Plugin harness resolves skill base directory to marketplaces/ instead of cache/ path

Bug

The plugin harness resolves a skill's base directory to ~/.claude/plugins/marketplaces/<repo>/ instead of the installPath recorded in ~/.claude/plugins/installed_plugins.json (which points to ~/.claude/plugins/cache/).

Observed behavior

When invoking a plugin skill (e.g. /kk:document), the harness injects a "Base directory" that points to the marketplaces path:

Base directory for this skill: ~/.claude/plugins/marketplaces/claude-toolbox/klaude-plugin/skills/document

But installed_plugins.json records the plugin's install path under cache:

{
  "plugins": {
    "kk@claude-toolbox": [
      {
        "scope": "project",
        "installPath": "~/.claude/plugins/cache/claude-toolbox/kk/0.17.3",
        "version": "0.17.3"
      }
    ]
  }
}

The agent then uses the marketplaces path for all subsequent Read calls within that skill invocation. This means ${CLAUDE_PLUGIN_ROOT} (substituted at plugin-load time) may resolve to a different directory than what installed_plugins.json reports.

Expected behavior

The harness should resolve the skill's base directory consistently — ideally to the installPath recorded in installed_plugins.json.

Impact

• LLMs that need the plugin root at runtime (outside of ${CLAUDE_PLUGIN_ROOT} substitution context) have no single reliable source for the path. They sometimes derive cache/ and sometimes marketplaces/, leading to ENOENT errors or reading stale content.
• Hook scripts that reference ${CLAUDE_PLUGIN_ROOT} may resolve to a different location than runtime Read calls within the same session.

Environment

• Claude Code CLI (latest stable as of 2026-06-01)
• Plugin installed via: claude plugin install kk@claude-toolbox --scope project
• OS: Linux

This issue is created by claude-code from a session where I observed that it read from marketplaces:

❯ I noticed when you were running document skill you also read from 'marketplace' dir. Why is that?

Ha — that's the exact problem you're fixing, caught in the wild. Look at the skill's base directory from that invocation:

Base directory for this skill: /home/sergio/.claude/plugins/marketplaces/claude-toolbox/klaude-plugin/skills/document

But installed_plugins.json says the install path is:

/home/sergio/.claude/plugins/cache/claude-toolbox/kk/0.17.3

The harness loaded the skill from marketplaces/ instead of cache/. That's the inconsistency your PR is addressing — and it happened right in front of us during this session.

✻ Brewed for 11s

❯ So you're saying the current harness points you to marketplaces?

Yes — that's what the skill-load output showed. The harness handed me this as the base directory:

/home/sergio/.claude/plugins/marketplaces/claude-toolbox/klaude-plugin/skills/document

And that's the path I used for the Read call to shared-profile-detection.md. I didn't choose it — the harness resolved it there.

Meanwhile installed_plugins.json records the install at /home/sergio/.claude/plugins/cache/claude-toolbox/kk/0.17.3.

So it's not just LLMs guessing wrong — the harness itself is pointing to two different locations depending on the context.