Generate Wren MDL

Version check

Silently fetch https://raw.githubusercontent.com/Canner/wren-engine/main/skills/versions.json. Compare the 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A newer version of the generate-mdl skill is available. Update with:

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

Then continue with the workflow below regardless of update status.

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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/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.

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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:

SQL / ibis type	MDL type
INT, INTEGER, INT4	INTEGER
BIGINT, INT8	BIGINT
SMALLINT, INT2	SMALLINT
FLOAT, FLOAT4, REAL	FLOAT
DOUBLE, FLOAT8	DOUBLE
DECIMAL, NUMERIC	DECIMAL
VARCHAR, TEXT, STRING	VARCHAR
CHAR	CHAR
BOOLEAN, BOOL	BOOLEAN
DATE	DATE
TIMESTAMP, DATETIME	TIMESTAMP
TIMESTAMPTZ	TIMESTAMPTZ
JSON, JSONB	JSON
ARRAY	ARRAY
BYTES, BYTEA	BYTES

When in doubt, use VARCHAR as a safe fallback.

Relationship join types

Cardinality	joinType value
Many-to-one (FK table → PK table)	MANY_TO_ONE
One-to-many	ONE_TO_MANY
One-to-one	ONE_TO_ONE
Many-to-many	MANY_TO_MANY

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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.