PreToolUse hook with exit 0 and valid hookSpecificOutput displayed as 'hook error'

Description

When a PreToolUse hook exits with code 0 and returns valid JSON with hookSpecificOutput.additionalContext, the Claude Code UI displays it as "hook error", which is confusing — the hook succeeded and the tool operation was not blocked.

Steps to reproduce

1. Configure a PreToolUse hook in ~/.claude/settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "/path/to/hook.sh",
            "statusMessage": "Checking..."
          }
        ]
      }
    ]
  }
}

1. The hook script outputs valid JSON and exits 0:

#!/bin/bash
INPUT=$(cat 2>/dev/null)
FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty' 2>/dev/null)

case "$FILE_PATH" in
  */.claude/memory/*)
    jq -n --arg msg "Some context to inject" \
      '{hookSpecificOutput:{additionalContext:$msg}}'
    ;;
esac

1. Verification:

$ echo '{"tool_input":{"file_path":"~/.claude/memory/test.md"}}' | ./hook.sh; echo "exit: $?"
{
  "hookSpecificOutput": {
    "additionalContext": "Some context to inject"
  }
}
exit: 0

1. When Claude triggers an Edit or Write to a matching path, the UI shows:

⎿  PreToolUse:Edit hook error

Expected behavior

The UI should not display "hook error" when the hook exits 0 with valid output. Either show nothing, or show the status message ("Checking...") briefly.

Actual behavior

Displays "PreToolUse:Edit hook error" / "PreToolUse:Write hook error" every time the hook fires, even on success.

Environment

• macOS (Darwin 25.4.0)
• Claude Code CLI