---
title: "Fail-closed: definition | KYA glossary"
url: https://knowyouragent.ch/en/knowledge/glossary/fail-closed
lang: en
updated: 2026-10-12
license: © Innopulse Consulting GmbH
---

# Fail-closed

Fail-closed means a system denies when in doubt: if information is missing or a check fails, the action is not executed.

For agents this is the only defensible default. A [verifier](https://knowyouragent.ch/en/knowledge/glossary/verifier) that lets actions through on error grants a [blanket authority](https://knowyouragent.ch/en/knowledge/glossary/blanket-authority) whenever something breaks. The verification model follows the same principle: what could not be checked never counts as proven.

## Example

A decision service cannot fetch a mandate's [status list](https://knowyouragent.ch/en/knowledge/glossary/status-list). Following the fail-closed principle, it denies the payment instead of allowing it for lack of counter-evidence.

## How it differs

Fail-open lets things through when in doubt; fail-closed stops them. For payments and legally binding actions, fail-closed is the cautious choice; for pure information, fail-open can be acceptable. What matters is that the rule is chosen deliberately and documented for every check.

## In practice

- For every check, define what happens when information is missing. Unclear cases should never silently count as allowed.
- Distinguish between a technical failure and a negative result, and log both separately. This shows whether a system is disrupted or an agent tried something it was not allowed to do.
- Test failure behavior regularly, for example with a deliberately unreachable status list, and confirm that the action is actually denied.

## Role in KYA

KYA applies the principle to the verdict: what is not proven does not count as valid. If essential evidence is missing, the verdict is “indeterminate”, never “valid”.

A term from the flow, not a check of its own. [To the method](https://knowyouragent.ch/en/method)

[Verify now: open a matching example](https://knowyouragent.ch/en/verify?beispiel=ohne-statusliste)

## Frequently asked questions

### What does fail-closed mean?

In case of an error or missing information, an action is denied rather than allowed.

### Why does fail-closed matter for agents?

Because otherwise an agent could act unchecked whenever something fails, such as an unreachable status list.

### How does KYA implement fail-closed?

If core evidence is missing, such as the revocation check, the verdict is not “valid” but “indeterminate”, with reasons in the certificate.

### Is fail-closed always the right choice?

For actions with consequences, such as payments or contracts, yes. For pure read requests, a less strict rule can be acceptable if it is chosen deliberately.

### How does KYA tell invalid from indeterminate?

Invalid means evidence is disproven, for example by a wrong signature. Indeterminate means nothing is disproven, but not everything essential is proven.

### Does fail-closed make systems unreliable?

It makes disruptions visible rather than dangerous. A denied action can be repeated once the disruption is fixed; a payment allowed by mistake often cannot be recovered. Good monitoring keeps such denials rare.

## Sources

- [KYA verification model](https://knowyouragent.ch/en/method/checks)

## Related terms

- [Verifier](https://knowyouragent.ch/en/knowledge/glossary/verifier)
- [Decision token](https://knowyouragent.ch/en/knowledge/glossary/decision-token)

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Quelle: https://knowyouragent.ch/en/knowledge/glossary/fail-closed · KYA – Know Your Agent