Stop hooks with type: "agent" don't appear to fire

I configured a Stop hook with type: "agent" in settings.json to run a session-end review workflow. The config is accepted without error, but the agent doesn't seem to execute when the session ends (v2.1.85).

{
  "type": "agent",
  "prompt": "Review this session and store important findings.",
  "timeout": 120
}

Is type: "agent" supported for Stop hooks? The docs mention it but I can't get it to fire. Command-type Stop hooks work fine.