Use the same rental_request_id for tenant and owner so both threads attach to the same rental (peer SSE + forwarding). Threads open automatically after login. With EXTERNAL_API_MOCK=true, defaults above match fixtures/mock_rental_requests.json key 1 and app MOCK_RENTAL_* defaults — change all three together if you customize.

Split hosts (recommended): run API on :8000, FE on :3000 (python scripts/serve_frontend_test.py from repo root). API base defaults to http://localhost:8000. Set CORS_ORIGINS=http://localhost:3000 (see .env.example). Same host: set ENABLE_TEST_UI=true and open /test-ui/ on :8000. SSE uses fetch with Bearer. Use JWT_ALLOW_UNVERIFIED=true and (recommended) EXTERNAL_API_MOCK=true for local dev.