On a hybrid model (full-attention + Mamba/GDN layers, e.g. Qwen3.6-35B-A3B),
enabling MTP/EAGLE speculative decoding together with prefix caching corrupts
the recurrent state, and the server serves wrong answers. #43559 reports
tool-eval accuracy dropping from ~90% to ~50%. Nothing raises an exception
and nothing shows up in the logs, so an operator has no signal that anything
is wrong, and prefix caching then spreads it, because the corrupted state is
hash-reachable and every later request sharing that prefix restores from it.
The root cause is the find_longest_cache_hit method on MambaManager,
at vllm/v1/core/single_type_kv_cache_manager.py:1049 on main, which takes
drop_eagle_block in its signature and never acts on it. Its body searches
right-to-left for the last cached block, builds [null]*i + [cached], and
breaks, referencing neither drop_eagle_block nor .pop(.
Signed-off-by: MOVZX <movzx@yahoo.com>
MOVZX
5da84df6b7