Fix PD decode radix cache double-counting cached_tokens (#25973)

python/sglang/srt/disaggregation/decode.py

@@ -1436,6 +1436,13 @@ class DecodeTransferQueue:
         # Success - commit the transfer
         decode_req.req.output_ids.append(output_id[0].item())
         decode_req.req.cached_tokens = cached_tokens[0].item()
+        # The prefill node already reported its prefix-cache hit in
+        # cached_tokens[0]. Seed already_computed with it so that
+        # prepare_for_prebuilt's `cached_tokens += pre_len - already_computed`
+        # only adds decode-side reuse *beyond* what prefill counted, instead of
+        # double-counting the shared prompt prefix (which would make
+        # cached_tokens exceed prompt_tokens when decode radix cache is on).
+        decode_req.req.already_computed = decode_req.req.cached_tokens
         decode_req.req.cached_tokens_device = cached_tokens[1].item()
         decode_req.req.cached_tokens_host = cached_tokens[2].item()
         decode_req.req.cached_tokens_storage = cached_tokens[3].item()

python/sglang/srt/disaggregation/decode_schedule_batch_mixin.py

@@ -62,7 +62,13 @@ class ScheduleBatchDisaggregationDecodeMixin:
                 ), f"seq_len={seq_len}, pre_len={pre_len}, req.extend_input_len={req.extend_input_len}"
 
             if not req.retracted_stain:
-                req.cached_tokens += pre_len - req.already_computed
+                # Clamp to avoid double-counting: already_computed is seeded from
+                # the prefill-reported cached_tokens in _commit_transfer_to_req, so
+                # a decode-side prefix shorter than the prefill report must not
+                # subtract from cached_tokens.
+                delta = max(0, pre_len - req.already_computed)
+                req.cached_tokens += delta
+                req.cached_tokens_device += delta
                 req.already_computed = seq_len
             req.is_retracted = False
             pre_lens.append(pre_len)

python/sglang/test/kits/cache_hit_kit.py

@@ -344,6 +344,15 @@ def run_multiturn_cache_hit_test(
             print(msg)
 
             assert resp.cached_tokens >= expected_cached
+            # Upper bound: cached tokens are a subset of the prompt, so they can
+            # never exceed prompt_len. In PD disaggregation with decode radix
+            # cache, the shared prefix was previously counted on both the prefill
+            # and the decode node, making cached_tokens exceed prompt_len.
+            assert resp.cached_tokens <= resp.prompt_len, (
+                f"Round {round_num}, client {i}: cached_tokens="
+                f"{resp.cached_tokens} exceeds prompt_len={resp.prompt_len} "
+                f"(double-counted prefix across prefill/decode)"
+            )
 
             # Record this round's prompt_len for next round's expected calc
             prev_prompt_lens[i] = resp.prompt_len