Plugin skill variable substitution: ${CLAUDE_SKILL_DIR} not expanded in skill instructions

Environment

• OS: macOS (Darwin 25.5.0)
• Claude Code version: 2.1.220

Reproducer

1. Invoke a plugin-provided skill using the namespace syntax: /plugin-name:skill-name [args]
2. The skill's instructions/reference docs contain a reference to ${CLAUDE_SKILL_DIR} (e.g., "Reference material is found in ${CLAUDE_SKILL_DIR}")
3. Observe the rendered instructions delivered to Claude

Expected behavior

The literal string ${CLAUDE_SKILL_DIR} should be substituted with the actual filesystem path to the plugin skill's directory.

Actual behavior

The variable ${CLAUDE_SKILL_DIR} appears unresolved in the skill instructions sent to Claude — it remains as the literal string ${CLAUDE_SKILL_DIR}, causing the skill to abort when it cannot locate its reference files per its own safety guards.

Impact

Plugin-provided skills that reference their own bundled files or supporting documentation using ${CLAUDE_SKILL_DIR} cannot function — they fail with "path not provided by the harness, stopping per instructions."

Additional notes

• The variable is documented at https://code.claude.com/docs/en/skills.md as supported in skill markdown content and allowed-tools frontmatter
• The bug is reproducible and repeatable with the same skill and invocation syntax