bug: PreToolUse hook updatedInput is ignored for the Agent tool

Summary

PreToolUse hooks that return hookSpecificOutput.updatedInput work for tools like Bash but are silently ignored for the Agent tool. This prevents programmatic model selection for subagents.

Steps to Reproduce

1. Configure a PreToolUse hook for Agent tool calls:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Agent",
        "hooks": [{
          "type": "command",
          "command": "python3 /path/to/model_tier_hook.py",
          "timeout": 5
        }]
      }
    ]
  }
}

1. Hook script returns:

{
  "hookSpecificOutput": {
    "hookEventName": "PreToolUse",
    "permissionDecision": "allow",
    "updatedInput": {
      "subagent_type": "python-engineer",
      "prompt": "...",
      "description": "...",
      "model": "sonnet"
    }
  }
}

1. Hook fires correctly (confirmed via file logging)
2. Agent spawns with parent model (Opus) instead of sonnet

Expected Behavior

The updatedInput should be applied to the Agent tool call, causing the subagent to use model: "sonnet".

Actual Behavior

updatedInput is silently ignored. The Agent tool uses the parent model regardless of what the hook returns.

Related

• #44385 — agent frontmatter model: field is also ignored
• Both issues together mean there is NO programmatic way to set subagent models — only the calling agent's explicit model parameter works

Environment

• Claude Code CLI (latest)
• macOS Darwin 25.3.0