generate-mdl skill writes bare-list relationships.yml that the loader silently ignores

[Sean-Liu-GitHub]

Bug summary

The generate-mdl skill template (core/wren/src/wren/skills_content/generate-mdl/SKILL.md) instructs agents to write relationships.yml as a bare YAML list:

# relationships.yml (what the skill template shows)
- name: orders_customers
  models:
    - orders
    - customers
  join_type: many_to_one
  condition: "orders.customer_id = customers.customer_id"

But load_relationships() in context.py expects a dict with a top-level relationships: key:

# context.py — load_relationships()
data = yaml.safe_load(rel_file.read_text(encoding="utf-8")) or {}
rels = data.get("relationships") if isinstance(data, dict) else None
if not isinstance(rels, list):
    return []

A bare YAML list parses as a Python list, so isinstance(data, dict) is False, and the function returns []. The relationships are silently dropped.

The CLI's own writer (_convert_mdl_to_project_files) correctly uses the wrapper format:

yaml.dump({"relationships": rels_snake}, ...)

Expected format

# relationships.yml (what the loader expects)
relationships:
  - name: orders_customers
    models:
      - orders
      - customers
    join_type: many_to_one
    condition: "orders.customer_id = customers.customer_id"

Steps to reproduce

1. Follow the quickstart guide to generate an MDL project using Claude Code with the /wren skill
2. The skill generates relationships.yml as a bare list (following the SKILL.md example)
3. Run wren context validate — reports 0 relationships
4. Run wren context build — target/mdl.json has "relationships": []

Impact

• Any project generated via the generate-mdl skill will have relationships silently ignored
• wren context validate reports 0 relationships without any warning
• Queries still work because agents can write explicit JOIN ... ON clauses, so the bug is hidden
• AI agents lose relationship context from wren memory fetch, reducing their ability to generate correct JOINs automatically

Suggested fix

Update the YAML example in core/wren/src/wren/skills_content/generate-mdl/SKILL.md (Phase 4, Step 3) to include the relationships: top-level key, matching the format that load_relationships() expects.

Environment

• wren CLI installed via pip install wrenai
• Discovered while debugging a DuckDB-backed project where relationships were defined but missing from the compiled MDL