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Guardrail

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A runtime control that inspects or mediates a model's or agent's inputs, outputs or tool calls and blocks, rewrites or escalates what breaks a policy, logging each decision as evidence. Guardrails are deterministic code or classifiers in the call path, unlike a guardian agent, which is itself an AI system.

Developed in
ch. 05, Pattern: Runtime Guardrail
ch. 23, Runtime guardrails for tool calls
Chapters
ch. 04, The Stack · ch. 05, Patterns · ch. 23, AI Agents
Contrast with
Guardian agent
Source
Defined by this Body of Knowledge: the section it is developed in is the source

Where it is used

21 chapters of the Body of Knowledge use the term. Each link opens the first section that does.

Patterns that use this term

12 pattern pages use the term; the 10 that use it most:

  • Failure posture

    What a guardrail, guardian agent or tool gateway does when it cannot reach a decision: fail open lets the call through, fail closed blocks it.

  • Small language model (SLM)

    A language model small enough to run close to the user, for example on a phone.

  • Manufacturing defect

    In product liability, a departure of a unit from its own design.

  • Multimodal model

    A model that takes or produces more than one modality (text, image, audio, video).

  • Prompt injection

    An input that alters a model's behaviour or output in ways its designers did not intend.

  • Version pinning

    Fixing, in the registry entry, the exact versions of the model, prompts, retrieval corpus and guardrails a deployed system uses, so what ran is known and any…

  • Design defect

    In product liability, a defect inherent in the design of every unit, judged by consumer expectations or by weighing risk against utility.

Sources

No external source: the term is coined or used in a specific sense by this Body of Knowledge, and the section linked under "Developed in" is its source.

Definitions of legal terms paraphrase the cited text, which governs. Dated statements are as of .

Cite this term

García Aibar, J. (2026). Guardrail. In AI Governance Engineering: The Thesis & Body of Knowledge (v0.5.0), Glossary. https://doi.org/10.5281/zenodo.22956197. https://aigovernanceengineer.com/glossary/guardrail. CC BY 4.0

BibTeX

@misc{aige2026guardrail,
  author  = {Jorge García Aibar},
  title   = {{Guardrail}},
  note    = {Glossary, AI Governance Engineering: The Thesis \& Body of Knowledge, version 0.5.0},
  year    = {2026},
  doi     = {10.5281/zenodo.22956197},
  url     = {https://aigovernanceengineer.com/glossary/guardrail}
}