Your current environment

Your output of `python collect_env.py` here

🐛 Describe the bug

# Eval script used
python tests/evals/gsm8k/gsm8k_eval.py

# With EAGLE: 0% prefix cache hit rate
vllm serve openai/gpt-oss-120b \
  --speculative-config '{"model": "nvidia/gpt-oss-120b-Eagle3-long-context", "num_speculative_tokens": 3, "method": "eagle3"}'
# Prefix cache hit rate: 0.0%

# Without EAGLE: ~80% hit rate as expected
vllm serve openai/gpt-oss-120b
# Prefix cache hit rate: 88.7%

# Workaround: disable hybrid KV cache manager
vllm serve openai/gpt-oss-120b \
  --speculative-config '...' \
  --disable-hybrid-kv-cache-manager
# Prefix cache hit rate: ~80%

Qwen3 with EAGLE works fine (not a hybrid attention model).

Root Cause

In HybridKVCacheCoordinator.find_longest_cache_hit():

1. Full attention finds N blocks, does EAGLE drop → returns N-1 blocks
2. This sets max_length = (N-1) * block_size for sliding window
3. Sliding window searches range 0 to N-2, but its cached blocks may be at positions N-k to N-1 (within the window)
4. The last cached block (N-1) is now outside the search range
5. Sliding window can't find enough contiguous blocks to meet its threshold → returns 0
6. Sliding window also does its own EAGLE drop, causing cascading reductions in the convergence loop

Potential Solutions

Option 1: Coordinator-level EAGLE handling

• Remove EAGLE logic from individual managers
• Coordinator applies EAGLE drop once at the end after all managers converge
• Cleanest separation of concerns, but requires careful handling of per-manager threshold requirements

Option 2: EagleMode enum (implemented in my experimental PR #32801)

• Single eagle_mode parameter: DISABLED, PRIMARY, SECONDARY
• PRIMARY: This manager handles EAGLE (threshold+1, do pop)
• SECONDARY: Another manager handled EAGLE (threshold-1, no pop)
• Coordinator assigns: full attention = PRIMARY, others = SECONDARY
• If no full attention, all managers = PRIMARY (this is the part I'm least sure about)

Option 3: Skip EAGLE for non-full-attention in hybrid models

• In HybridKVCacheCoordinator, only pass EAGLE flag to full attention
• Other managers just find best match within the (already reduced) range
• Simpler but may need threshold adjustment for sliding window

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