Response schema

The LLM must respond with JSON containing:

{
  "ok": true | false,
  "reason": "Explanation for the decision",
  "impossible": true | false
}

Field	Description
ok	true to allow. For false, see the per-event behavior below
reason	Required when ok is false
impossible	Optional. The model returns it with ok: false when it judges the condition can never be satisfied. On Stop and SubagentStop, Claude Code then lets the turn end instead of feeding the reason back. Agent hooks and other events ignore it

What happens on ok: false depends on the event:

• Stop and SubagentStop: the reason is fed back to Claude as its next instruction and the turn continues, unless the response also sets impossible: true, in which case Claude Code allows the stop and the turn ends
• PreToolUse: the tool call is denied; by default the turn ends and the deny reason appears in the chat as a warning line. Set continueOnBlock: true to instead return the reason to Claude as the tool error so it can adjust and continue, equivalent to a command hook's permissionDecision: "deny". Before v2.1.210, the deny reason was returned to Claude as the tool error and the turn continued
• PostToolUse: by default the turn ends and the reason appears in the chat as a warning line. Set continueOnBlock: true to feed the reason back to Claude and continue the turn instead
• PostToolBatch, UserPromptSubmit, and UserPromptExpansion: the turn ends and the reason appears as a warning line. These events end the turn on decision: "block" regardless of continue
• PostToolUseFailure and TaskCreated: the reason is returned to Claude as a tool error and the turn continues, regardless of continueOnBlock
• TaskCompleted: when it fires because a task is marked completed during a turn, the reason is returned to Claude as a tool error and the turn continues, regardless of continueOnBlock. When it fires because a teammate stops, it behaves like TeammateIdle and halts the teammate by default
• TeammateIdle: by default the teammate stops and the reason appears as a warning line. Set continueOnBlock: true to feed the reason back to the teammate and keep it working instead
• PermissionRequest: ok: false has no effect. To deny an approval from a hook, use a command hook returning hookSpecificOutput.decision.behavior: "deny"
• PermissionDenied: ok: false has no effect because the denial already happened. The only output this event reads is hookSpecificOutput.retry, which prompt and agent hooks can't set. They run on this event, but their output is discarded. Use a command hook to return retry

If you need finer control on any event, use a command hook with the per-event fields described in Decision control.