Found while answering a question from the operator, relayed via brokkr-smithy-dev,
about whether a recorded Qwen3.8 degeneracy at ~1,700 tokens relates to a length
sensitivity just measured on the tuned Gemma-4. The record is §3.7 and the number is
~2,000 -- but reading it to answer that question surfaced that the section is stale.
§3.7 presented "disable prefix caching, keep MTP" as THE MITIGATION, resolved
2026-08-17, and stated the gen seat runs that config. It does not and has not since
that same day: APC-off passed a synthetic 7-turn probe and the operator still saw
severe degeneration in real use, so it was reverted. The multi-day hunt resolved to
the AEON W4A4 quant being defective, with MTP / prefix-caching / gateway merely
amplifying it (§3.8 records the corrected causal story; §3.7 was never updated to
match).
Verified against the live container rather than against the compose file alone:
vllm-gen runs --enable-prefix-caching with qwen3_5_mtp / num_speculative_tokens 3.
stacks/gen-seat/compose.yaml carries the full corrected history inline and is the
current authority.
§3.7's superseded text is kept and fenced rather than deleted -- it is the history of
a mitigation that looked right and was not. Added a dated row to §7 per the standing
rule that a wrong playbook claim gets a superseded-claims entry, not just a fix.
The lesson inside the lesson is worth more than the correction: §3.7's own standing
rule is "gate MTP on a multi-turn coherence probe, not just single-shot acceptance."
The APC-off mitigation was gated on exactly that probe, passed it, and still failed in
real use -- the multi-turn probe was itself too small to gate on. A passing probe is
not sufficient evidence at any size that has not been calibrated against real use.
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