[BUG] 2.1.261 macOS arm64: session hangs, main thread 100% CPU in JIT loop while waiting on a background subagent (sample attached)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Mid-session (about 32 minutes in, right after dispatching a background subagent), the TUI stops repainting and accepts no input. The process stays alive but is wedged:

• Main thread R+ at 98–100% CPU continuously; the other 10 threads idle (ps -M).
• RSS flat at ~750 MB (physical footprint 401 MB, peak 423 MB) — not a memory leak.
• Zero child processes. No hook, MCP server or statusline process running.
• All three HTTPS sockets to the API in CLOSED state; a healthy sibling session on the same machine shows 5 ESTABLISHED.
• System memory fine (81% free, swap 1.1 GB). Two other Claude Code sessions on the same Mac kept working normally.
• On an earlier occurrence (2026-08-27, second Mac) SIGTERM was ignored and SIGKILL was required. claude --resume recovers the transcript.

Same signature as #89062 (regression since ~2026-08-21, subagent-heavy sessions) and the older #51560. Per #89062, BUN_JSC_useDFGJIT=0 still hangs and BUN_JSC_useJIT=0 crashes at startup, so JIT tuning is not a workaround.

sample <pid> 5 — main thread, 3637/3637 samples in one identical stack

Load address 0x1041f0000 (binary 2.1.261). Offsets from load address, outermost → innermost:

+0x112efd4 +0x104eb84 +0x1053528 +0xdfa30c +0xfe7b40 +0x79bf4c +0x79b9d8
+0x15ab938 +0x15ab4d4 +0x15abfa0 +0x11286f4 +0x13a0d38 +0x6ed9d4 +0x1a73c8
+0x2bde880 +0x28affe4 +0x35418b4
  [JIT 0x116316224] [JIT 0x1163161f4]
+0x588f44 +0x31d337c +0x3074cbc +0x2dd5068 +0x2dd9b58 +0x35418b4
  [JIT 0x1173ead10]
+0x3564f90 +0x3564f90 +0x3564f90            <- same frame three times
  [JIT 0x115905978] [JIT 0x1169eace8] [JIT 0x115cb3fbc] [JIT 0x1163a7060]
+0x219819c +0x2198318 +0x30d4224 +0x30d4224  <- same frame twice
  leaf (JIT, 100% of samples within 0x115904880–0x115904944):
    2247 0x1159048dc | 1162 0x1159048b4 | 66 0x115904930 | 57 0x11590493c | 51 0x1159048a4 | 43 0x115904880

The leaf never leaves a ~200-byte JIT region, i.e. a tight loop in JavaScript (regex backtracking / O(n²) string or array processing), not a lock or a syscall. Full sample output attached under Additional Information; the process was still spinning after the 5 s window.

Ruled out on this machine

• Fullscreen renderer: reproduced with "tui": "default" (and earlier with fullscreen).
• Hooks / MCP: no child processes at freeze time; project hooks measured 45–100 ms each.
• Memory pressure: 81% free, RSS flat.
• Terminal emulator: seen in Zed, VS Code integrated terminal and Warp.

What Should Happen?

The parent session should keep rendering and accept input while a backgrounded subagent runs, and surface the subagent's completion. If the main thread is starved for more than a few seconds, a watchdog should log the JS stack (and SIGTERM/SIGINT should still work) instead of requiring kill -9.

Ask: symbolicate the offsets above against the 2.1.261 arm64 build.

Error Messages/Logs

No error message: the TUI simply stops updating. Process state at capture time (~32 min after launch, ~25 min after the freeze):

  PID  PPID  %CPU  RSS    STAT ELAPSED TTY      COMM
10745 10578 100.0 750512 R+   32:17   ttys001  claude

ps -M -p 10745 (main thread vs others):
USER   PID   TT   %CPU STAT PRI     STIME     UTIME COMMAND
jm   10745 s001  98.6 R    31T   0:07.37  12:45.65 claude
     10745         0.0 S    31T   0:01.33   0:08.95
     10745         0.0 S    31T   0:00.20   0:00.06
     ... (8 more idle threads)

lsof -i -p 10745 (frozen):
TCP 192.168.0.101:51528->160.79.104.10:https (CLOSED)
TCP 192.168.0.101:50169->165.66.149.34.bc.googleusercontent.com:https (CLOSED)
TCP 192.168.0.101:50158->160.79.104.10:https (CLOSED)
lsof -i -p 8598 (healthy session, same machine, same minute): 5 x TCP ...:https (ESTABLISHED)

===== sample 10745 5 -f  (main-thread excerpt; full 120 KB file available on request) =====
Analysis of sampling claude (pid 10745) every 1 millisecond
Process:         2.1.261 [10745]
Path:            /Users/USER/*/2.1.261
Load Address:    0x1041f0000
Identifier:      2.1.261
Code Type:       ARM64
Parent Process:  zsh [10578]
Date/Time:       2026-09-06 06:58:19.163 +1000
Launch Time:     2026-09-06 06:25:43.724 +1000
OS Version:      macOS 26.6.2 (25G83)
Physical footprint:         401.6M
Physical footprint (peak):  423.3M

Call graph:
    3637 Thread_124058   DispatchQueue_1: com.apple.main-thread  (serial)
    + 3637 start  (in dyld) + 6992  [0x185dc44e4]
    +   3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x112efd4  [0x10531efd4]
    +     3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x104eb84  [0x10523eb84]
    +       3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x1053528  [0x105243528]
    +         3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0xdfa30c  [0x104fea30c]
    +           3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0xfe7b40  [0x1051d7b40]
    +             3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x79bf4c  [0x10498bf4c]
    +               3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x79b9d8  [0x10498b9d8]
    +                 3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x15ab938  [0x10579b938]
    +                   3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x15ab4d4  [0x10579b4d4]
    +                     3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x15abfa0  [0x10579bfa0]
    +                       3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x11286f4  [0x1053186f4]
    +                         3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x13a0d38  [0x105590d38]
    +                           3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x6ed9d4  [0x1048dd9d4]
    +                             3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x1a73c8  [0x1043973c8]
    +                               3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x2bde880  [0x106dce880]
    +                                 3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x28affe4  [0x106a9ffe4]
    +                                   3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x35418b4  [0x1077318b4]
    +                                     3637 ???  (in <unknown binary>)  [0x116316224]
    +                                       3637 ???  (in <unknown binary>)  [0x1163161f4]
    +                                         3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x588f44  [0x104778f44]
    +                                           3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x31d337c  [0x1073c337c]
    +                                             3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x3074cbc  [0x107264cbc]
    +                                               3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x2dd5068  [0x106fc5068]
    +                                                 3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x2dd9b58  [0x106fc9b58]
    +                                                   3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x35418b4  [0x1077318b4]
    +                                                     3637 ???  (in <unknown binary>)  [0x1173ead10]
    +                                                       3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x3564f90  [0x107754f90]
    +                                                         3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x3564f90  [0x107754f90]
    +                                                           3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x3564f90  [0x107754f90]
    +                                                             3637 ???  (in <unknown binary>)  [0x115905978]
    +                                                               3637 ???  (in <unknown binary>)  [0x1169eace8]
    +                                                                 3637 ???  (in <unknown binary>)  [0x115cb3fbc]
    +                                                                   3637 ???  (in <unknown binary>)  [0x1163a7060]
    +                                                                     3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x219819c  [0x10638819c]
    +                                                                       3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x2198318  [0x106388318]
    +                                                                         3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x30d4224  [0x1072c4224]
    +                                                                           3637 ???  (in 2.1.261)  load address 0x1041f0000 + 0x30d4224  [0x1072c4224]
    +                                                                             2247 ???  (in <unknown binary>)  [0x1159048dc]
    +                                                                             1162 ???  (in <unknown binary>)  [0x1159048b4]
    +                                                                             66 ???  (in <unknown binary>)  [0x115904930]
    +                                                                             57 ???  (in <unknown binary>)  [0x11590493c]
    +                                                                             51 ???  (in <unknown binary>)  [0x1159048a4]
    +                                                                             43 ???  (in <unknown binary>)  [0x115904880]
    +                                                                             5 ???  (in <unknown binary>)  [0x115904928]
    +                                                                             3 ???  (in <unknown binary>)  [0x1159048b0]
    +                                                                             2 ???  (in <unknown binary>)  [0x1159048d0]
    +                                                                             1 ???  (in <unknown binary>)  [0x115904944]
[other 10 threads (mi-scavenger, Bun Pool 0-8): all idle in __ulock_wait / __ulock_wait2 / kevent64]

Binary Images:
       0x1041f0000 -        0x107fef9bf +2.1.261 (???) <4C4C4401-5555-3144-A13C-E92490FFBCBC> /Users/*/2.1.261
       0x11b874000 -        0x11b98fb4f +.bun-501-b163af53257b65ef.node (0) <DFCF136E-D26C-3F3F-99BF-059726A371DE> /var/folders/*/.bun-501-b163af53257b65ef.node

Steps to Reproduce

No deterministic reproducer; it happens 30 min to 2 h into sessions that use subagents (same as #89062). Exact sequence of the captured occurrence:

1. cd into a project whose .claude/settings.json defines PreToolUse, UserPromptSubmit and SessionStart command hooks (bash + jq); start claude in auto permission mode, classic renderer ("tui": "default"), from Zed's integrated terminal (also seen in VS Code terminal and Warp).
2. Prompt: "pull the latest version" → Claude runs one Bash call (git pull --ff-only … && git log … && git status) and answers. Status line shows "1 shell still running".
3. Run a project slash command (/pm-loop) whose instructions make Claude run three short Bash calls, CronList, then dispatch a background subagent via the Agent tool (project-manager, model Opus 5 with 1M context). The UI shows "Backgrounded agent" and Claude prints "Waiting for the tick's completion notification", spinner "Waiting for 1 background agent to finish".
4. The screen never updates again. The background agent's result is never surfaced. Ctrl+C does nothing.
5. From another terminal: ps -M -p <pid> shows the main thread at ~100% CPU; sample <pid> 5 shows one identical stack for every sample (above).
6. Only kill -9 <pid> ends it; claude --resume recovers the transcript.

So the hang happens while the parent turn is idle waiting on a backgrounded subagent (plus one still-running background shell), i.e. in the background-agent / task-notification path, not while streaming. Possibly related to the 2.1.261 changelog entry "Fixed sustained high CPU usage when a background agent could not be resumed".

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown. Did not happen before ~mid/late August 2026 on either of two Macs; #89062 dates the same regression to ~2026-08-21 (2.1.231+). Reverting to a much older build was tried but that build was unusable for other reasons.

Claude Code Version

2.1.261 (Claude Code) — native install; also reproduced on 2.1.252, 2.1.258, 2.1.259

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

Environment

• Hardware: MacBook Air M1 (MacBookAir10,1), 8 cores, 16 GB. Also seen on a second Mac cloned from the same config.
• macOS 26.6.2 (25G83); earlier occurrences on macOS 26.3 (25D125).
• Install: native (claude.ai/install.sh), binary ~/.local/share/claude/versions/2.1.261, autoUpdates: false.
• Terminal: Zed 1.18.1 integrated terminal (login -> zsh -> claude). Previously reproduced in VS Code integrated terminal and Warp, so terminal-independent.
• Renderer: "tui": "default" at the time of this capture; earlier freezes with "tui": "fullscreen".
• Permission mode: auto. Output style: Brief. Status line: a bash+jq script (~120 ms).
• Project-level hooks (all bash+jq, measured): PreToolUse (no matcher, 79 ms; Bash matcher, 97 ms), UserPromptSubmit (1.3 s), SessionStart (2.7 s). No MCP servers configured in this project. Plugins enabled: superpowers, typescript-lsp, security-guidance (official marketplace).
• Frequency: several times per day across sessions in this and sibling projects; always during or after subagent use; two other sessions on the same machine at the same time were unaffected.

The main-thread excerpt of sample <pid> 5 -f (1 ms interval, 3637 samples) is in the Logs field above, with the Binary Images UUID for symbolication. The full 120 KB sample (all threads) is saved locally and I can attach it on request.

A second, older sample from 2026-08-27 (different Mac, 2.1.24x) showed the same shape: 100% CPU main thread, flat RSS, SIGTERM ignored. Happy to run any diagnostic build or --debug flags on the next occurrence — it recurs within hours.

Not a duplicate of #83237 (there the main thread is idle and mi-scavenger spins; here the main thread spins and every other thread is idle).