Skill tags missing for slash-command-invoked skills

[cindersax]

Description

collect_skill_tags() in hooks/langfuse_hook.py is supposed to add a skill:<name> tag to every trace where a skill ran during the turn, gated by CC_LANGFUSE_SKILL_TAGS (default true). It only detects skills invoked via a tool_use block literally named "Skill".

Claude Code has a second invocation path that never produces that block: slash commands mapped to a skill (e.g. /some-skill). These instead appear as a <command-name>/<command-message> block on the user row, followed by assistant rows carrying a top-level attributionSkill field (sibling of type/uuid/message) set to the skill's slug.

langfuse_hook.py never reads attributionSkill, and trace_metadata in emit_turn() is built from a fixed field allowlist, so the value can't pass through incidentally. Result: any skill invoked via slash command is invisible to skill-based tagging/filtering in Langfuse, regardless of which skill it is.

Steps to Reproduce

1. Configure the plugin with valid Langfuse credentials; leave CC_LANGFUSE_SKILL_TAGS at its default.
2. Invoke any skill via its slash-command form (e.g. /<skill-name>) and let the turn complete.
3. In the session transcript, confirm no tool_use block named "Skill" exists for the invocation, but a following assistant row has top-level "attributionSkill": "<skill-name>".
4. Query Langfuse: langfuse api traces list --tags "skill:<skill-name>".

Expected Behavior

The trace(s) for that turn carry a skill:<skill-name> tag, and the query in step 4 returns them.

Actual Behavior

The trace ships with only the generic claude-code tag; the tag-filtered query returns zero results. The tag is missing, not the trace itself — metadata and content are otherwise complete.

Environment

• Plugin: langfuse-observability v1.0.0 (github.com/langfuse/Claude-Observability-Plugin)
• Langfuse SDK: langfuse>=4.0,<5
• OS: macOS