Wren MCP Setup

Set Up Wren MCP via Docker

Version check

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A newer version of the wren-mcp-setup skill is available.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

npx skills add Canner/wren-engine --skill wren-mcp-setup --agent claude-code

Then continue with the workflow below regardless of update status.

Runs the Wren Engine ibis-server + MCP server together in a single Docker container and connects it to Claude Code (or another MCP client) over streamable-http.

Step 1 — Ask for workspace path

Ask the user:

What directory on your host machine should be mounted as the MCP workspace? This is where MDL files and YAML project directories will be read and written. (Example: ~/wren-workspace)

If the user has no preference, suggest creating a dedicated directory such as ~/wren-workspace:

mkdir -p ~/wren-workspace

Save the answer as <WORKSPACE_PATH> (use the absolute path, e.g. /Users/me/wren-workspace) for use in the next steps.

Step 2 — Prepare workspace and start the container

Check for a newer image

Before starting the container, verify whether a newer wren-engine-ibis image is available on the registry.

If the image has never been pulled (first-time setup), just pull it:

docker pull ghcr.io/canner/wren-engine-ibis:latest

If the image already exists locally, compare the local digest with the remote one to detect updates:

# Save current local digest (empty string if image not present)
LOCAL_DIGEST=$(docker inspect --format='{{index .RepoDigests 0}}' ghcr.io/canner/wren-engine-ibis:latest 2>/dev/null || echo "")

# Pull from registry (downloads only if remote digest differs)
docker pull ghcr.io/canner/wren-engine-ibis:latest

# Compare digests
NEW_DIGEST=$(docker inspect --format='{{index .RepoDigests 0}}' ghcr.io/canner/wren-engine-ibis:latest 2>/dev/null || echo "")

if [ "$LOCAL_DIGEST" != "$NEW_DIGEST" ]; then
  echo "✓ New image pulled — container will use the updated version."
else
  echo "✓ Already up to date. No update was needed."
fi

Note: If a wren-mcp container is already running and a new image was pulled, stop and remove the old container before proceeding:

docker rm -f wren-mcp

The workspace directory is mounted at /workspace inside the container. Place your compiled MDL at <WORKSPACE_PATH>/target/mdl.json so the container can load it at startup via MDL_PATH.

Recommended workspace layout:

<WORKSPACE_PATH>/
└── target/
    └── mdl.json           # Compiled MDL (from wren-project build)

Note: Connection info is managed via the MCP server Web UI (see Step 3) and persisted to ~/.wren/connection_info.json. You can also pre-configure this file before starting the container.

Run the following command, substituting <WORKSPACE_PATH> with the path from Step 1:

docker run -d \
  --name wren-mcp \
  -p 8000:8000 \
  -p 9000:9000 \
  -p 9001:9001 \
  -e ENABLE_MCP_SERVER=true \
  -e MCP_TRANSPORT=streamable-http \
  -e MCP_HOST=0.0.0.0 \
  -e MCP_PORT=9000 \
  -e WREN_URL=localhost:8000 \
  -e MDL_PATH=/workspace/target/mdl.json \
  -v <WORKSPACE_PATH>:/workspace \
  ghcr.io/canner/wren-engine-ibis:latest

Example with a concrete path:

docker run -d \
  --name wren-mcp \
  -p 8000:8000 \
  -p 9000:9000 \
  -p 9001:9001 \
  -e ENABLE_MCP_SERVER=true \
  -e MCP_TRANSPORT=streamable-http \
  -e MCP_HOST=0.0.0.0 \
  -e MCP_PORT=9000 \
  -e WREN_URL=localhost:8000 \
  -e MDL_PATH=/workspace/target/mdl.json \
  -v /Users/me/wren-workspace:/workspace \
  ghcr.io/canner/wren-engine-ibis:latest

If MDL_PATH is not set or the file doesn't exist yet, the container starts without a loaded MDL. You can deploy later using the deploy MCP tool or the Web UI.

This starts the container using the image ghcr.io/canner/wren-engine-ibis:latest with:

The workspace directory is mounted at /workspace inside the container.

Verify the container is running:

docker ps --filter name=wren-mcp
docker logs -f wren-mcp

Step 3 — Configure connection info via Web UI

Open the Web UI at http://localhost:9001.

The Web UI provides:

• Read-only Mode — toggle to prevent the AI agent from calling deploy, deploy_manifest, list_remote_tables, or list_remote_constraints. Useful for read-only query access.
• MDL Status — shows the currently deployed MDL (models, columns, data source).
• Connection Info — select the data source type and fill in credentials. Click Save Connection to test and persist.
• MDL Editor — view and edit the live MDL JSON inline. Click Save & Deploy to update.

Docker host hint

Critical: The container cannot reach the host's localhost directly. If your database runs on the host machine, use host.docker.internal instead of localhost or 127.0.0.1 as the hostname.

The Web UI shows a hint for this when it detects you may be connecting to a local database:

Fill in the connection credentials in the Web UI form, apply the Docker host hint if needed, and click Save Connection.

Step 4 — Configure Claude Code MCP

Claude Code uses streamable-http transport to connect to the containerized MCP server.

Via Claude Code CLI (recommended):

claude mcp add --transport http wren http://localhost:9000/mcp

Or add manually to ~/.claude/settings.json under mcpServers:

{
  "mcpServers": {
    "wren": {
      "type": "http",
      "url": "http://localhost:9000/mcp"
    }
  }
}

After adding, restart Claude Code for the new MCP server to be loaded into the session. Confirm with:

claude mcp list

Note: The config file is ~/.claude/settings.json, not ~/.claude.json. Adding the MCP server while a session is already running has no effect until the session is restarted.

Step 5 — Configure other MCP clients

Cline / Cursor / VS Code MCP Extension

These clients also support HTTP transport. Add to their MCP settings:

{
  "mcpServers": {
    "wren": {
      "type": "streamable-http",
      "url": "http://localhost:9000/mcp"
    }
  }
}

Claude Desktop (stdio fallback)

Claude Desktop does not support HTTP transport natively. Use a local stdio proxy or run the MCP server locally instead.

Step 6 — Verify the connection

Ask the AI agent to run a health check:

Use the health_check() tool to verify Wren Engine is reachable.

Expected response: SELECT 1 returns a successful result.

If health check fails, see Troubleshooting below.

Troubleshooting — MCP server not healthy

1. Check container status and logs

docker ps --filter name=wren-mcp
docker logs wren-mcp

Look for startup errors or crash loops. If the container exited, logs will show the last output before it stopped.

2. Port already in use

The container exposes ports 8000 (ibis-server), 9000 (MCP), and 9001 (Web UI). If any port is already bound by another process on the host, the docker run command will fail with a bind error.

Check what is using the port:

# macOS / Linux
lsof -i :9000
lsof -i :8000
lsof -i :9001

If another process is occupying the port you have two options:

Option A — stop the conflicting process:

# macOS: kill by port
kill $(lsof -ti :9000)

Option B — remap to different host ports:

Stop and remove the existing container first, then re-run with different host-side ports:

docker rm -f wren-mcp
docker run -d \
  --name wren-mcp \
  -p 18000:8000 \
  -p 19000:9000 \
  -p 19001:9001 \
  -e ENABLE_MCP_SERVER=true \
  -e MCP_TRANSPORT=streamable-http \
  -e MCP_HOST=0.0.0.0 \
  -e MCP_PORT=9000 \
  -e WREN_URL=localhost:8000 \
  -e WEB_UI_PORT=9001 \
  -e MDL_PATH=/workspace/target/mdl.json \
  -v <WORKSPACE_PATH>:/workspace \
  ghcr.io/canner/wren-engine-ibis:latest

Then update the MCP client URL and Web UI address to match:

claude mcp add --transport http wren http://localhost:19000/mcp
# Web UI: http://localhost:19001

3. Container started but MCP endpoint returns an error

• Confirm the container was started with -e ENABLE_MCP_SERVER=true, -e MCP_TRANSPORT=streamable-http, and -e MCP_HOST=0.0.0.0
• Try curling the endpoint directly:

  curl -v http://localhost:9000/mcp
  

  A 405 Method Not Allowed response means the endpoint is reachable but expects a POST — that is normal and indicates the MCP server is up.

4. MCP tools fail with "Session not found" after container restart

Container restarts invalidate all active MCP sessions. The AI client still holds the old session ID, so every MCP call returns "Session not found". Start a new Claude Code session (or restart your MCP client) to reconnect.

5. Database connection refused inside the container

If health_check() passes but queries fail with a connection error, the database host is likely still set to localhost. Open the Web UI at http://localhost:9001, edit the connection info, and change the host to host.docker.internal.

Step 7 — Load an MDL and start querying

Option A — Auto-load at startup (recommended):

Place your compiled mdl.json in <WORKSPACE_PATH>/target/mdl.json before starting the container. The container reads MDL_PATH at startup and logs:

Loaded MDL /workspace/target/mdl.json (9 models, 47 columns)

Option B — Deploy via Web UI:

Open http://localhost:9001, paste or edit your MDL JSON in the MDL Editor, and click Save & Deploy.

Option C — Deploy via MCP tool (after container is running):

In the AI client (after MCP is connected):

deploy(mdl_file_path="/workspace/target/mdl.json")

Or deploy from an in-memory dict:

deploy_manifest(mdl=<manifest dict>)

To generate an MDL from scratch, use the wren-generate-mdl skill. It will introspect your database schema via ibis-server and build the manifest for you.

You're ready!

Start a new Claude Code session (or restart your MCP client). The Wren tools (health_check, query, deploy, setup_connection, etc.) are now available.

Try it:

Use health_check() to verify the connection.

Once confirmed, you can start querying your data through Wren Engine — use /wren-generate-mdl to scaffold an MDL from your database, or /wren-sql for help writing queries.

Source: Canner/wren-engine — distributed by TomeVault.