Glossary
Fail closed
A failure policy in which, if a safety check cannot run, the action is held or blocked rather than allowed.
Checks run on our GPUs in Switzerland. Request payloads are not stored.Security
What Fail closed means
A system fails closed when, if a control cannot give an answer, it defaults to the safe outcome: the door stays locked. The opposite, failing open, lets the action through when the check is unavailable.
For a pre-execution guard, the question is what your agent does when the guard times out, returns an error or cannot be reached. For low-impact, read-only calls, failing open may be an acceptable trade for availability. For destructive or externally visible actions, treating an unavailable guard as ask or block is usually the right default.
MCP Guard returns a 503 on retryable failures and charges nothing for them, so a short retry followed by a fail-closed fallback is a reasonable pattern. The choice is yours to make and document; the guard cannot make it for you.
Related terms
How Fail closed connects to the rest of the vocabulary.