[BUG] VSCode Remote SSH: Extremely slow tool execution / UI unresponsive (regression in 2.1.20+)

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?

Description

Claude Code becomes extremely slow on Remote SSH connections. Tool calls that should complete quickly take very long or appear to hang indefinitely. In some cases, clicking "Stop" or reloading VSCode causes outputs to appear: suggesting work may have completed but wasn't displayed. In other cases, it's unclear if the slowness is actual processing time or a UI/streaming issue.

Environment

• Claude Code Version: 2.1.25 (also affects 2.1.23, 2.1.22, 2.1.21, 2.1.20)
• Working Version: 2.1.19 and earlier
• OS: Linux (SLURM cluster login node)
• VSCode: 1.108.2 via Remote SSH extension
• Filesystem: Home on ext4, working directory on BeeGFS (parallel network FS)

Symptoms

• Extension shows working indicator ("Tinkering...", "Wibbling...") for extended periods
• Simple tool calls (file reads, bash commands) take minutes instead of seconds
• Sometimes outputs appear after clicking Stop/reload
• Extension logs show high-frequency polling (~2000+ get_session_request calls per session)

What Should Happen?

Tool calls should execute and display results in real-time, as they do in version 2.1.19 and when running locally (not over Remote SSH).

Error Messages/Logs

During initial downgrade attempt to 2.1.20:
API Error: 400 due to tool use concurrency issues.

Extension logs show excessive polling:
- 2000+ `get_session_request` calls per session
- Constant `Getting authentication status` / `OAuth tokens found in secure storage` spam

Steps to Reproduce

1. Connect to a remote Linux server via VSCode Remote SSH
2. Install Claude Code extension version 2.1.20 or later
3. Open Claude Code panel and start a conversation
4. Send a prompt that triggers tool use (e.g., "list files in current directory")
5. Observe: UI shows working indicator but no output appears for extended time
6. Click Stop or reload VSCode window
7. Observe: Tool outputs may suddenly appear, indicating work completed but wasn't displayed

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.19

Claude Code Version

2.1.25

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

Diagnostic Steps Taken

Things I tested that were NOT the cause:

| Check | Result |
|-------|--------|
| Network latency to Anthropic API | ✅ Fine (~120ms) |
| API connectivity | ✅ Established connections |
| System memory | ✅ 83GB available |
| CPU limits (cgroups) | ✅ 4 cores, no throttling |
| ulimits | ✅ 1M open files |
| Disk space | ✅ Sufficient |
| ~/.claude location | ✅ On ext4 /home, not slow network FS |
| OOM kills / kernel throttling | ✅ None |
| File locks | ✅ Normal |
| Process state | ✅ Running, not stuck |

Suspicious findings (not root cause):

• Multiple Claude processes running simultaneously (3)
• Large session files (17MB, 10MB, 3.4MB .jsonl)
• 817 files in ~/.claude/todos/
• Excessive polling in logs

My attempts before diagnosis:

• Removed files and reduced home directory size
• Checked CPU usage
• Killed SLURM jobs
• Reloaded VSCode window multiple times
• Symlinked ~/.claude to working directory (did not help)

Downgrade attempts:

1. Downgrade to 2.1.20 → Got "API Error: 400 due to tool use concurrency issues"
2. Downgrade to 2.1.19 → Issue resolved ✅

Related Issues

• #11689 - VSCode Extension UI freezes 26+ minutes on Remote SSH (closed NOT_PLANNED)
• #10181 - Bash tool runs extremely slowly on Linux (open, 27 👍)
• #20571 - Tool use concurrency error after Jan 23-24 update
• #21825 - Broken in v2.1.25, works in v2.1.19
• #21097 - API Error 400 on every tool call
• #20632 - MCP server causing concurrency 400 errors

Workaround

Downgrade to version 2.1.19 resolves the issue.