WorktreeCreate hook: $CLAUDE_PROJECT_DIR not expanded when Agent uses isolation: "worktree"

Summary

When spawning a subagent via the Agent tool with isolation: "worktree", the WorktreeCreate hook fails because $CLAUDE_PROJECT_DIR is not expanded (or not set) in the hook command string. Interactive worktree creation from the same project works fine — the failure is specific to the Agent-tool-spawned path.

Repro

.claude/settings.json:

{
  "hooks": {
    "WorktreeCreate": [
      {
        "matcher": "",
        "hooks": [
          {
            "type": "command",
            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/create-worktree.sh",
            "timeout": 60
          }
        ]
      }
    ]
  }
}

.claude/hooks/create-worktree.sh exists and is executable (chmod +x).

Spawn an Agent with isolation: "worktree".

Expected

Hook runs with $CLAUDE_PROJECT_DIR expanded to the project root; script executes.

Actual

WorktreeCreate hook failed: .claude/hooks/create-worktree.sh: /bin/sh: .claude/hooks/create-worktree.sh: No such file or directory

Note the error shows the relative path .claude/hooks/create-worktree.sh — $CLAUDE_PROJECT_DIR/ was stripped/empty, so the shell tried a relative lookup from whatever CWD the hook runner was in, which was not the project root.

Workarounds considered

• Hardcoding an absolute path breaks for users with concurrent worktrees at different paths.
• cd "$CLAUDE_PROJECT_DIR" && ... hits the same expansion problem.

Environment

• Claude Code (Opus 4.6, 1M context)
• Darwin 25.3.0
• Project is a git repo with worktree-based subagent workflow

Impact

Blocks background Agent tool calls that require isolation: "worktree" whenever a WorktreeCreate hook references $CLAUDE_PROJECT_DIR. Forces fallback to un-isolated agents, losing the safety worktrees provide.