[BUG] Fix misleading plugin install error for fabric-collection marketplace

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?

Installing plugins from the "fabric-collection" marketplace (https://github.com/microsoft/skills-for-fabric.git) fails with a misleading error: "This plugin uses a source type your Claude Code version does not support. Update Claude Code and try again." This happens even on the latest version (2.1.222), so the suggested update is a dead end.

Root cause (diagnosed by diffing marketplace.json): all plugin source fields are identical (./plugins/<name>), so source type is not the actual differentiator. The plugins that fail all declare an mcpServers entry with "type": "http" (remote server, with url/headers), while the one plugin that succeeds (powerbi-authoring) uses "type": "stdio" (local npx command). This strongly suggests the installer's mcpServers schema parsing doesn't recognize the http server type and falls through to a catch-all "source type not supported" error, misattributing the failure to source.

This matches the same underlying bug class as three already-closed issues (all closed as "not planned" without a fix landing):

• #60884 — object-form skills[] entries trip the same error
• #64431 — "source": "." (repo-root plugin) falls outside the switch's handled cases
• #56043 — marketplace name elisa-gepa itself misclassifies plugins as remote

This appears to be a fourth, previously unreported trigger for the same underlying bug: a narrow switch/schema-shape check throws a generic "unsupported source type, please update" error for any unrecognized shape, rather than identifying the actual field/value that failed to parse, and rather than distinguishing "unsupported schema" from "you're on an old version."

What Should Happen?

Either mcpServers.type: "http" should be supported by the plugin/marketplace schema parser, or at minimum the error message should name the specific field/value that failed to parse instead of the generic "update Claude Code" message.

Error Messages/Logs

Failed to install plugin "fabric-authoring@fabric-collection": This plugin uses a source type your Claude Code version does not support. Update Claude Code and try again.

Steps to Reproduce

1. Add marketplace: fabric-collection (points at https://github.com/microsoft/skills-for-fabric.git)
2. Attempt to install any of: fabric-authoring, fabric-consumption, fabric-operations, fabric-skills — either via the Claude Desktop plugin UI or via CLI: claude plugin install fabric-authoring@fabric-collection
3. Observe failure with the error above
4. Run claude update — confirms 2.1.222 is already the latest version, so the suggested fix is a dead end
5. As a comparison, install powerbi-authoring@fabric-collection from the same marketplace — this one installs successfully

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.222 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

Marketplace source file for reference: https://raw.githubusercontent.com/microsoft/skills-for-fabric/main/.claude-plugin/marketplace.json

Related closed issues describing the same underlying bug class (different triggers):

• https://github.com/anthropics/claude-code/issues/60884
• https://github.com/anthropics/claude-code/issues/64431
• https://github.com/anthropics/claude-code/issues/56043