Agent tool with isolation: "worktree" stacks worktrees inside existing worktree sandbox #485

Symptom

The Claude Code Agent tool, when invoked with isolation: "worktree", creates the new git worktree relative to the calling agent's current working directory. When that calling agent is itself running inside a worktree the harness previously created (i.e. cwd is already .claude/worktrees/<parent>/), the new worktree lands at:

.claude/worktrees/<parent>/.claude/worktrees/<child>/

Each additional level of sub-agent compounds the nesting. git worktree list in this repo currently shows a 4-deep stack on disk:

.claude/worktrees/split-graph/.claude/worktrees/split-analyzer/.claude/worktrees/rebase-472/.claude/worktrees/fix-lpc-stubs

Total path length on that one is ~220 characters before any source file is appended.

Why it matters

On Windows, paths past MAX_PATH (260) cause cargo build scripts (notably cc-rs, when compiling bzip2-sys / libz-sys and similar) to fail with fatal error C1081: 'file name too long'. cc-rs then retries and re-emits the kilobyte-sized PATH env var on every retry per source file. That stderr stream was being piped back to Claude verbatim by the Stop hook, which overflowed the 1M context window — the user's session became unrecoverable mid-task (see #481).

#482 truncated the hook stderr (symptom). #483 added a PreToolUse guard hook on Agent that refuses isolation: "worktree" when cwd is already inside .claude/worktrees/... (local workaround). Neither addresses the root cause: the harness's Agent worktree placement.

Suggested fix (upstream)

The Claude Code Agent tool should anchor isolation: "worktree" paths at the repo root (git rev-parse --show-toplevel), not at the calling agent's cwd. New worktrees would then always land at <repo_root>/.claude/worktrees/<name>/ regardless of where the spawning agent is running.

Alternative: refuse to create a worktree if git rev-parse --show-toplevel differs from the cwd's repo top-level (i.e. cwd is itself a worktree, not the main checkout) and surface a clear error.

Workaround in this repo

The PreToolUse hook in #483 (ci/hooks/worktree_guard.py) detects the nested case and denies it before the harness creates the worktree. This is fbuild-local and should be retired once the harness behavior is fixed.

Links

• Trigger incident: Hook stderr unbounded: cc-rs build-script + MAX_PATH C1081 overflow Claude context #481
• Symptom fix: fix(hooks): truncate subprocess output fed back to Claude #482
• Local workaround: fix(hooks): refuse Agent worktree-isolation calls from nested worktrees #483