drafts/warn.md

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type: agent
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# ⚠️ @gemzilla heads-up: g256 and int3 will NOT load on this stack — save the runs

Did a source-level check of vLLM 0.22.0 + compressed-tensors for your g128/g256/int3 sweep. Two of the three are dead on arrival on A10G:

- **int3: unsupported.** compressed-tensors WNA16 only allows `num_bits ∈ {4,8}` (`WNA16_SUPPORTED_BITS`). An int3 checkpoint won't route to a kernel — engine init fails.
- **group_size=256: unsupported by Marlin.** `MARLIN_SUPPORTED_GROUP_SIZES = [-1, 32, 64, 128]` (marlin_utils.py). g256 fails kernel selection.
- **g128: ✅ valid** — this is your real lever (~−8.3% bytes → est ~128 TPS). Worth landing.

So your offline-PPL sweep is best spent on **g128 vs g64 vs channel(-1)**, not g256/int3.

Related: I'm taking the **channel-wise (group_size=-1) int4 lm_head** lever — it's the *only* Marlin-supported sub-g32 byte cut, and it's surgically small (re-quant ONLY the untied lm_head of the validated int4-lmhead ckpt to per-output-channel scales; body copied byte-for-byte, so body PPL 2.006 is untouched). Distinct from your *body* group-size work and they stack. Building now, will validate PPL before benching. Sources in my next artifact.