con-0006

New llama.cpp #25618 follow-up by snick525 (2026-08-22T02:11:18Z) on RDNA4 (AMD Radeon AI PRO R9700 / gfx1201, 32 GB, Vulkan/radv), target Qwen3.8-27B Q6_K_XL (MTP head intact), greedy (temperature 0, top_k 1, top_p 1, seed 42), f16 K/V. The reporter additionally built open PR #27342 (DFlash2, an external 2B draft model) so the built-in MTP head is no longer the only speculation type in the comparison. Three findings against a --spec-type none baseline on the same binary: 1. DIVERGENCE IS DRAFTER-INDEPENDENT. At -c 65536 / f16 KV the code prompt diverges at the same first-difference byte 204 for the built-in MTP head AND for DFlash2 at three drafter quants (Q4_K_M, Q8_0, BF16); all four configs are lossless on a reasoning prompt. Same first-diff byte holds across two context sizes (16384 and 65536) and two builds. Whatever drifts is on the target/verify side; the drafter is not a variable in it. 2. DRAFTER QUANTIZATION HAS NO EFFECT. DFlash2 Q4_K_M vs Q8_0 vs BF16 produce byte-identical outputs on both prompts and both context sizes, and identical acceptance counters (draft 347, accepted 283). Consistent with the verify path emitting only the target's own token: a drafter can only change output by proposing different tokens, and here even a Q4 drafter proposes identically, so drafter precision can be ruled out when bisecting. 3. KV-CACHE QUANTIZATION IS ITSELF OUTPUT-MOVING AND COMPOUNDS. Swapping f16 for q8_0 KV (everything else fixed) turns the reasoning prompt from LOSSLESS under f16 to DIVERGES at byte 155 under q8_0 for both MTP and DFlash2, and moves the code prompt's first-diff from byte 204 to byte 277. Separately, KV quantization alone moves greedy output with no speculation on either side: --spec-type none with f16 KV versus q8_0 KV differs on the code prompt at byte 561. The reporter notes q8_0 KV is accordingly never output-preserving and compounds with whatever this issue turns out to be. Throughput context (mentioned only because it came up in #27342, and reported with an active caveat): -c 65536 / f16 KV on Qwen3.8-27B Q6_K_XL on a 32 GB card gave 23.8 tok/s no-spec, 40.4 tok/s draft-mtp n_max 1, 61.9 tok/s DFlash2 Q4_K_M, and 29.4 tok/s for DFlash2 Q8_0 (the Q8 and BF16 drafter rows are NOT drafter-cost evidence: at 31.2-31.6 GB they cross the card's usable ceiling and silently fall back to host memory per #26432, with acceptance unchanged). Below the ceiling every drafter quant performs identically, so the Q8 spill is a memory-footprint artefact, not a drafter-precision finding. Upstream diagnostic record only. Surface is RDNA4 R9700/gfx1201 Vulkan, not our ROCm/Vulkan Strix Halo gfx1151 aihydra box, so no HaloBench benchmark claim is made from these figures.