[Bug] MCP Server Fails Due to Internal API Mismatch Between `ibis-server` and `java-engine

[mknaepen-mycsn]

Describe the bug
When using the Wren MCP Server with a standalone Trino database, any attempt to run a SQL query from an MCP client fails with a generic connection error ('ConnectError' object has no attribute 'text').

The root cause is an internal API incompatibility between the ibis-server and java-engine containers. The ibis-server receives the request but then calls an API endpoint on the java-engine that does not exist, resulting in an HTTP 404 Not Found error within the backend, which breaks the query process. This is not a user configuration or networking issue, as all container networking has been verified to be working correctly.

To Reproduce
Steps to reproduce the behavior:

1. Set up a standalone Trino container using docker-compose.
2. Set up the Wren Engine services (ibis-server, java-engine) using the default docker-compose.yml from the repository.
3. Create a shared external Docker network and attach all three containers to it to ensure they can communicate.
4. Configure the wren-engine/mcp-server with the correct .env, mdl.json, and trino_connection.json files, pointing to the Trino container via its service name (trino_parquet_server).
5. Launch the MCP server via uv run app/wren.py.
6. Connect an AI agent (e.g., Cline, Claude Desktop) and attempt to run a SQL query (e.g., "how many channels are there").
7. Observe that the query fails with a connection error.

Expected behavior
The query should be successfully proxied from the ibis-server to the java-engine, executed on the Trino database, and the results returned to the MCP client. The health_check tool should report that the Wren Engine is healthy.

Screenshots
The user-facing error in the MCP client:

The error from the java-engine logs, which shows the true root cause:

2025-07-16T13:37:58.279Z  WARN  http-worker-22  io.wren.main.web.WrenExceptionMapper    Exception, type: class jakarta.ws.rs.NotFoundException, message: HTTP 404 Not Found
jakarta.ws.rs.NotFoundException: HTTP 404 Not Found
        at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:271)
        at org.glassfish.jersey.internal.Errors$1.call(Errors.java:248)
        ... (stack trace continues)

Desktop:

• OS: Linux Ubuntu 24 LTS (Docker Desktop)

Wren AI Information

• Version: latest tags as of July 16, 2025, for ghcr.io/canner/wren-engine-ibis and ghcr.io/canner/wren-engine.

Additional context
An exhaustive troubleshooting process was performed to isolate this bug. Here is the key evidence that proves all networking and configuration is correct:

1. Host-to-Ibis-Server Connection Works: A curl http://localhost:8000/v1/health from the host machine successfully reaches the ibis-server and receives a {"detail":"Not Found"} response, proving the port mapping is correct.

2. Ibis-Server-to-Trino Connection Works: A curl from inside the ibis-server container to the Trino container's service name and port (http://trino_parquet_server:8080) connects successfully and receives a valid HTTP 303 See Other response. This proves the Docker network bridge and DNS are working.

3. Ibis-Server-to-Java-Engine Connection Works: A curl from inside the ibis-server to the java-engine (http://java-engine:8080/v1/health) connects successfully and receives the HTTP 500 / 404 Not Found error shown in the screenshot above.

This combination of evidence definitively proves that all network links are functional. The failure occurs because the ibis-server makes a request to a URL path on the java-engine that does not exist, causing the java-engine to throw the NotFoundException and break the chain.