Auto-discover workspace ID to set DATABRICKS_GATEWAY_HOST automatically

[datasciencemonkey]

Summary

Auto-construct DATABRICKS_GATEWAY_HOST from the auto-injected DATABRICKS_WORKSPACE_ID env var, eliminating it as a manually-configured env var in app.yaml.

Key Finding

Databricks Apps auto-inject DATABRICKS_WORKSPACE_ID into every container. The AI Gateway URL follows a predictable pattern:

https://{DATABRICKS_WORKSPACE_ID}.ai-gateway.cloud.databricks.com

Implementation Plan

1. Add a helper that constructs the gateway URL from DATABRICKS_WORKSPACE_ID
2. Use DATABRICKS_GATEWAY_HOST env var as an explicit override (if set)
3. Fall back to constructed URL when DATABRICKS_GATEWAY_HOST is not set but DATABRICKS_WORKSPACE_ID is
4. Final fallback: DATABRICKS_HOST/serving-endpoints (existing behavior)
5. Remove DATABRICKS_GATEWAY_HOST from app.yaml (no longer needed)
6. Update docs

Files to update

• utils.py — add get_gateway_host() helper
• app.py — use new helper
• setup_claude.py, setup_codex.py, setup_gemini.py, setup_opencode.py, setup_proxy.py — use new helper
• app.yaml — remove DATABRICKS_GATEWAY_HOST env entry
• README.md, docs/deployment.md — document auto-discovery, note override option