Operator correction to the prior 3.8 framing (d28a371), which over-blamed
AEON and dismissed the vLLM bug as a mere amplifier. Both were real and
compounded:
- Cause 1 (real, upstream): the qwen3_5_mtp x GDN partial-accept bug
(#51113), architectural across vLLM/SGLang/llama.cpp, genuinely improved
by the nightly fix -- not just an amplifier.
- Cause 2 (real, quant): AEON is FULL W4A4 (A4 activations on attention),
the bottom of the KNOWN activation-precision gradient already in 1
(W4A4 < W4+FP8 < W4+bf16) -- mildly subpar, not 'defective'. On top of
Cause 1 it degenerated ~15-20% of real multi-turn generations.
The mixed FP8-attention build sits a rung up that gradient and is coherent;
a W4+bf16 build would be higher still at a prefill cost. Process lessons
retained (two causes mask each other; stochastic degeneration is invisible
to n=1 probes; isolate weights in parallel with serving flags -- but the
weight swap alone would NOT have found the real vLLM bug).
Root cause of the multi-day degeneration hunt, operator-confirmed: the AEON
NVFP4 W4A4 quant (sakamakismile/Qwen3.8-27B-AEON-ULTIMATE-UNCENSORED-NVFP4,
full W4A4 incl. attention) went degenerate ~15-20% of generations in real
multi-turn use and forced regenerates. MTP, prefix-caching, and the gateway
all merely AMPLIFIED it, which is why MTP-off, APC-off, and the vLLM #51113
fix each 'helped' a synthetic probe without fixing it -- three plausible
false root-causes, each passing one clean run then failing in real use.
The fix was the WEIGHTS: the in-house JonathanColetti/Heretic mixed
NVFP4+FP8 build (qwen38-27b-uncensored-nvfp4-mixed, FP8 attention not W4A4,
same base, same MTP) is coherent through long multi-turn with MTP ON. W4A4
*attention* was the defect; FP8 attention is not.
This commit:
- GEN_MODEL -> the mixed FP8-attn build (primary gen until DavidAU 3.8 lands)
- GEN_IMAGE pinned to vllm/vllm-openai:nightly-311b3513... (v0.27.2rc1.dev150,
carries #51113; pinned by sha so it does not drift on the next pull)
- AEON weights PURGED from /tank (no-good), safety-checked not-in-use first
- playbook 3.8: the stochastic-W4A4-degeneration lesson + isolate-weights-early
+ do-not-declare-a-fix-from-one-probe (it validated three non-fixes)
AEON is re-pullable from HF if ever needed, but the operator ruled it no-good.
The qwen3_5_mtp corruption (playbook 3.7) is gated on MTP x prefix-caching
TOGETHER (vllm#43559 / #47194), per both cross-frontier peers. Disabling
prefix caching (--no-enable-prefix-caching; vLLM V1 defaults it ON, so the
explicit --no- form is required) forces the GDN cache into a mode where the
partial-accept align-path bug is inert, so MTP can stay on.
Verified on our stack (AEON W4A4): MTP on + prefix-caching off -> the 7-turn
varied series stays coherent through 3.9k tokens, zero cross-turn bleed, at
104.6 tok/s / 53.6% acceptance -- the FULL MTP speedup restored (vs ~half
with MTP off), losing only prefix-cache reuse. All 7 aliases route.
Ruled out on the way: num_speculative_tokens=1 (corruption is
depth-independent, n=1 and n=2 both corrupt); switching to SGLang (vLLM /
SGLang / llama.cpp mainline all share the architectural GDN-rollback bug).
Proper upstream fix (#51113) is in main / v0.27.2rc0 only, not stable, so we
hold at APC-off rather than jump the fleet gateway to an RC.
Supersedes the MTP-off config from 7bd38b3.
The single hardest bug of the night, and invisible to the existing
acceptance gate: a LOADED, healthy-accepting MTP head still corrupts
Qwen3.8-27B multi-turn output past ~2k cumulative tokens (length collapse +
cross-turn content bleed), while single-turn is perfect. Model-independent
across all three of our Qwen3.8 quants; Qwen3.6 on the same qwen3_5_mtp
method is clean; disabling MTP fixes it. New rule: gate MTP on a multi-turn
coherence probe, not just single-shot acceptance.
Quants are hard-fought and we keep re-paying for the same lessons. A survey
found quant knowledge scattered across 18 files in four trees, with three
documents having independently discovered and recorded overlapping
"landmines" sections — and one of them now actively misleading.
Adds docs/pfi/model-quantization-playbook.md as the single home for the
TRANSFERABLE lessons, with per-model artifacts demoted to worked examples
that link up to it. Contents:
- scheme decision table, incl. that a literal "W4A8" NVFP4 checkpoint is
unservable on vLLM (two legal activation settings, FP8 is not one)
- the reference mixed-precision recipe and the three parts of it that are
load-bearing and easy to drop
- the recurring landmines, ordered by cost: the loader-class trap
(rediscovered THREE times), the three separate ways to lose the MTP head,
toolchain deadlocks, vision configs, memory/device placement
- pipeline shape: prove targets before spending GPU time; mandatory
post-steps that verify rather than assume
- the acceptance gate, and the three ways measurement has lied to us —
prefix caching faking both speed metrics, prompt_logprobs going uniform
under speculative decoding, and a 0600 .env making compose silently no-op
- hardware/co-residency, including that a SMALLER model can starve its
neighbour because gpu-memory-utilization is a fraction of the whole card
- a superseded-claims table, and measured negatives not to re-chase
The superseded table earns its place immediately: the heretic2 runbook tells
readers to use modelopt because "compressed-tensors can't load the BF16 MTP
head, 0% acceptance". That symptom was real but the cause was not the format
-- it was the missing re:^mtp.* ignore entry. compressed-tensors gives
47.7-83.2% acceptance in production. A fresh session following that doc would
be sent down the modelopt path that current memory calls dependency hell, so
the runbook now carries a stale-warning header pointing here.
Wires discovery: an orientation.md "Where to look for what" row, pointers
from the gen-seat / heretic2 / mistral artifacts, and a CLAUDE.md maintenance
rule so the playbook gets fed instead of going stale -- model-agnostic
lessons land in the playbook, model-specific ones stay put, and a wrong
claim earns a dated superseded row rather than a silent edit.
Motivated by Qwen3.8 having just released: the next model swap will need a
requant, and this is what that session should read first.
vh