[BUG] [Windows] /plugin marketplace add reports success but marketplace directory is not created; marketplace validator is also inconsistent

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?

On Windows, /plugin marketplace add mocreo-iot/skills reports success, but the marketplace is not actually installed to the expected local directory.

Claude Code prints:

Successfully added marketplace: mocreo-marketplace

However, the target directory is not created:

C:\Users\<user>\.claude\plugins\marketplaces\mocreo-marketplace

At the same time, Claude Code does write partial marketplace state into local config:

• ~/.claude/plugins/known_marketplaces.json
• ~/.claude/settings.json

So the result is a half-installed state:

• Claude believes the marketplace was added
• local config records it as added
• but the marketplace files are not actually present on disk

I also found a related inconsistency in the official validator:

• claude plugin validate passes for the plugin package itself
• but fails for the marketplace manifest with:

Unrecognized keys: "$schema", "description"

• those same keys are also present in Anthropic's own built-in marketplace manifest

That suggests two related problems:

1. marketplace add can report success before installation is fully materialized on disk
2. marketplace validation appears inconsistent with the marketplace format Claude Code itself uses

I found related previously reported plugin/marketplace issues, but those threads are now closed and I cannot comment there. I am filing this new issue because I have a fresh reproducible case on Windows plus an additional validator inconsistency that may help narrow down the root cause.

What Should Happen?

• /plugin marketplace add <repo> should only report success after the marketplace has actually been downloaded and written to the expected local install directory.
• If the marketplace content cannot be fetched, unpacked, or validated, Claude Code should not leave partial state in known_marketplaces.json or settings.json.
• Marketplace installation should be atomic: either fully installed and usable, or fully rolled back.
• claude plugin validate <marketplace-path> should accept the same marketplace manifest format that Claude Code itself uses for built-in marketplaces.
• Error output should clearly indicate whether failure happened during repo fetch, local extraction, manifest parsing, or plugin discovery.

Error Messages/Logs

Steps to Reproduce

• On Windows, run:

```bash
/plugin marketplace add mocreo-iot/skills
Observe that Claude Code reports:
Successfully added marketplace: mocreo-marketplace
Check:
C:\Users\<user>\.claude\plugins\known_marketplaces.json
C:\Users\<user>\.claude\settings.json
C:\Users\<user>\.claude\plugins\marketplaces\
Observe that:
known_marketplaces.json contains mocreo-marketplace
settings.json contains extraKnownMarketplaces.mocreo-marketplace
but C:\Users\<user>\.claude\plugins\marketplaces\mocreo-marketplace does not exist
Run marketplace validation:
claude plugin validate E:\MOCREOUSER\mocreoskill\skills
Observe validation error:
Unrecognized keys: "$schema", "description"
Compare with Anthropic's built-in marketplace manifest under:
C:\Users\<user>\.claude\plugins\marketplaces\claude-plugins-official\.claude-plugin\marketplace.json
Validate Anthropic's built-in marketplace and observe the same error

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.79

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

The plugin package itself validates successfully:
claude plugin validate E:\MOCREOUSER\mocreoskill\skills\plugins\mocreo-api
Result: validation passed.
The marketplace root does not validate, but the reported invalid keys are also present in Anthropic's own built-in marketplace manifest.
I also tested a temporary copy of the marketplace manifest with only $schema and description removed; that copy validated successfully, which suggests the validator is inconsistent rather than the marketplace structure being fundamentally broken.
This seems related to other Claude Code plugin/marketplace state issues:
#9426
#9537
#10364