PRD — Smallest from-scratch vision-language toy on consumer hardware

Goal

Train a LLaVA-style VL model from scratch on M5 Pro: pre-trained ViT encoder (frozen first cut) + small projector + small-from-scratch TinyGPT decoder + cross-attention or projector. Evaluate on TextVQA / VQAv2-mini. Publish “smallest from-scratch VL model runnable on a Mac” with a reproducible recipe.

Tier-5 research arc. ~2 weeks. The publishable artifact is the same shape as 5.1 / 5.2: smallest-X-on-consumer-hardware. The practical artifact is a vision-conditioned text-generation path that future Tier-B work (vision specialist, screen-understanding agent) can reuse.

Why now

• The Pace planner / agent layer wants a vision route for screen-understanding (factory-vision-specialist.md), and the cleanest unblock for that is: do we know how to fuse a vision encoder into our decoder at all from scratch? V1 doesn’t have to be production-grade, just functional.
• The smallest VL toys in the literature are ~100M (MobileVLM, etc.). “Smallest TinyGPT-class from-scratch VL” is novel enough to publish — and ahead of TTS/diffusion in the Tier-5 ordering because the cross-attention infra is closer to shipping.
• TinyGPT’s CrossAttention.swift already exists (used for YOCO); fusing into a vision-encoder K/V source is a small extension.

Scope — in

• Vision encoder: load a pretrained ViT-S/16 (any reasonable CLIP-class encoder) via swift-transformers; frozen for V1.
• Projector: 2-layer MLP from ViT hidden dim → TinyGPT d_model. Trained.
• Decoder: flagship-small or flagship-huge (TinyGPT from-scratch architecture). Token-soup the projector output before the text tokens, à la LLaVA-1.5.
• New file: Sources/TinyGPTModel/VLForward.swift — projector
  • token-soup forward path; routes image embedding to the prefix.
• Training: Sources/TinyGPT/TrainVL.swift — extend SFT with VL data loading. Stage 1: train projector only with frozen decoder. Stage 2: unfreeze decoder and continue. Standard LLaVA recipe.
• Data: LLaVA-Instruct (~80K), TextVQA (~30K), MS-COCO captions (~80K) — all open. Total ~200K samples, fits a 1-day training budget at 22M.
• Eval: tinygpt eval-vqa (new) on VQAv2-mini + TextVQA-test subsets. Compare to “decoder-only with image-as-text-description” baseline (CLIP caption → decoder).
• Artifact: docs/research/vl-toy-results.md — paper-shaped.

Scope — out

• Unfrozen vision encoder. V1 frozen. Joint training is V2 research.
• Multi-image inputs. One image per prompt for V1.
• Vision agent loop (screen + tools). That’s the production arc (factory-vision-specialist.md); this PRD is research.
• High-resolution images. 224×224 fixed for V1, like the early LLaVA papers.

Files to touch

Don’t touch

• The existing CrossAttention.swift (used by YOCO) — V1 LLaVA-style uses token-soup, not cross-attention. CrossAttention path is the V2 fusion option.

Acceptance criteria

• tinygpt train-vl --vit <vit-s.safetensors> --decoder flagship-small.tinygpt --data llava+textvqa+coco --stage 1 --steps 5000 --out vl-stage1.tinygpt runs and produces a checkpoint.
• Stage 2 (unfrozen decoder) continues; final model captions COCO images coherently (manual rubric inspection on 20 samples).
• tinygpt eval-vqa <model> --task vqav2-mini --limit 500 scores ≥ the caption-baseline by ≥ 5pp.
• Total model size on disk ≤ 500 MB (decoder + projector; ViT loaded separately).
• docs/research/vl-toy-results.md reports the recipe + numbers
  • “smallest from-scratch VL on M5 Pro” claim with the size comparison to MobileVLM / TinyLLaVA.

Reference patterns

• LLaVA-1.5 paper — the recipe template. Two-stage projector-then-decoder training.
• Sources/TinyGPTModel/HFModel.swift — HF safetensors reader; reuse for ViT.
• Sources/TinyGPT/Sft.swift — the training-loop template. VL training is SFT with image inputs.

Open questions

• Which ViT to import. Recommendation: ViT-S/16 (DINO) — small, works for VQA; SigLIP is the modern fancy option but larger. Document the choice.
• Whether to ship the trained model on HF / R2 alongside the docs. Recommendation: yes — the artifact is the proof.