[BUG] Desktop (Windows): file preview for WSL sessions broken since 1.37937.1 — "Couldn't preview this file. It couldn't be read from the remote session."

Environment

• Claude Desktop for Windows 1.37937.3 (28dcf5), MSIX/Store install (C:\Program Files\WindowsApps\Claude_1.37937.*), auto-update
• Bundled Claude Code daemon (CCD): 2.1.246
• Windows 11 + WSL2 (Debian), sessions run in WSL via the Desktop app's WSL integration ([SSH] Using remote spawn function ... on wsl:Debian)
• Last working version: Desktop 1.34493.1 (CCD 2.1.237)

Summary

Files sent by an agent via SendUserFile (e.g. an HTML report with display: "render") from a WSL session no longer render in the Desktop chat. The file card appears, but opening the preview shows:

Couldn't preview this file It couldn't be read from the remote session. Check that the session is connected, then open the file again.

This worked reliably until the auto-update from 1.34493.1 to 1.37937.1 (installed 2026-08-26 10:34 local). It is still broken in 1.37937.3.

Evidence from Desktop logs (%LOCALAPPDATA%\Claude\logs\main.log)

Before the update, each rendered HTML file spawned a dedicated preview server; e.g.:

2026-08-26 07:59:18 [info] [Launch] Updating active servers store { count: 1, servers: '[{"serverId":"html-preview-1787723958149-0","name":"2026-08-25.html","sessionId":"local_984f3533-...","port":0,"status":"running","filePath":"/home/re/workday-summaries/2026-08-25.html"}]' }

That was the last successful render. The update landed the same day:

2026-08-26 10:34:58 [info] [updater] Version changed since last launch: 1.34493.1 → 1.37937.1

After the update:

• No html-preview-* server is ever created again.
• The startup line [CoworkFilePreview] Protocol handler registered (present at every app start on 1.34493.1) no longer appears on 1.37937.x. ([SessionMedia] claude-media:// protocol registered appears on both versions, so that's not the change.)
• Clicking the preview produces zero log output in main.log and no request on the WSL side (the SSH bridge log shows no file read, no error) — the failure is entirely inside the chat webview before any IPC/bridge call is made.
• New on 1.37937.x: [MSIX] Filesystem virtualization active — C:\Users\...\Packages\Claude_...\LocalCache\Roaming\Claude exists (possibly related if the new preview path does direct FS access).

Repro

1. Windows + WSL2, Claude Desktop ≥ 1.37937.1, session running in WSL.
2. Have the agent send any HTML file via SendUserFile with display: "render" (WSL-native path like /home/re/report.html).
3. Preview fails with the error above.

Not path-dependent

Sending the identical file from a Windows-drive path visible in WSL (/mnt/c/Users/<user>/Downloads/report.html) fails with the same error — the preview resolves every path through the session, and it's the webview→session byte channel that's broken.

Related (but not identical) open issues

• #67869 — SendUserFile images never render in Desktop chat (local session)
• #48466 — preview pane unavailable in remote-control sessions
• #72715 — local preview failures on Windows + WSL

Those describe a long-standing gap; this report is specifically a regression: WSL-session previews demonstrably worked on 1.34493.1 (log evidence above) and stopped with 1.37937.1.