# eRP dual-seat overhaul — MeroMero-v2 + Dark-Scarlett, NVFP4A16 @ 256K on ana-ml2

`[2026-08-12]` Replaced the two legacy char-rp seats with home-quantized NVFP4A16 vLLM
seats. Operator-driven, end to end this session.

## What landed

| Seat (LiteLLM alias) | Model | Role | GPU | Context |
|---|---|---|---|---|
| `char-rp` (:8016) | **G4-MeroMero-v2-31B** (Gemma-4) | non-thinking PROSE, **multimodal (vision)** | GPU0 | 256K @ 2.07× (util 0.52) |
| `char-rp-reasoning` (:8018) | **Dark-Scarlett-v1.0-27B** (Qwen3.6) | THINKING (default) | GPU1 | 256K @ 1.62× (util 0.44) |

- Both **NVFP4A16 weight-only** (llm-compressor, `compressed-tensors`), `--kv-cache-dtype fp8`.
- Replace: `char-rp-gguf` (Magidonia-24B GGUF/llama.cpp, :8016) + `heretic2-charrp-reasoning`
  (DavidAU Qwen3.6-27B-Heretic2 modelopt NVFP4+MTP, :8018). Old stacks/containers **stopped +
  retained** for rollback.
- Compose-ified: `stacks/meromero-charrp` + `stacks/darkscarlett-charrp-reasoning` (ana-ml2
  `/opt/docker/compose/`, mirrored to eshpfi, commit **`f08b6cb`**) → survive reboot.
- Research that drove picks: `docs/pfi/erp-thinking-finetunes-2026.md` (from the `gecko-65` Booth).

## Load-bearing lessons (the whole point of this file)

1. **Load via the ConditionalGeneration WRAPPER class, never `AutoModelForCausalLM`.** For a
   multimodal-capable base (Gemma-4, Qwen3.6), `AutoModelForCausalLM.from_pretrained` +
   `save_pretrained` writes a FLAT text config (`Qwen3_5TextConfig`, `model.layers.*`) that
   **both vLLM AND SGLang reject** (SGLang: "Qwen3_5ForCausalLM has no SGLang implementation";
   vLLM wants `Qwen3_5ForConditionalGeneration`). Loading via `Qwen3_5ForConditionalGeneration` /
   `Gemma4ForConditionalGeneration` keeps the wrapper config they accept. **This was the DS
   blocker** — re-quant via the wrapper fixed it (`Dark-Scarlett-...-NVFP4A16-wrapper`).
2. **NVFP4A16 is weight-only → DATA-FREE.** llm-compressor infers `DataFreePipeline`; calibration
   data is unused (only matters for W4A4 activation quant). W4A16 chosen per NVIDIA's sm_120
   long-context guidance (W4A4 KLD 2-4× worse past ~10k ctx).
3. **Load on CPU (`device_map=None`)** so llm-compressor onloads one layer at a time. `device_map=
   "auto"` packs the whole model onto the GPU and OOMs when the card isn't fully free.
4. **Both models are KV-EFFICIENT — the "dense = KV-hungry" worry was WRONG.** MeroMero (Gemma-4)
   uses **sliding-window attention** (most layers cache only a bounded window); DS (Qwen3.6) uses
   **hybrid GatedDeltaNet linear-attention** (3:1 linear:full, linear layers carry no KV). Both
   hit full native 256K easily. (MeroMero KV pool ~542K tokens at util 0.52.)
5. **MeroMero vision reconstruction.** The finetune ships `processor_config.json` (image_processor
   inline, `Gemma4ImageProcessor`) but NOT `preprocessor_config.json` — the old-format file vLLM's
   feature-extractor loader wants. **Even google/gemma-4-31B-it (ungated!) ships only
   processor_config.json.** FIX: extract the `image_processor` section → write
   `preprocessor_config.json` verbatim, serve WITHOUT `--language-model-only`. Verified (model
   correctly ID'd a red circle). Audio is config-declared but WEIGHTLESS (0 audio tensors).
6. **GPU placement.** Match the KV-heavier model to the roomier GPU. GPU0 (gen neighbor, ~54GB
   free) > GPU1 (utility cluster, ~45GB free). Swapped MeroMero→GPU0, DS→GPU1. Pins via compose
   `deploy.resources.reservations.devices`.

## Dead ends (tried + abandoned)

- **DS via llm-compressor `AutoModelForCausalLM`** → flat config vLLM/SGLang reject. → wrapper class.
- **DS via NVIDIA ModelOpt** → modelopt↔transformers **version deadlock**: current transformers
  supports `qwen3_5` but crashes modelopt's sparse-moe plugin (`issubclass()` on a non-class);
  modelopt 0.43.0 pulls an old transformers that can't load `qwen3_5` at all. Abandoned.
- **DS via SGLang** → `Qwen3_5ForCausalLM has no SGLang implementation`. Abandoned, but it REVEALED
  that both engines need the wrapper (→ the fix in lesson 1).
- **`device_map="auto"` for the quant** → CUDA OOM in the weight observer. → `device_map=None`.

## granite retired + gateway repoint

- `vllm-granite` (granite-4.1-8b, fleet summarizer, GPU1) **`docker stop`ped** (reversible) to
  reclaim ~13.6GB GPU1 for RP context.
- LiteLLM (`ana-docker:/opt/docker/conf/litellm/config.yaml`, backed up
  `.bak-pre-granite-down-*`): **`granite-4.1-8b` alias RETIRED** — commented out, now 404s cleanly
  (the `*` wildcard→llama-swap was decommissioned 2026-06-20, so no fallthrough). **`summarizer` +
  `classifier` REPOINTED to gen** (`hosted_vllm/qwen3.6-35b-a3b-heretic` @ :8015,
  `enable_thinking:false`) — both verified. ⚠ This LiteLLM change is **server-only / not
  version-controlled** (a follow-up).

## MTP — deferred

DS's MTP heads were dropped by the CausalLM loader; **deferred, not restored** (spec-decode is
net-negative at RP temps: ~38-52% accept at temp 0.8-1.25, below vLLM's 0.5 cutoff). The
splice-back path (`splice_mtp.py` in the heretic2 work dir) exists if ever wanted. MeroMero
(Gemma-4) has no MTP by architecture.

## On-disk / where things live

- Quant pipelines: `ana-ml2:/tank/aimodels/meromero-v2-nvfp4-work/` +
  `/tank/aimodels/darkscarlett-nvfp4-work/` (scripts, BF16 source, NVFP4 outputs).
- Compose stacks: `ana-ml2:/opt/docker/compose/{meromero-charrp,darkscarlett-charrp-reasoning}/`.
- Gateway aliases (unchanged, port-based): `char-rp`→:8016, `char-rp-reasoning`→:8018. (char-rp was
  also fixed from the stale `magidonia-24b-v4.3` backend model name → `char-rp`.)

## Open follow-ups

1. LiteLLM granite/repoint change NOT version-controlled (server + backup only).
2. eshpfi unpushed (many commits this session incl. `f08b6cb`, `7bd7375`, `398b58a`).
3. MTP deferred (see above).
4. DS thinks verbosely (~13:1 reasoning:content) — eval item; consumers need generous `max_tokens`.
5. MeroMero full 256K needs util 0.55 (GPU0 ~1.8GB free, tight); ran at 0.52 for headroom (~4.6GB).
