---
title: "Failure posture"
description: "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."
canonical: https://aigovernanceengineer.com/glossary/failure-posture
author: "Jorge García Aibar"
license: "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)"
doi: https://doi.org/10.5281/zenodo.22956197
version: "0.5.0"
updated: 2026-09-25
---

# 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. The reference guardian of the OWASP Agent Control Standard starts at proceed unless set to deny [1]. The posture is a governance decision, set per operation class and recorded in the agent's Policy Card.

- Developed in: [ch. 23, Runtime guardrails for tool calls](https://aigovernanceengineer.com/bok/governing-agents#runtime-guardrails-for-tool-calls)
- Chapters: [ch. 23, AI Agents](https://aigovernanceengineer.com/bok/governing-agents)
- In the glossary chapter: https://aigovernanceengineer.com/bok/glossary#t-failure-posture

## Sources

[1] Agent Control Standard (ACS) repository (a wire specification that lets a separate guardian agent permit, deny or modify an agent's action before it happens; the reference guardian's failure posture defaults to proceed, overridable to deny; donated to the OWASP GenAI Security Project, announced 1 Sep 2026). OWASP GenAI Security Project (GitHub). 2026-09-01. https://github.com/GenAI-Security-Project/agent-control-standard (verified: primary)