Evidence and further reading

- [Infrastructure noise moves eval scores more than model margins](https://www.anthropic.com/engineering/infrastructure-noise)
- [Same Signal, Different Semantics](https://arxiv.org/abs/2605.18332)