[BUG] Desktop (Windows): a session and its linked sessions each keep a claude.exe process at 100% of one core after their turns end; quit warns a task is active

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?

Environment: Claude Desktop 2.42304.0 (MSIX build, bundled Claude Code 2.1.299), Windows 11 Pro 10.0.26200, x64. Remote Control on for the affected sessions.

What happens: a Code-tab session (Opus, effort xhigh, permission mode auto) started three linked sessions with start_session (Sonnet, effort high). Each linked session wrote one doc and ended its turn between 23:52 and 23:53 UTC, and the parent ended its turn at 00:15 UTC. The session list then reported all four as isRunning: false, but their four claude.exe --output-format stream-json ... processes each stayed at about one full core for over two hours:

| Process | Started (local) | CPU time | CPU over a 5 s sample |
|---|---|---|---|
| Parent (Opus, xhigh) | 7:45:06 PM | ~7,600 s | 5.3 s |
| Linked session 1 (Sonnet, high) | 7:49:16 PM | ~7,570 s | 5.5 s |
| Linked session 2 (Sonnet, high) | 7:49:16 PM | ~7,560 s | 5.5 s |
| Linked session 3 (Sonnet, high) | 7:49:16 PM | ~7,550 s | 5.6 s |

• Each process's CPU time roughly matched its age, so each was spinning from soon after it started, through its turn and long after it.
• Each process's only child was conhost.exe; no shell, MCP server or tool process was running under it.
• Memory stayed flat at ~220–270 MB per process.
• The three linked sessions were all launched with --add-dir=%APPDATA%\Claude\side-session-notes\local_<parent-id>.
• Other idle Code-tab session processes running at the same time on the same Claude Code version, including linked sessions of other parents, stayed near 0% CPU.
• Quitting the app showed a prompt saying a task was still active, although no session was running.

Workaround: killing the four PIDs. The transcripts were already saved, so nothing was lost, and total CPU across Claude processes fell from ~4.4 cores to ~0.5.

What Should Happen?

When a session's turn ends, its process goes idle (near 0% CPU). Quitting doesn't warn about an active task when no session is running.

Error Messages/Logs

No relevant logs. The desktop app logs and ~/.claude/debug had nothing from this run.
Each session's transcript ends normally with a stop_hook_summary entry (preventedContinuation: false, no hookErrors).

Steps to Reproduce

I haven't reproduced it a second time. These are the steps that led to it:

1. In Claude Desktop on Windows, open a Code-tab session with Opus at effort xhigh, permission mode auto, Remote Control on.
2. Have it start three linked sessions with start_session (Sonnet, effort high), each writing one doc into the repo.
3. Let all four finish their turns.
4. In Task Manager, each of the four session claude.exe processes stays at about 100% of one core. Quitting the app warns that a task is active.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.299 (bundled with Claude Desktop 2.42304.0)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Terminal: the Claude Desktop Code tab, not a terminal. I can share the transcripts privately if that helps.

Possibly related: #19393 (Claude Code instances at 100%+ CPU while idle). #96378 involves V8 GC thrash with growing memory, but memory stayed flat here.