[BUG]

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?

Title: Claude Desktop 1.12603.1 (Windows): extension installation silently fails for all extensions after auto-update

Summary

After Claude Desktop auto-updated from 1.11847.5 to 1.12603.1 on Windows, installing
any extension no longer works — both Anthropic directory extensions and local .mcpb
bundles. There is no error message; the extension simply never installs and does not
appear in Settings → Extensions. Downgrading to 1.11847.5 restores normal behavior.

Environment

• OS: Windows 11 (win32, x64)
• Claude Desktop: 1.12603.1 (broken), built-in Node v24.16.0
• Previous working version: 1.11847.5, built-in Node v24.15.0
• AV: Microsoft Defender only (no third-party EDR)
• Org allowlist: disabled (Organization allowlist enabled: false)
• No Claude policies in registry (HKLM/HKCU SOFTWARE\Policies\Claude empty)

Steps to reproduce

1. On Claude Desktop 1.12603.1, go to Settings → Extensions.

2a. Install a directory extension (e.g. Filesystem / PDF server): click Install.
2b. Or Advanced settings → Install Extension… → select a local .mcpb file.

1. Observe.

Expected

Extension downloads/unpacks, MCP server launches, extension appears and is usable.

Actual

• Directory install: bundle downloads to Temp, log shows Installing unsigned extension,

then nothing — no server launch, extension never registered.

• Local .mcpb install: file picker accepts the file, but nothing happens at all

(no error dialog, no log entry, no install).

• Affects every extension tried, including Anthropic-published ones.

main.log evidence (directory install, version 1.12603.1)

2026-06-12 10:09:12 [info] [download:2] Starting download from .../dxt/extensions/ant.dir.cursortouch.windows-mcp/download
2026-06-12 10:09:13 [warn] Installing unsigned extension from C:\Users\<USER>\AppData\Local\Temp\dxt-download-1781251752823.mcpb
--> no further lines; no "Launching MCP Server", no "Initializing server"; extension absent
2026-06-12 10:12:07 [info] [download:3] Starting download from .../dxt/extensions/ant.dir.gh.anthropic.pdf-server-mcp/download
2026-06-12 10:12:09 [warn] Installing unsigned extension from C:\Users\<USER>\AppData\Local\Temp\dxt-download-1781251927383.mcpb
--> same: hangs after this line, nothing installs

For local .mcpb install via Settings, main.log shows no install-related lines at all
(only routine blocklist refreshes and Checking 0 extensions).

Additional observation right after the update

Immediately after updating to 1.12603.1 the network service crashed and startup was
severely degraded (this persisted into a ~31s startup):
2026-06-12 08:41:53 [error] 'Utility' process exited with 'crashed'
2026-06-12 08:41:53 [error] Network Service process gone (reason=crashed, exitCode=-1073741205) outside app quit — main-process networking is broken until relaunch
A clean restart fixed the network crash, but extension installation remained broken.

Things ruled out

• Bundle integrity: .mcpb is a valid zip; manifest valid (manifest_version 0.4, node >=18).
• Mark-of-the-Web: downloaded Temp .mcpb has no Zone.Identifier stream.
• Defender: no detections/blocks at the relevant times; Controlled Folder Access off;

no ASR rules configured.

• Folder permissions: write test to %APPDATA%\Claude\Claude Extensions succeeds.
• Registry policies: none.
• Third-party EDR: none installed.
• Full uninstall + reinstall of Claude Desktop 1.12603.1 did NOT fix it.

Workaround

Downgrading to 1.11847.5 restores extension installation. Auto-update had to be
manually disabled to stay on the working version.

Impact

On 1.12603.1 no MCP extensions can be installed at all, including official ones.

What Should Happen?

Expected behavior

Installing an extension on 1.12603.1 should behave exactly as it does on 1.11847.5:

• Directory extension (e.g. Filesystem / PDF server): after clicking Install, the

bundle downloads, Claude Desktop unpacks it into the local extensions directory,
launches its MCP server, and the extension appears in Settings → Extensions and is
immediately usable in conversations. The Installing unsigned extension log line
should be followed by Launching MCP Server / Initializing server / a successful
connection — not silence.

• Local .mcpb (Advanced settings → Install Extension…): after selecting the file,

an installation UI should appear showing the extension's details and permissions, and
after confirming, the extension installs and registers the same way. Selecting the file
should never be a silent no-op.

• In either case, if installation cannot proceed, Claude Desktop should surface a

clear error message to the user and write a corresponding error to main.log — rather
than failing silently with no feedback and no log entry.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce

Preconditions:

• Windows 11 (x64), Microsoft Defender only, no third-party EDR.
• Claude Desktop on version 1.12603.1 (built-in Node v24.16.0).
• No org allowlist, no SOFTWARE\Policies\Claude registry policies.

Reproduction A — directory extension:

1. Open Claude Desktop and go to Settings → Extensions.
2. Pick any directory extension (e.g. Filesystem, or the PDF server).
3. Click "Install".
4. Observe: the bundle downloads to %LOCALAPPDATA%\Temp, main.log logs

Installing unsigned extension from ...\dxt-download-*.mcpb, and then nothing
further happens — no server launch, no error, and the extension never appears
in Settings → Extensions.

Reproduction B — local .mcpb:

1. Settings → Extensions → Advanced settings → "Install Extension…".
2. Select a valid local .mcpb file (valid zip, manifest_version 0.4, node >=18).
3. Observe: the file picker closes but nothing happens — no installation UI, no

error dialog, and no related entry is written to main.log at all.

Both paths fail the same way on 1.12603.1 and affect every extension tried,
including Anthropic-published ones.

Verification of the version dependency:

1. Uninstall 1.12603.1 and install 1.11847.5.
2. Repeat A and B → both succeed normally (extension installs, server launches,

appears in Settings).

1. Re-updating to 1.12603.1 reintroduces the failure.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

1.12603.1

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_