[BUG] Claude Desktop: extension install fully broken — "Install Extension" no-ops and drag-and-drop falls through the modal into the chat

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: [BUG] Claude Desktop: extension install fully broken — "Install Extension" no-ops and drag-and-drop falls through the modal into the chat

Preflight

• [x] Searched existing issues; did not find this specific symptom
• [x] Single bug report (two related failures on the same surface)
• [x] Reproduced across multiple users/machines

What's wrong

On the current Claude Desktop build, there is no working way to install a custom .mcpb desktop extension. Both supported install paths are non-functional:

1. Settings → Extensions → Advanced settings → Install Extension… — selecting a .mcpb in the file picker produces no response at all. No Preview/details pane renders, no install dialog appears, no error is shown.
2. Drag-and-drop onto the Extensions page — the drop is not captured by the Extensions modal. The file falls through to whatever chat is behind the modal, attaching the .mcpb to the conversation instead of installing it.

This is reproducible across multiple users, so it appears to be a client-side regression in the extension-install UI rather than an environmental or bundle-specific issue. The same .mcpb bundle installed successfully on a prior build.

Environment

• Claude Desktop version: ⟨FILL IN — list every affected user's build number; note if they share one⟩
• OS / version: ⟨FILL IN — e.g. macOS 15.x (Apple Silicon / Intel); note any Windows machines in the set⟩
• Plan: ⟨FILL IN — Free / Pro / Team / Enterprise⟩
• Desktop extension allowlist: not enabled (the Advanced settings → Install Extension developer path is present)

Steps to reproduce

1. Open Settings → Extensions → Advanced settings → Install Extension…
2. Select a valid custom .mcpb file
3. Observe: nothing happens (no Preview pane, no dialog, no error)
4. Alternatively, drag the same .mcpb onto the Extensions page
5. Observe: the file is dropped into the chat behind the modal rather than installed

Expected

Selecting a .mcpb loads the extension details/Preview pane and offers Install; dragging a .mcpb onto the Extensions page begins installation.

Actual

Neither path initiates installation. The button path is a silent no-op; the drag path misroutes the file into the active conversation.

Logs

⟨FILL IN — paste the lines written on the failed attempt from
~/Library/Logs/Claude/ or ~/Library/Application Support/Claude/logs/.
Look for desktop_mcp_unexpected_close or
"not found in installed extensions" entries.⟩

Impact

Custom/internally-distributed .mcpb extensions cannot be installed or updated at all on this build. This blocks teams distributing private MCP servers. Sequence that exposed it: attempted to update an installed extension (failed), uninstalled it, then could no longer reinstall the previously-working bundle.

Workaround

Bypass the extension UI entirely by configuring the server manually in claude_desktop_config.json (translate the server block from the bundle's manifest.json). This restores function but loses the GUI config and keychain-backed secret storage.

Possibly related

Same extension-management/stale-state machinery as anthropics/claude-code#31864 and #30954.

What Should Happen?

Expected

Selecting a .mcpb loads the extension details/Preview pane and offers Install; dragging a .mcpb onto the Extensions page begins installation.

Error Messages/Logs

No response, appears to be a silent failure. This exact flow was working reliably for weeks until today.

Steps to Reproduce

Steps to reproduce

1. Open Settings → Extensions → Advanced settings → Install Extension…
2. Select a valid custom .mcpb file
3. Observe: nothing happens (no Preview pane, no dialog, no error)
4. Alternatively, drag the same .mcpb onto the Extensions page
5. Observe: the file is dropped into the chat behind the modal rather than installed

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude 1.12603.1 (3df4fd) 2026-06-11T16:57:36.000Z

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

THis happens the same on both windows and mac. Seems to be a bug with all desktop builds.