Streaming, tool use (index-based delta accumulation), tool name sanitization (fs.read → fs_read), StreamOptions.IncludeUsage for token tracking. Hardcoded model list (gpt-4o, gpt-4o-mini, o3, o3-mini). Wired into CLI with OPENAI_API_KEY env support. Live verified: text streaming + tool calling with gpt-4o.