HOOK · Anthropic docs

Block when a hook fails (onFailure)

Set onFailure: "block" on a command or HTTP hook so a missing script, crash or timeout blocks the action instead of letting it through.

proof: logs blocks: verification hook silently failing open

https://code.claude.com/docs/en/hooks
Checked 2026-10-16. Read the original before installing.

Harness	Status	How
Cursor	adapted	Use failClosed: true.
Claude Code	native	Requires Claude Code v2.1.295+. Note: no effect on Stop hooks.
Grok Build	adapted	Not documented for Grok Build; hooks fail open.
Codex	untested	Untested.
AGENTS.md generic	unsupported	Harness feature.

"source" = the source targets it; "untested" = not verified by ProofOfDone yet.