[BUG] Desktop app (Windows): Electron main process pins a full CPU core during agent work — whole app freezes; reproduces across 1.32885 and 1.49585

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

• App: Claude desktop app 1.49585.0.0 (MSIX, Claude_pzs8sxrjxfjjc); also reproduced on 1.32885.1.0
• Bundled Claude Code runtime: 2.1.260 (previously reproduced on 2.1.234)
• OS: Windows 11 Pro 25H2, build 10.0.26200, September 2026 cumulative installed
• Hardware: desktop, 20 logical cores, Intel UHD 770 (driver 32.0.101.7088)

Symptoms

Intermittent freezes of the entire app — input, scrolling, all UI — correlated
with agent activity (file generation, code execution, streamed tool output).
Typing lag between freezes. Ongoing since ~mid-August 2026 across app updates.

Measurements

Per-process CPU deltas sampled over 6 s during freeze episodes
(TotalProcessorTime; process types from --type= flags):

App 1.49585 / runtime 2.1.260 (2026-09-09), 3 session tabs open:

| Process | Mem (MB) | CPU-sec in 6 s |
|-----------------------------|----------|----------------|
| Electron main (no --type) | 520 | 6.31 |
| other app processes (each) | 122–461 | ≤ 0.55 |
| session children (each) | ~280–296 | ≤ 0.11 |

App 1.32885 / runtime 2.1.234 (2026-09-05), 4 session tabs open:

| Process | Mem (MB) | CPU-sec in 6 s |
|-------------------|----------|----------------|
| Electron main | 378 | 4.61 |
| renderer | 447 | 0.27 |
| gpu-process | 226 | 0.17 |
| session children | 420–489 | ≤ 0.06 |

The main process saturates ≥1 core while renderer and GPU are essentially
idle; since window input and IPC route through the Electron main process,
this explains the whole-app freeze. Each open session tab runs a
claude.exe --output-format stream-json --verbose ... child whose streamed
output (including full tool results) the main process ingests single-threaded;
freezes coincide with bursts of this traffic.

Ruled out on this machine (tested individually)

• Norton 360: fully uninstalled (was injecting aswhook.dll; removal did not fix)
• Gigabyte kernel drivers (gdrv3/uiomap): unloaded and disabled — no change
• Grammarly / screen-recorder hooks: exited — no change
• GPU driver: updated Oct-2024 → Jun-2026 build — no change
• Windows: Aug + Sep 2026 cumulatives — no change
• GPU/disk/network load during freezes: all near idle (5% GPU, 0.2 MB/s disk)
• A concurrent Adobe Acrobat freeze issue on the same machine was separately

traced to an Adobe-acknowledged Acrobat 26.x bug — unrelated

Clear Cache and Restart gives temporary relief; lag/freezes return with use.

Suggested direction

Move stream-json ingestion/persistence for session children off the Electron
main thread, or batch/throttle it. Severity scales with number of open session
tabs and with session length.

Possibly related

#55149, #42045, #31643

What Should Happen?

No freeze should happen

Error Messages/Logs

Steps to Reproduce

1. On Windows 11, open the Claude desktop app with 3–4 session tabs (each tab spawns a claude.exe --output-format stream-json child process).
2. In one session, ask Claude to do output-heavy agent work — e.g. generate a PowerPoint/large file, or run code producing large tool results.
3. While output streams, interact with the app: type in the input box, scroll the conversation.
4. Observe: typing lags by seconds, then the entire app window becomes unresponsive (all tabs — input, scrolling, buttons).
5. In Task Manager, observe the Claude process group at ~5% CPU on a 20-core machine (= one core pinned) with "Very high" power usage. Per-process sampling shows the Electron main process (Claude.exe, no --type= flag) consuming a full core while renderer, GPU, and session-child processes stay near idle.

Frequency: intermittent but reliably triggered by heavy streamed output; worsens with session length and number of open tabs. Temporarily relieved by Help → Troubleshooting → Clear Cache and Restart; returns with use.

Claude Model

Not sure / Multiple models

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.260 (bundled in Claude desktop app 1.49585.0.0, Windows MSIX). Also reproduced on 2.1.234 / app 1.32885.1.0.

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_