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# What is fail closed?
- URL: https://www.agentsecbrief.com/what-is-fail-closed/
- Published: 2026-10-09T18:09:05.000Z
- Updated: 2026-10-09T18:09:05.000Z
- Description: A fail-closed control blocks an action when it cannot reach a decision, because it crashed, timed out or lacked data, instead of letting the action through.
- Author: Angela Milash
- Tags: Glossary, Architecture & Harnesses

**A fail-closed control blocks an action when it cannot reach a decision, because it crashed, timed out or lacked data, instead of letting the action through.**

## Why it matters

AI agent guardrails such as approval checks, policy hooks and permission rules often fail open by default: if the check errors, the agent carries on. An attacker who can make a guard crash, time out or see truncated input then gets past it without ever beating its logic.

## Example

On 8 October 2026 Claude Code 2.1.295 added an onFailure: "block" option so hooks that fail to start, time out or exit unexpectedly deny the action, and fixed guards receiving silently truncated input ([release notes](https://github.com/anthropics/claude-code/releases/tag/v2.1.295?ref=agentsecbrief.com)). Days earlier Microsoft's Agent Framework switched approvals to fail closed ([python-1.20.0](https://github.com/microsoft/agent-framework/releases/tag/python-1.20.0?ref=agentsecbrief.com)).

## How to defend against it

- List every guard that must block, and set it to deny on error or timeout ([checklist](https://www.agentsecbrief.com/checklist/)).
- Test guards with malformed, oversized and deeply nested input.
- Alert on guard errors, not just on denials.

## Related terms

Policy enforcement point, approval fatigue, [repo-planted agent config](https://www.agentsecbrief.com/what-is-repo-planted-agent-config/).

## Sources

- [Claude Code v2.1.295 (8 Oct 2026)](https://github.com/anthropics/claude-code/releases/tag/v2.1.295?ref=agentsecbrief.com)
- [Microsoft Agent Framework python-1.20.0](https://github.com/microsoft/agent-framework/releases/tag/python-1.20.0?ref=agentsecbrief.com)

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