Fix token leak with logprob_start_len=0 in streaming sessions (#20557)

python/sglang/srt/mem_cache/session_aware_cache.py

@@ -181,8 +181,26 @@ class SessionAwareCache(BasePrefixCache):
 
         slot.restore_to_req(req)
 
-        max_prefix_len = len(params.key.token_ids)
+        # For streaming sessions, ignore the logprob_start_len truncation on
+        # the key and reuse the full committed KV from the slot. The key may
+        # have been truncated (e.g. to length 0 when logprob_start_len=0), but
+        # the session slot already holds those KV tokens — skipping them would
+        # orphan allocated memory. Use fill_ids length (the actual input for
+        # this turn) as the upper bound instead of the truncated key.
+        # Also clamp logprob_start_len so the scheduler doesn't expect logprobs
+        # for tokens that are already in the session's committed KV.
+        input_len = (
+            len(req.fill_ids) if req.fill_ids is not None else len(params.key.token_ids)
+        )
+        max_prefix_len = max(input_len - 1, 0)
         prefix_len = min(req.kv_committed_len, max_prefix_len)
+
+        if (
+            req.return_logprob
+            and req.logprob_start_len >= 0
+            and req.logprob_start_len < prefix_len
+        ):
+            req.logprob_start_len = prefix_len
         device_indices = self.req_to_token_pool.req_to_token[
             req.req_pool_idx, :prefix_len
         ].to(dtype=torch.int64)