Current State (2592 lines, 6 files)

regular/responses/
├── mod.rs          (20 lines)   # Module exports
├── context.rs      (70 lines)   # ResponsesContext
├── types.rs        (18 lines)   # Just BackgroundTaskInfo ← merge into context.rs
├── conversions.rs  (428 lines)  # responses_to_chat, chat_to_responses
├── handlers.rs     (812 lines)  # Entry points + non-MCP execution + load_conversation_history
└── tool_loop.rs    (1244 lines) # ALL MCP tool loop logic (too big!)

Proposed Structure (mirroring harmony pattern)

regular/responses/
├── mod.rs            (~30 lines)  # Module exports
├── context.rs        (~90 lines)  # ResponsesContext + BackgroundTaskInfo (merged)
├── conversions.rs    (428 lines)  # Unchanged
├── common.rs         (~250 lines) # NEW: Shared helpers
│                                  #   - ToolLoopState (from tool_loop.rs)
│                                  #   - prepare_chat_tools_and_choice
│                                  #   - extract_all_tool_calls_from_chat
│                                  #   - convert_mcp_tools_to_chat_tools
│                                  #   - build_mcp_list_tools_item
│                                  #   - build_mcp_call_item
│                                  #   - generate_mcp_id
│                                  #   - load_conversation_history (from handlers.rs)
├── handlers.rs       (~100 lines) # THIN: Entry points only
│                                  #   - route_responses (dispatches to sync/streaming)
│                                  #   - route_responses_sync
│                                  #   - route_responses_streaming
├── non_streaming.rs  (~400 lines) # NEW: Non-streaming execution
│                                  #   - route_responses_internal (from handlers.rs)
│                                  #   - execute_tool_loop (from tool_loop.rs)
│                                  #   - execute_without_mcp (from handlers.rs)
└── streaming.rs      (~700 lines) # NEW: Streaming execution
                                   #   - execute_tool_loop_streaming (from tool_loop.rs)
                                   #   - execute_tool_loop_streaming_internal
                                   #   - convert_and_accumulate_stream
                                   #   - ChatResponseAccumulator
                                   #   - convert_chat_stream_to_responses_stream (from handlers.rs)
                                   #   - process_and_transform_sse_stream (from handlers.rs)
                                   #   - StreamingResponseAccumulator (from handlers.rs)