[Bugfix] Align hybrid model prefix cache hit lengths to prevent block missing on EAGLE

[huaiyann]

Purpose

Fix the accuracy drop when both using --enable-prefix-caching and MTP on mamba and full attn hybrid model like Qwen 3.5/3.6 family.

Associated issue: 43559

Root Cause Analysis

The reason is HybridKVCacheCoordinator.find_longest_cache_hit returns mismatched hit_length and full attn's hit blocks, and tokens under this mismatching are lost and do not participate in the full attention calculation when prefilling.

Technical Details & Example:

Detailed, when the mamba groups hit N blocks (N > 0) and the hit_length is N * block_size, unfortunately the full attn group only hit N-1 blocks, because the MTP/EAGLE is enabled and the last block is popped. In this case, the prefill forward will resume from token hit_length+1 (N*block_size+1), whereas the correct index should be (N-1)*block_size +1, so the tokens at block N are totally lost and not participated in the calculation.

Regression Source

This issue was introduced in PR #40860. It changed the while loop's max_length value in HybridKVCacheCoordinator.find_longest_cache_hit from curr_hit_length to min(curr_hit_length + spec.block_size, max_cache_hit_length). Since this change, MambaManager returns one additional hit block and does not pop it, leading to a mismatch with the hit block count from FullAttentionManager.

Test Plan

New Test Case: test_hybrid_model_full_attn_missing_block_on_eagle

Added to tests/v1/core/test_prefix_caching.py, this new test case verifies that the hit block counts from different attention groups in computed_blocks and the num_computed_tokens returned by get_computed_blocks exactly match and align with each other.

Third-Party Tool Verification: tool-eval-bench

An issue comment reports that this tool can easily replicate the accuracy drop. Evaluation using tool-eval-bench with Qwen 3.5 4B on an NVIDIA RTX 5070 Ti demonstrates that applying this patch successfully recovers the model's accuracy.

Test Result

Test Case

code version	pytest ./tests/v1/core/test_prefix_caching.py
unpatched	1 failed, 80 passed, 16 warnings FAILED tests/v1/core/test_prefix_caching.py::test_hybrid_model_full_attn_missing_block_on_eagle - assert 16 == 0
patched	81 passed, 16 warnings

Tool tool-eval-bench

Environment: Qwen3.5-4B on an NVIDIA RTX 5070 Ti

Core Parameters:

--tensor-parallel-size 1
--max-model-len 10000
--reasoning-parser qwen3
--enable-prefix-caching
--kv-cache-dtype bfloat16
--speculative-config '{"method": "mtp", "num_speculative_tokens": 1}'
--served-model-name "Qwen/Qwen3.5-4B"
--enable-auto-tool-choice
--tool-call-parser qwen3_coder

Test Command: tool-eval-bench --short

unpatched code score

• Final Score: 63 / 100
• Total Points: 19 / 30

patched code score

• Final Score: 90 / 100
• Total Points: 27 / 30

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @huaiyann.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[huaiyann]

already fixed at #46384