[Bugfix][Core] Cache output blocks of reasoning requests (#1338)

#1297 stopped caching the output blocks of requests that start in reasoning
mode, assuming chat templates drop past reasoning. Qwen3.8 renders past
`reasoning` back verbatim by default, and keeps it inside a tool-call loop
even with preserve_thinking=false, so a client that returns it sends the
previous output back as the next prompt prefix.

The rule only applied in KVCacheManager.allocate_slots; the async
scheduler, used by the Qwen3.8 release profile, caches every finalized
token anyway. Remove it so both schedulers cache finalized output blocks.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/v1/core/test_partial_prefix_cache.py

@@ -71,7 +71,7 @@ def test_eviction_releases_block_references():
     assert pool.get_num_free_blocks() == free_before
 
 
-def _request(boundaries, reasoning_ended=None):
+def _request(boundaries):
     return Request(
         request_id="r",
         prompt_token_ids=list(range(1000)),
@@ -79,7 +79,6 @@ def _request(boundaries, reasoning_ended=None):
             max_tokens=4, extra_args={"prefix_cache_boundaries": boundaries}
         ),
         pooling_params=None,
-        reasoning_ended=reasoning_ended,
     )
 
 
@@ -89,12 +88,6 @@ def test_request_boundaries_align_to_recurrent_chunks():
     assert request.prefix_cache_boundaries == (align, 3 * align, 999 // align * align)
 
 
-def test_reasoning_requests_do_not_cache_output_tokens():
-    assert not _request([], reasoning_ended=False).cache_output_tokens
-    assert _request([], reasoning_ended=True).cache_output_tokens
-    assert _request([]).cache_output_tokens
-
-
 class _Tokenizer:
     """ChatML-like template: one token per role marker and content word."""
 

vllm/v1/core/kv_cache_manager.py

@@ -739,9 +739,7 @@ class KVCacheManager:
         # "finalized" tokens are cached.
         num_tokens_to_cache = min(
             total_computed_tokens + num_new_tokens,
-            request.num_tokens
-            if request.cache_output_tokens
-            else request.num_prompt_tokens,
+            request.num_tokens,
         )
         self.coordinator.cache_blocks(request, num_tokens_to_cache)
 

vllm/v1/request.py

@@ -174,10 +174,6 @@ class Request:
         )
         # Partial prefix-cache entry selected by the last cache lookup.
         self.partial_prefix_entry: Any = None
-        # A request that starts in reasoning (thinking) mode: chat templates
-        # drop the reasoning from later turns, so its output blocks can never
-        # be a prefix of another request and are not cached.
-        self.cache_output_tokens = reasoning_ended is not False
 
         # Multi-modal related
         self.mm_features = mm_features or []