[Bug] Multiple concurrent instances cause version downgrade (2.0.70 → 2.0.65)

[mithro]

Description

When running multiple Claude Code instances simultaneously, the auto-update mechanism can downgrade the symlink to an older version instead of maintaining the newest installed version.

Observed Behavior

• Version 2.0.70 was downloaded and installed at 10:22
• At 10:48, the symlink was changed to point to 2.0.65 (an older version)
• 8 concurrent Claude Code instances were running at the time

Evidence

Installed versions:

$ ls -la ~/.local/share/claude/versions/
-rwxr-xr-x 1 tim tim 211547692 Dec  4 08:34 2.0.58
-rwxr-xr-x 1 tim tim 205340223 Dec 11 15:16 2.0.65
-rwxr-xr-x 1 tim tim 207455556 Dec 12 14:00 2.0.67
-rwxr-xr-x 1 tim tim 207576789 Dec 13 11:31 2.0.69
-rwxr-xr-x 1 tim tim 212229481 Dec 16 10:22 2.0.70  ← newest

Symlink points to older version:

$ ls -la ~/.local/bin/claude
lrwxrwxrwx 1 tim tim 45 Dec 16 10:48 /home/tim/.local/bin/claude -> /home/tim/.local/share/claude/versions/2.0.65

Concurrent instances running:

$ pgrep -a claude
1891488 claude
2553510 claude --resume
2554915 claude --resume
2555979 claude --resume
2557322 claude --resume
2558340 claude --resume
2563536 claude
2563997 claude

Lock files show activity for both versions:

$ ls -la ~/.local/state/claude/locks/
drwxrwxr-x 2 tim tim 4096 Dec  9 14:44 2.0.58.lock
drwxrwxr-x 2 tim tim 4096 Dec 16 10:51 2.0.65.lock
drwxrwxr-x 2 tim tim 4096 Dec 16 10:40 2.0.70.lock

Expected Behavior

When multiple instances race to update the symlink, the updater should:

1. Compare version numbers before changing the symlink
2. Never downgrade to an older version
3. Use atomic operations or proper locking to prevent races

Root Cause Hypothesis

The update mechanism appears to:

1. Download a new version
2. Update the symlink to point to the downloaded version

When multiple instances run concurrently, an older instance that started its update earlier may complete its symlink update after a newer instance, overwriting the symlink with an older version.

Suggested Fix

Before updating the symlink, compare the target version against the current symlink target:

# Pseudocode
current_version = readlink(~/.local/bin/claude) | extract_version
new_version = downloaded_version
if semver_compare(new_version, current_version) > 0:
    atomic_symlink_update(new_version)
else:
    skip_symlink_update()  # Already on newer version

Environment

• Platform: Linux (Ubuntu 24.04)
• Kernel: 6.14.0-36-generic
• Architecture: x86_64
• Node.js: v18.19.1
• Shell: bash
• Installation method: Native installer (symlink-based)

Related Issues

• #12444 - Auto-update fails with ENOTEMPTY when multiple CLI instances are running
• #7373 - (duplicate target of #12444)
• #2561 - Something keeps installing old version of Claude every day
• #13280 - Multiple Claude Code instances fail after update to 2.0.60
• #13705 - Update process leaves stale lock file, blocking subsequent updates

Workaround

Manually fix the symlink:

ln -sf ~/.local/share/claude/versions/2.0.70 ~/.local/bin/claude