Main-thread CPU spin (100-140%+) persists for hours after a long background Bash task completes (v2.1.222, macOS arm64)

Summary

After a long-running background Bash task with a large subprocess tree completes (e.g. a ~25–40 min test suite launched via run_in_background), the Claude Code CLI's main thread enters a sustained CPU spin (100–140%+) that never settles — it keeps the machine hot for hours until the session process is killed manually. Observed repeatedly across sessions and days; reproduced today with diagnostics attached.

Environment

• Claude Code: 2.1.222 (desktop-app-managed binary: ~/Library/Application Support/Claude/claude-code/2.1.222/claude.app/Contents/MacOS/claude --output-format stream)
• macOS 15.5, Apple Silicon (arm64)

Repro (as observed, several occurrences over recent days)

1. In an interactive session, run a long shell command as a background task (run_in_background: true). In our case: a Python unittest suite of ~6,200 tests taking 25–40 min, which itself forks many short-lived subprocesses. Stdout is redirected to a file, so the harness-visible output is small (the task output files are ≤345 KB — output volume is not the trigger).
2. Let the task complete (exit 0, completion notification delivered normally).
3. Watch the claude process afterward: CPU stays pegged at 100–140%+ indefinitely — including while the session sits idle between turns — until the process is killed.

Diagnostics from today's occurrence

• ps: the session's claude process showed CPU time 55:59 against an elapsed lifetime of 42:29 — i.e. >130% average for its entire life, far exceeding the actual interactive work done.
• No orphaned children: the suite's subprocess tree was fully reaped (no leftover python/node/chromium processes). The burn is in the CLI process itself.
• sample <pid> 3: all 2,406 samples on com.apple.main-thread, in a tight call graph inside the (stripped) claude binary — a pure CPU spin, no syscall/IO wait at the top of the stack:

2406 Thread  DispatchQueue_1: com.apple.main-thread  (serial)
+ 2406 start (in dyld)
+   2406 ??? (in claude) load address 0x104ea0000 + 0xa61388
+     2406 ??? (in claude) + 0x10f6e68
+       2406 ??? (in claude) + 0x10fcf70
+         2406 ??? (in claude) + 0x5735ec
+           2406 ??? (in claude) + 0x117a6ec
+             2288 ??? (in claude) + 0xeef3bc
+             ! 2208 ??? (in claude) + 0x10b8314
+             ! : 1582 ??? (in claude) + 0x830ec0
...

(Full sample output available on request.)

Impact

A part-time operator's laptop runs hot for hours after every heavy background task; multiple long-lived sessions compound it (any of them can enter this state after such a task). Workarounds in use: running scheduled test suites outside Claude Code entirely (launchd), launching in-session long jobs fully detached (nohup … & disown) instead of harness-tracked background tasks, and closing sessions promptly.

Expected

After a background task completes and its notification is delivered, the CLI returns to an idle event loop (~0% CPU).