PostToolUse hooks show "hook error" despite successful execution (exit 0, valid JSON)

Environment

• Claude Code version: 2.1.39
• OS: Linux (WSL2) - Linux 6.6.87.2-microsoft-standard-WSL2
• Project: /mnt/c/Users/MarcoAniballi/Radicle/alpha-delta-context

Issue

PostToolUse:Bash hooks display "PostToolUse:Bash hook error" in the UI even though all hooks execute successfully.

Evidence of Success

• ✅ All hooks return valid JSON: {"hookSpecificOutput": null, "decision": null}
• ✅ Debug logging shows SUCCESS! after each execution
• ✅ Exit code: 0
• ✅ No stderr errors
• ✅ Hooks complete within timeout (3s/5s/10s)
• ✅ Functionality works correctly (compression, instinct extraction, audit logging all functional)

Hooks Configuration

{
  "matcher": "Bash",
  "hooks": [
    {
      "type": "command",
      "command": "python3 \"$CLAUDE_PROJECT_DIR/.claude/hooks/posttool-output-compression.py\"",
      "timeout": 10
    },
    {
      "type": "command",
      "command": "python3 \"$CLAUDE_PROJECT_DIR/.claude/hooks/posttool-instinct-extractor.py\"",
      "timeout": 5
    },
    {
      "type": "command",
      "command": "python3 \"$CLAUDE_PROJECT_DIR/.claude/hooks/posttool-audit-logger.py\"",
      "timeout": 3
    }
  ]
}

Observed Behavior

Every Bash command shows 3x "PostToolUse:Bash hook error" messages despite hooks working correctly.

Example output:

● Bash(echo "test")
  ⎿  test
  ⎿  PostToolUse:Bash hook error
  ⎿  PostToolUse:Bash hook error
  ⎿  PostToolUse:Bash hook error

Expected Behavior

No error messages when hooks exit 0 with valid JSON.

Related Issues

Similar to #17088 (PreToolUse hooks showing false errors)

Contact

marco@novarra.vc