Toy is a pure-Ruby transformer LM that compiles to a native binary via Spinel. Inference (KV-cache decode on CPU/CUDA/Metal), training (LoRA/full-FT/from-scratch via tinynn-FFI'd ggml), GGUF load + save, Tao-compatible events.jsonl emission. As a gem, it exposes the primitives a research project composes: tinynn FFI bridges, the sequence-forward training graph, ViT-Tiny, the Llama / SmolLM2 / Qwen2.5 inference cache, the GGUF loader, drift/grad/CKA observability, cosine LR schedules, and the GGUF checkpoint writer. Filed as toy#19; pairs with bundler-spinel / spinelgems for the consumer vendor flow.