Headless claude -p deadlocks in uninterruptible openat on macOS; interactive sessions unaffected

Date: 2026-08-24

Summary

Every headless invocation (claude -p) on this machine deadlocks partway into the session:
main thread blocked forever in openat$NOCANCEL, 0% CPU, no TCP connections, no child
processes. Interactive sessions on the same machine, same account, are unaffected.
8/8 reproductions: 5 via a no-mistakes pipeline daemon spawning claude -p --output-format
stream-json --json-schema ... --dangerously-skip-permissions, and 3 spawned directly from a
shell with stdout redirected to a file (with and without --debug, with and without
CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1).

Signature

• sample <pid>: main thread parked in openat$NOCANCEL (1500+/1500+ samples), JIT (JS)

frames above; worker threads idle in kevent64/pool waits.

• No TCP sockets open (lsof); not a network hang.
• No child processes; not a tool-subprocess hang.
• SIGKILL sometimes fails to reap the process (uninterruptible syscall).
• Freeze point varies by run: after ~2 tool rounds (transcript event 19, last =

total_tokens_reminder attachment), at ai-title (event 9), or at atis-latch (event 20).
Always coincides with concurrent background activity (session-title request in flight,
statsig/bootstrap fetch, atomic ~/.claude.json rewrite).

• With --debug: log stops mid-stream right after two concurrent /v1/messages requests

(source=generate_session_title + source=sdk) both received first bytes.

Ruled out

• no-mistakes daemon (reproduces with plain claude -p from a shell)
• claude version regression (2.1.235 and 2.1.241 both freeze)
• permission-prompt deadlock (--dangerously-skip-permissions present in pipeline repros)
• stdout backpressure (reproduces with stdout redirected to a regular file)
• hung file provider (Google Drive + fileproviderd restarted; CloudStorage lists fine)
• FIFOs / dead symlinks in cwd, skills, agents, plugins trees (none exist)
• keychain-db file locks (no holders)
• nonessential traffic (CLAUDE_CODE_DISABLE_NONESSENTIAL_TRAFFIC=1 still freezes)

Not yet done (needs sudo)

fs_usage/spindump or lldb attach to capture the blocked path argument — lldb attach is
denied by SIP for a same-user attach on this machine.

Repro

cd <any git repo>
echo 'Read README.md, then read pyproject.toml, then reply with one sentence.' | claude -p
# hangs forever; sample the pid to see openat$NOCANCEL

Impact

Any local tooling that drives claude -p (review pipelines, cron jobs, CI-style local gates)
wedges silently. Workaround in use: pipeline agent switched to codex; Claude review still
applied via GitHub Actions (claude-review.yml) and interactive sessions.