Can't add with CoWork, but can add with Code

[anteksiler]

Using Claude Desktop, I am seeing the below issue under Desktop, but not under Code:

[drewburchfield]

@anteksiler I've been having the same issue with other marketplaces also when I tried a few yesterday. Will look into it

[chaseclemons]

Our team is also running into this one. It's on a new Macbook setup.

Happy to help out with any testing or log checks!

[drewburchfield]

Thanks for the extra detail @chaseclemons, and for being willing to help test.

I dug into the Claude Desktop logs and found the specific error. When CoWork tries to sync the marketplace, the server-side validation rejects the plugin with:

MARKETPLACE_ERROR:REMOTE_SYNC_FAILED
{"plugins": [{"name": "helpscout-navigator", "error": "External plugin sources are not yet supported."}], "message": "0 plugins found, 1 failed validation"}

The issue is that our marketplace.json points to the plugin in a separate GitHub repo (an "external source"). CoWork's remote sync doesn't support this yet, even though it's a documented and widely-used pattern for Claude Code plugins. External sources work fine in Claude Code CLI but fail server-side validation when CoWork tries to sync.

This also explains why it worked briefly in earlier versions of CoWork and then stopped. The validation appears to have been added or tightened recently.

Working on a fix now to inline the plugin directly into the marketplace repo so CoWork's sync will accept it. Will update here when that's ready.

[drewburchfield]

Update (March 31): I've spent the day digging into this. Here's where things stand.

What I fixed:
The original "External plugin sources are not yet supported" error is resolved. I inlined the plugin directly into the marketplace repo so CoWork's sync accepts it.

What's still broken (upstream Anthropic platform bugs):

Plugin persistence: Third-party plugins don't persist across app restart. The RemotePluginManager wipes non-Anthropic plugins on every server sync. Skills load on first install but disappear after restarting Claude Desktop.

• Desktop Cowork: RemotePluginManager removes 3rd-party GitHub marketplace plugins on every sync anthropics/claude-code#38429
• Cowork: Personal marketplace plugin installation lost after app restart anthropics/claude-code#40600

Credential injection: CoWork VM doesn't receive environment variables for MCP server credentials. Shell env vars, launchctl setenv, and userConfig with sensitive: true all fail to propagate into the CoWork sandbox.

• Cowork sessions don't inject user_config env vars into MCP server processes anthropics/claude-code#39125
• [BUG] Plugin userConfig values not prompted on enable anthropics/claude-code#39455

userConfig prompting: Even in Claude Code CLI, plugins with userConfig don't prompt at enable time.

• [BUG] Plugin userConfig values not prompted on enable anthropics/claude-code#39455

MCPB extension forwarding: Desktop Extensions (MCPB) are intermittently dropped from CoWork sessions. Race condition in the VM tool forwarding means extension tools are sometimes available and sometimes not across sessions.

• [BUG] Desktop Extension MCP servers not passed to Cowork VM despite being enabled in session config anthropics/claude-code#26259
• [BUG] DXT Extension Tools Not Permitted in Claude Desktop Co-work Sessions anthropics/claude-code#39674

These affect all third-party CoWork plugins and extensions with local MCP servers, not just ours. Every official Anthropic plugin uses remote HTTP servers with OAuth, which sidesteps all of these. The third-party local MCP server story in CoWork is unfortunately unstable right now.

What works today:

• Claude Code CLI: Full plugin with skills + MCP server. Set your env vars, add the marketplace, install the plugin. This is the most reliable path.
• Claude Desktop Chat mode: Install the MCPB extension (download .mcpb file, double-click). Has a built-in credential configuration UI. Note: if you also use CoWork, the MCPB extension tools may not reliably appear in CoWork sessions due to the forwarding race condition mentioned above.

I'm tracking an installer script that writes directly to CoWork's local plugin cache to bypass both the sync-wipe and credential issues, similar to what others in the community have done. Will update here when that's ready.

Remote HTTP MCP server with OAuth (which would solve all of these) is on the roadmap.