Agent hooks fail: Messages are required for agent hooks

Bug Description

Agent hooks of "type": "agent" configured in PreToolUse fail with the error:

PreToolUse:Bash hook error
Failed to run: Messages are required for agent hooks. This is a bug.

The error message itself acknowledges this is a bug ("This is a bug").

Steps to Reproduce

1. Configure an agent hook in settings.json:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Bash",
        "hooks": [
          {
            "type": "agent",
            "if": "Bash(gh pr-create:*)",
            "prompt": "Review the PR before creation...",
            "model": "claude-sonnet-4-6",
            "timeout": 180,
            "statusMessage": "Pre-PR architectural review..."
          }
        ]
      }
    ]
  }
}

(Note: the actual if pattern uses pr create with a space — adjusted here to avoid triggering commit hooks in this issue body)

1. During a conversation, trigger a PR creation command
2. The agent hook fires but immediately fails with the error above

Expected Behavior

The agent hook should receive the conversation messages and execute the sub-agent review as configured.

Actual Behavior

The hook errors with "Messages are required for agent hooks. This is a bug." The underlying tool call still executes successfully — the hook error is non-blocking — but the agent-based review never runs.

Environment

• Claude Code version: 2.1.104
• OS: macOS (Darwin 24.2.0)
• Shell: zsh

Additional Context

• The error appears to originate from the agent hook runner not receiving/passing conversation messages to the sub-agent
• Other hook types ("type": "command") on the same matcher work fine
• The "if" conditional pattern matching works correctly (hook only fires on PR creation commands)
• The hook is non-blocking, so the tool call succeeds despite the error