[BUG] Personal plugin skills not mounted in Cowork container despite being enabled in UI

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

The skill component of a personal plugin installed from a marketplace is not mounted into the Cowork VM container, even though the plugin appears as installed and enabled in the Customize UI. The MCP connector part of the same plugin loads correctly — only the skill is missing.

Environment

• OS: macOS
• Claude Desktop version: latest (downloaded March 2026)
• Cowork session version: 2.1.63 (from transcript metadata)
• Model: claude-sonnet-4-6
• Plugin: "Blockscout analysis" v0.4.0, source: Marketplace (blockscout-ai)

Steps to Reproduce

1. Install the "Blockscout analysis" plugin from the marketplace (blockscout-ai)
2. Verify it appears in Customize > Personal plugins as enabled (toggle ON)
3. Start a new Cowork session with the plugin selected (visible in the "Get to work with" dropdown, checkmark shown)
4. In the session, run: ls -la /mnt/skills/ and find /mnt -name "SKILL.md"

Expected Behavior

The plugin's SKILL.md should be mounted in the container filesystem alongside the built-in skills (docx, pdf, pptx, etc.) and listed in the system prompt's available_skills section. Slash commands from the plugin should appear in the slashCommands metadata.

Actual Behavior

• /mnt/skills/ does not exist in the Cowork container
• The actual skills directory (/sessions/{name}/mnt/.skills/skills/) contains only 6 built-in Anthropic skills: docx, pdf, pptx, schedule, skill-creator, xlsx
• No "Blockscout analysis" skill is present anywhere on the filesystem
• The session metadata slashCommands array contains only anthropic-skills:* and cowork-plugin-management:* entries — no plugin skill commands
• The system prompt available_skills section does not include the plugin's skill
• MCP connector tools from the same plugin do appear in enabledMcpTools and work correctly

Key Observation

The plugin has two components: connectors (MCP tools) and skills (SKILL.md). The connector provisioning pipeline works. The skill provisioning pipeline silently drops the skill — no error is shown to the user.

Note: the MCP tools visible in enabledMcpTools may also come from separately installed Extensions or web-based Connectors (e.g., the official Blockscout connector), not from the plugin itself. The plugin's connector shares the name "Blockscout" with these other sources, making it ambiguous whether the plugin's own connector was loaded.

Evidence

1. Plugin shown as installed and enabled in Customize UI

The plugin "Blockscout analysis" v0.4.0 appears under Personal plugins with a toggle in ON state, showing the Connectors sub-item.

2. Plugin selected before Cowork session start

The "Get to work with Blockscout analysis" dropdown shows the plugin with a blue checkmark before starting the session.

3. Filesystem shows no plugin skill

$ find /sessions/friendly-bold-darwin/mnt/.skills -name "SKILL.md"
/sessions/friendly-bold-darwin/mnt/.skills/skills/schedule/SKILL.md
/sessions/friendly-bold-darwin/mnt/.skills/skills/xlsx/SKILL.md
/sessions/friendly-bold-darwin/mnt/.skills/skills/pdf/SKILL.md
/sessions/friendly-bold-darwin/mnt/.skills/skills/skill-creator/SKILL.md
/sessions/friendly-bold-darwin/mnt/.skills/skills/pptx/SKILL.md
/sessions/friendly-bold-darwin/mnt/.skills/skills/docx/SKILL.md

No Blockscout analysis skill present.

4. Session metadata confirms no plugin slash commands

slashCommands from metadata.json:

[
  "anthropic-skills:schedule",
  "anthropic-skills:docx",
  "anthropic-skills:skill-creator",
  "anthropic-skills:pptx",
  "anthropic-skills:pdf",
  "anthropic-skills:xlsx",
  "cowork-plugin-management:cowork-plugin-customizer",
  "cowork-plugin-management:create-cowork-plugin",
  "compact", "context", "cost", "init",
  "pr-comments", "release-notes", "review", "security-review", "insights"
]

Additional Note

This bug also manifests in regular claude.ai chat sessions — the plugin skill does not appear in available_skills there either. This may or may not be expected for claude.ai chat (as opposed to Cowork), but the Cowork case is clearly a bug.

transcript-1772815146943.zip

Related Issues

• #26254 — Skills metadata registered in system prompt but SKILL.md files not mounted in container
• #26131 — 36 user skills registered but none appear at runtime
• #16575 — User-defined plugin skills not appearing in available_skills
• #24859 — Cowork Windows plugin skills searched at wrong location