curl -X POST http://localhost:8080/apps/linux_admin/users/alice/sessions/dev_session -H 'Content-Type: application/json' -d '{}'

Step one of the two-step ADK HTTP API call: creates a session for the given app, user, and session ID with an empty body. Without this step, POST /run returns 404 'Session not found'. The session ID must be a valid Python identifier (underscores, no hyphens). Sessions persist — reuse the ID for later turns.