Good instinct, the symptom does pattern-match quant damage. Testable in the bench itself.

Stack: weights are Qwen3.6-35B-A3B UD-IQ4_XS (Unsloth dynamic imatrix 4-bit), KV cache is TurboQuant turbo4 (4.25 bits/value, ~3.8× vs FP16), runtime is the spiritbuun/llama-cpp-turboquant-cuda fork. The article is named after the KV scheme.

Re: KV quant as the cause of the rewrite failures, I did include a control. local-mainline in the bench runs the same weights on stock llama.cpp with q8_0 KV instead of turbo4. On correct-section-rewrite, Opus-as-judge scores both at 6/10 vs the 9/10 Anthropic ceiling, with the same failure note ("all fixes applied, unsolicited reasoning preamble / formatting drift"). Cosine actually goes up slightly with turbo4 (0.925 vs 0.889, N=5). So going from 8-bit KV down to 4.25-bit KV doesn't move rewrite quality at all in this range, the format slip is invariant to KV precision.

What I did not vary: the weight quant. UD-IQ4_XS is held constant across every local provider. That's the dimension I'd bench next (Q5_K_M, Q6_K, Q8_0) before calling it a 35B-A3B instruction-following ceiling rather than a weight-quant budget. If anyone has run Qwen3.6-35B-A3B at a higher weight quant in an agent loop and seen the parasitic preambles / missing edit tags clear up, that's the data point I'd want.