Generate Wren MDL

Version check

Silently fetch https://raw.githubusercontent.com/Canner/wren-engine/main/skills/versions.json. Compare the wren-generate-mdl key with this skill's version (from the frontmatter above). If the remote version is newer, notify the user before proceeding:

A newer version of the wren-generate-mdl skill is available. Update with:

Copy

npx skills add Canner/wren-engine --skill wren-generate-mdl --agent claude-code

Then continue with the workflow below regardless of update status.

Generates a Wren MDL manifest by using ibis-server to introspect the database schema — no local database drivers required. All schema discovery goes through ibis-server, which already has drivers for all supported data sources.

Workflow

Follow these steps in order. Do not skip steps or ask unnecessary questions between them.

Step 1 — Verify connection and choose data source

Connection info can ONLY be configured through the Web UI at http://localhost:9001. Do not attempt to set connection info programmatically via ibis-server API calls, curl, or any other method. The ibis-server does not expose a public API for writing connection info — only the Web UI can do this.

Confirm the MCP server has a working connection before proceeding:

health_check()

If the health check fails, or if the user has not yet configured a connection, direct them to the Web UI at http://localhost:9001 to enter their data source credentials. Wait for the user to confirm the connection is saved before continuing.

Ask the user for:

1. Data source type (e.g. POSTGRES, BIGQUERY, SNOWFLAKE, …) — needed to set dataSource in the MDL
2. Schema filter (optional) — if the database has many schemas, ask which schema(s) to include

After this step you will have:

• data_source: e.g. "POSTGRES"
• Optional schema_filter: used to narrow down results in subsequent steps

Step 2 — Fetch table schema

list_remote_tables()

Returns a list of tables with their column names and types. Each table entry has a properties.schema field — use it to filter to the user's target schema if specified.

If this fails:

1. Check that read-only mode is disabled in the Web UI (http://localhost:9001) — list_remote_tables() will fail when read-only mode is on, even if the connection is healthy.
2. Ask the user to verify connection info in the Web UI if read-only mode is already off.

Step 3 — Fetch relationships

list_remote_constraints()

Returns foreign key constraints. Use these to build Relationship entries in the MDL. If the response is empty ([]), infer relationships from column naming conventions (e.g. order_id → orders.id).

If this fails, verify that read-only mode is disabled in the Web UI (http://localhost:9001).

Step 4 — Build MDL JSON

Construct the manifest following the MDL structure below.

Rules:

• catalog: use "wren" unless the user specifies otherwise
• schema: use the target schema name (e.g. "public" for PostgreSQL default, "jaffle_shop" if user specified)
• dataSource: set to the enum value from Step 1 (e.g. "POSTGRES")
• tableReference.catalog: set to the database name (not "wren")
• Each table → one Model. Set tableReference.table to the exact table name
• Each column → one Column. Use the exact DB column name
• Mark primary key columns with "isPrimaryKey": true and set primaryKey on the model
• For FK columns, add a Relationship entry linking the two models
• Omit calculated columns for now — they can be added later

Step 5 — Validate

Deploy the draft MDL and validate it with a dry run:

deploy_manifest(mdl=<manifest dict>)
dry_run(sql="SELECT * FROM <any_model_name> LIMIT 1")

If dry_run succeeds, the MDL is valid. If it fails, fix the reported errors, call deploy_manifest again with the corrected MDL, and retry.

Step 6 — Save project (optional)

Ask the user if they want to save the MDL as a YAML project directory (useful for version control).

If yes, follow the wren-project skill (skills-archive/wren-project/SKILL.md) to write the YAML files and build target/mdl.json.

Step 7 — Deploy final MDL

deploy_manifest(mdl=<manifest dict>)

Confirm success to the user. The MDL is now active and queries can run.

MDL Structure

{
  "catalog": "wren",
  "schema": "public",
  "dataSource": "POSTGRES",
  "models": [
    {
      "name": "orders",
      "tableReference": {
        "catalog": "",
        "schema": "public",
        "table": "orders"
      },
      "columns": [
        {
          "name": "order_id",
          "type": "INTEGER",
          "isCalculated": false,
          "notNull": true,
          "isPrimaryKey": true,
          "properties": {}
        },
        {
          "name": "customer_id",
          "type": "INTEGER",
          "isCalculated": false,
          "notNull": false,
          "properties": {}
        },
        {
          "name": "total",
          "type": "DECIMAL",
          "isCalculated": false,
          "notNull": false,
          "properties": {}
        }
      ],
      "primaryKey": "order_id",
      "cached": false,
      "properties": {}
    }
  ],
  "relationships": [
    {
      "name": "orders_customer",
      "models": ["orders", "customers"],
      "joinType": "MANY_TO_ONE",
      "condition": "orders.customer_id = customers.customer_id"
    }
  ],
  "views": []
}

Column types

Map SQL/ibis types to MDL type strings:

When in doubt, use VARCHAR as a safe fallback.

Relationship join types

Connection setup

Connection info is configured exclusively via the MCP server Web UI at http://localhost:9001. There is no API endpoint for setting connection info — do not attempt to configure it programmatically. See the wren-mcp-setup skill for Docker setup instructions.

Note: If the Web UI is disabled (WEB_UI_ENABLED=false), connection info must be pre-configured in ~/.wren/connection_info.json before starting the container. Use /wren-connection-info in Claude Code for the required fields per data source.