Qwen3.8-Flash-Next-MLX-mixed-4_8bit
MLX (Apple Silicon) build of Qwen3.8-Flash-Next — 125B-A6B hybrid Gated-DeltaNet / sparse-attention MoE with a 51B-parameter hashed n-gram embedding — quantized to 4-bit experts / 8-bit everything else.
These files are modified: the weights are converted to MLX and quantized; the architecture is unchanged. The 4B multi-token-prediction head is not included. The vision tower is carried in bfloat16 but the runtime below is text-only.
Runtime
qwen4_exp is in no released mlx-lm (https://github.com/ml-explore/mlx-lm/pull/1788 is open and unmerged), so this repository ships its
own qwen4_exp.py and declares it via model_file:
pip install -U mlx-lm
mlx_lm.generate --model pipenetwork/Qwen3.8-Flash-Next-MLX-mixed-4_8bit --trust-remote-code \
--prompt "Write a Python function that merges overlapping intervals." --max-tokens 300
from mlx_lm import load, generate
model, tokenizer = load("pipenetwork/Qwen3.8-Flash-Next-MLX-mixed-4_8bit", trust_remote_code=True)
The bundled runtime is the open PR with three numerical fixes found while validating it against
transformers 5.16 (details and tests in https://github.com/PipeNetwork/qwen38-flash-next-mlx):
| what | reference | the PR as submitted | effect |
|---|---|---|---|
| RMSNorm variant | x/rms · (1 + w), zero-initialised |
x/rms · w |
half the channels of every residual read sign-flipped |
| n-gram hash seed | 1234 (transformers default; not in config.json) |
0 |
every bigram/trigram looks up an unrelated row |
| sparse-attention prefill | per-query blocks, own partial block visible, causal | global blocks; leaks future tokens, drops the query's own | wrong hidden states for prompts > 2048 tokens |
This checkpoint follows the mlx-lm convention for the norm fix: the +1 is folded into the stored
norm weights at conversion, and the runtime multiplies by w. Tiny-config parity against
transformers is 1e-7 on the dense path, exact on cached decode and chunked prefill.
Size and what is quantized
106.2 GB on disk (bfloat16 upstream: 360.0 GB).
| group | share of parameters | this build |
|---|---|---|
routed experts (switch_mlp) |
120.8B (96.6% of the 125B) | 4-bit, group 64 |
| n-gram embedding tables (128 shards × [2,500,012 × 160]) | 51.2B (separate) | 4-bit, group 32 |
attention, DeltaNet, hyper-connections, shared experts, embeddings, lm_head |
~4.2B | 8-bit, group 64 |
MoE router, shared_expert_gate, residual write gates, DeltaNet in_proj_a/b, indexer projection |
36M | bfloat16 |
| vision tower | 0.4B | bfloat16 (unused by this runtime) |
The n-gram tables need group size 32 because their row width (160) is not a multiple of 64; left in bfloat16 they alone would be 102 GB.
Quality
Perplexity on wikitext-2 (test), 296,815 tokens in 145 windows of 2048, every build scored on identical windows through this runtime. Perplexity varies far more between windows than between quants, so the comparison that means anything is paired: per-window NLL differences against bfloat16, bootstrapped over one shared index set (20,000 resamples).
| build | size | perplexity | ΔNLL/token vs bf16 [95% CI] | windows worse |
|---|---|---|---|---|
| bfloat16 (upstream) | 360.0 GB | 4.4708 | — | — |
| 8bit | 192.2 GB | 4.4749 | +0.0009 [−0.0003, +0.0021] | 73/145 |
| 6bit | 148.0 GB | 4.4767 | +0.0013 [−0.0003, +0.0029] | 81/145 |
| mixed-4_8bit | 106.2 GB | 4.5286 | +0.0128 [+0.0109, +0.0148] | 128/145 |
| 4bit | 103.8 GB | 5.3914 | +0.1872 [+0.1778, +0.1968] | 145/145 |
Read the interval, not the point estimate: an interval that straddles zero is a build that is statistically indistinguishable from bfloat16 on this corpus; "windows worse" counts how many of the 145 windows the build lost outright.
The mixed 4/8-bit build is the one to use at this size. Routed experts are 96.6% of the parameters, yet quantizing everything to 4-bit costs +20.6% while keeping only the ~3% of non-expert weights (attention, DeltaNet, hyper-connections, shared experts, embeddings) at 8-bit brings that to +1.3% for 2.4 GB more — the uniform 4-bit build is dominated and is published for completeness. 6-bit and 8-bit are statistically indistinguishable from bfloat16 on this corpus.
Where the 4-bit damage comes from
One group at a time moved back to 8-bit from the uniform 4-bit build, same windows, same runtime (the ablation builds are not published):
| everything 4-bit except… | perplexity | vs bfloat16 |
|---|---|---|
| — (uniform 4-bit) | 5.3914 | +20.6% |
hyper-connection read gates (input_mix_weight_down/up, 0.6B) at 8-bit |
4.9744 | +11.3% |
| attention, DeltaNet, shared experts, PLE projections (~2.3B) at 8-bit | 4.8969 | +9.5% |
embed_tokens and lm_head (1.3B) at 8-bit |
5.2843 | +18.2% |
| all three at 8-bit (= mixed-4_8bit) | 4.5286 | +1.3% |
No single group is responsible: the hyper-connection gates and the attention/DeltaNet projections each carry about half of the loss and the effects are roughly additive, so every non-expert weight is worth its 8 bits. Per parameter, these ~4B weights are roughly 20x more quantization-sensitive than the 121B of routed experts.
Greedy generation (a collapse detector, not a ranking) is coherent on every published build.
License
Qwen Community License 1.0, as the upstream model. Port code: https://github.com/PipeNetwork/qwen38-flash-next-mlx.